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From Wikipedia, the free encyclopedia

Human blood smear:
a – erythrocytes; b – neutrophil;
c – eosinophil; d – lymphocyte.
Blood circulation:
Red = oxygenated
Blue = deoxygenated
Human blood magnified 600 times
Frog blood magnified 600 times
Fish blood magnified 600 times

Blood is a specialized bodily fluid that delivers necessary substances to the body's cells – such as nutrients and oxygen – and transports waste products away from those same cells.

In vertebrates, it is composed of blood cells suspended in a liquid called blood plasma. Plasma, which comprises 55% of blood fluid, is mostly water (90% by volume),[1] and contains dissolved proteins, glucose, mineral ions, hormones, carbon dioxide (plasma being the main medium for excretory product transportation), platelets and blood cells themselves. The blood cells present in blood are mainly red blood cells (also called RBCs or erythrocytes) and white blood cells, including leukocytes and platelets. The most abundant cells in vertebrate blood are red blood cells. These contain hemoglobin, an iron-containing protein, which facilitates transportation of oxygen by reversibly binding to this respiratory gas and greatly increasing its solubility in blood. In contrast, carbon dioxide is almost entirely transported extracellularly dissolved in plasma as bicarbonate ion.

Vertebrate blood is bright red when its hemoglobin is oxygenated. Some animals, such as crustaceans and mollusks, use hemocyanin to carry oxygen, instead of hemoglobin. Insects and some molluscs use a fluid called hemolymph instead of blood, the difference being that hemolymph is not contained in a closed circulatory system. In most insects, this "blood" does not contain oxygen-carrying molecules such as hemoglobin because their bodies are small enough for their tracheal system to suffice for supplying oxygen.

Jawed vertebrates have an adaptive immune system, based largely on white blood cells. White blood cells help to resist infections and parasites. Platelets are important in the clotting of blood.[2] Arthropods, using hemolymph, have hemocytes as part of their immune system.

Blood is circulated around the body through blood vessels by the pumping action of the heart. In animals with lungs, arterial blood carries oxygen from inhaled air to the tissues of the body, and venous blood carries carbon dioxide, a waste product of metabolism produced by cells, from the tissues to the lungs to be exhaled.

Medical terms related to blood often begin with hemo- or hemato- (also spelled haemo- and haemato-) from the Ancient Greek word αἷμα (haima) for "blood". In terms of anatomy and histology, blood is considered a specialized form of connective tissue, given its origin in the bones and the presence of potential molecular fibers in the form of fibrinogen.



green = heme groups
red & blue = protein subunits

Blood performs many important functions within the body including:

Constituents of human blood

Two tubes of EDTA-anticoagulated blood.
Left tube: after standing, the RBCs have settled at the bottom of the tube.
Right tube: contains freshly drawn blood.

Blood accounts for 8% of the human body weight,[4] with an average density of approximately 1060 kg/m3, very close to pure water's density of 1000 kg/m3.[5] The average adult has a blood volume of roughly 5 liters (1.3 gal), composed of plasma and several kinds of cells (occasionally called corpuscles); these formed elements of the blood are erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes (platelets). By volume, the red blood cells constitute about 45% of whole blood, the plasma about 54.3%, and white cells about 0.7%.

Whole blood (plasma and cells) exhibits non-Newtonian fluid dynamics; its flow properties are adapted to flow effectively through tiny capillary blood vessels with less resistance than plasma by itself. In addition, if all human hemoglobin were free in the plasma rather than being contained in RBCs, the circulatory fluid would be too viscous for the cardiovascular system to function effectively.


One microliter of blood contains:

  • 4.7 to 6.1 million (male), 4.2 to 5.4 million (female) erythrocytes:[6] In most mammals, mature red blood cells lack a nucleus and organelles. They contain the blood's hemoglobin and distribute oxygen. The red blood cells (together with endothelial vessel cells and other cells) are also marked by glycoproteins that define the different blood types. The proportion of blood occupied by red blood cells is referred to as the hematocrit, and is normally about 45%. The combined surface area of all red blood cells of the human body would be roughly 2,000 times as great as the body's exterior surface.[7]
  • 4,000–11,000 leukocytes:[8] White blood cells are part of the immune system; they destroy and remove old or aberrant cells and cellular debris, as well as attack infectious agents (pathogens) and foreign substances. The cancer of leukocytes is called leukemia.
  • 200,000–500,000 thrombocytes:[8] thrombocytes, also called platelets, are responsible for blood clotting (coagulation). They change fibrinogen into fibrin. This fibrin creates a mesh onto which red blood cells collect and clot, which then stops more blood from leaving the body and also helps to prevent bacteria from entering the body.
Constitution of normal blood
Parameter Value

45 ± 7 (38–52%) for males
42 ± 5 (37–47%) for females

pH 7.35–7.45
base excess −3 to +3
PO2 10–13 kPa (80–100 mm Hg)
PCO2 4.8–5.8 kPa (35–45 mm Hg)
HCO3 21–27 mM
Oxygen saturation

Oxygenated: 98–99%
Deoxygenated: 75%


About 55% of whole blood is blood plasma, a fluid that is the blood's liquid medium, which by itself is straw-yellow in color. The blood plasma volume totals of 2.7–3.0 litres (2.8–3.2 quarts) in an average human. It is essentially an aqueous solution containing 92% water, 8% blood plasma proteins, and trace amounts of other materials. Plasma circulates dissolved nutrients, such as glucose, amino acids, and fatty acids (dissolved in the blood or bound to plasma proteins), and removes waste products, such as carbon dioxide, urea, and lactic acid.

Other important components include:

The term serum refers to plasma from which the clotting proteins have been removed. Most of the proteins remaining are albumin and immunoglobulins.


Blood pH is regulated to stay within the narrow range of 7.35 to 7.45, making it slightly alkaline.[3][9] Blood that has a pH below 7.35 is too acidic, whereas blood pH above 7.45 is too alkaline. Blood pH, partial pressure of oxygen (pO2), partial pressure of carbon dioxide (pCO2), and HCO3 are carefully regulated by a number of homeostatic mechanisms, which exert their influence principally through the respiratory system and the urinary system in order to control the acid-base balance and respiration. An arterial blood gas will measure these. Plasma also circulates hormones transmitting their messages to various tissues. The list of normal reference ranges for various blood electrolytes is extensive.

Bones are especially affected by blood pH as they tend to be used as a mineral source for pH buffering. Consuming a high ratio of animal protein to vegetable protein is implicated in bone loss in women.[10]

Blood in non-human vertebrates

Human blood is typical of that of mammals, although the precise details concerning cell numbers, size, protein structure, and so on, vary somewhat between species. In non-mammalian vertebrates, however, there are some key differences:[11]

  • Red blood cells of non-mammalian vertebrates are flattened and ovoid in form, and retain their cell nuclei
  • There is considerable variation in the types and proportions of white blood cells; for example, acidophils are generally more common than in humans
  • Platelets are unique to mammals; in other vertebrates, small, nucleated, spindle cells are responsible for blood clotting instead


Cardiovascular system

The circulation of blood through the human heart

Blood is circulated around the body through blood vessels by the pumping action of the heart. In humans, blood is pumped from the strong left ventricle of the heart through arteries to peripheral tissues and returns to the right atrium of the heart through veins. It then enters the right ventricle and is pumped through the pulmonary artery to the lungs and returns to the left atrium through the pulmonary veins. Blood then enters the left ventricle to be circulated again. Arterial blood carries oxygen from inhaled air to all of the cells of the body, and venous blood carries carbon dioxide, a waste product of metabolism by cells, to the lungs to be exhaled. However, one exception includes pulmonary arteries, which contain the most deoxygenated blood in the body, while the pulmonary veins contain oxygenated blood.

Additional return flow may be generated by the movement of skeletal muscles, which can compress veins and push blood through the valves in veins toward the right atrium.

The blood circulation was famously described by William Harvey in 1628.[12]

Production and degradation of blood cells

In vertebrates, the various cells of blood are made in the bone marrow in a process called hematopoiesis, which includes erythropoiesis, the production of red blood cells; and myelopoiesis, the production of white blood cells and platelets. During childhood, almost every human bone produces red blood cells; as adults, red blood cell production is limited to the larger bones: the bodies of the vertebrae, the breastbone (sternum), the ribcage, the pelvic bones, and the bones of the upper arms and legs. In addition, during childhood, the thymus gland, found in the mediastinum, is an important source of lymphocytes.[13] The proteinaceous component of blood (including clotting proteins) is produced predominantly by the liver, while hormones are produced by the endocrine glands and the watery fraction is regulated by the hypothalamus and maintained by the kidney.

Healthy erythrocytes have a plasma life of about 120 days before they are degraded by the spleen, and the Kupffer cells in the liver. The liver also clears some proteins, lipids, and amino acids. The kidney actively secretes waste products into the urine.

Oxygen transport

Basic hemoglobin saturation curve. It is moved to the right in higher acidity (more dissolved carbon dioxide) and to the left in lower acidity (less dissolved carbon dioxide)

About 98.5% of the oxygen in a sample of arterial blood in a healthy human breathing air at sea-level pressure is chemically combined with the Hgb. About 1.5% is physically dissolved in the other blood liquids and not connected to Hgb. The hemoglobin molecule is the primary transporter of oxygen in mammals and many other species (for exceptions, see below). Hemoglobin has an oxygen binding capacity of between 1.36 and 1.37 ml O2 per gram Hemoglobin,[14] which increases the total blood oxygen capacity seventyfold,[15] compared to if oxygen solely was carried by its solubility of 0.03 mL O2 per litre blood per mmHg partial pressure of oxygen (approximately 100 mmHg in arteries).[15]

With the exception of pulmonary and umbilical arteries and their corresponding veins, arteries carry oxygenated blood away from the heart and deliver it to the body via arterioles and capillaries, where the oxygen is consumed; afterwards, venules, and veins carry deoxygenated blood back to the heart.

Under normal conditions in humans at rest, hemoglobin in blood leaving the lungs is about 98–99% saturated with oxygen. In a healthy adult at rest, deoxygenated blood returning to the lungs is still approximately 75% saturated.[16][17] Increased oxygen consumption during sustained exercise reduces the oxygen saturation of venous blood, which can reach less than 15% in a trained athlete; although breathing rate and blood flow increase to compensate, oxygen saturation in arterial blood can drop to 95% or less under these conditions.[18] Oxygen saturation this low is considered dangerous in an individual at rest (for instance, during surgery under anesthesia. Sustained hypoxia (oxygenation of less than 90%), is dangerous to health, and severe hypoxia (saturations of less than 30%) may be rapidly fatal.[19]

A fetus, receiving oxygen via the placenta, is exposed to much lower oxygen pressures (about 21% of the level found in an adult's lungs), and, so, fetuses produce another form of hemoglobin with a much higher affinity for oxygen (hemoglobin F) in order to function under these conditions.[20]

Carbon dioxide transport

When blood flows through capillaries, carbon dioxide diffuses from the tissues into the blood. Some carbon dioxide is dissolved in the blood. Some carbon dioxide reacts with hemoglobin and other proteins to form carbamino compounds. The remaining carbon dioxide is converted to bicarbonate and hydrogen ions through the action of RBC carbonic anhydrase. Most carbon dioxide is transported through the blood in the form of bicarbonate ions.

Carbon dioxide (CO2), the main cellular waste product is carried in blood mainly dissolved in plasma, in equilibrium with bicarbonate (HCO3-) and carbonic acid (H2CO3). 86–90% of CO2 in the body is converted into carbonic acid, which can quickly turn into bicarbonate, the chemical equilibrium being important in the pH buffering of plasma.[21] Blood pH is kept in a narrow range (pH between 7.35 and 7.45).[22]

Transport of hydrogen ions

Some oxyhemoglobin loses oxygen and becomes deoxyhemoglobin. Deoxyhemoglobin binds most of the hydrogen ions as it has a much greater affinity for more hydrogen than does oxyhemoglobin.

Lymphatic system

In mammals, blood is in equilibrium with lymph, which is continuously formed in tissues from blood by capillary ultrafiltration. Lymph is collected by a system of small lymphatic vessels and directed to the thoracic duct, which drains into the left subclavian vein where lymph rejoins the systemic blood circulation.


Blood circulation transports heat throughout the body, and adjustments to this flow are an important part of thermoregulation. Increasing blood flow to the surface (e.g., during warm weather or strenuous exercise) causes warmer skin, resulting in faster heat loss. In contrast, when the external temperature is low, blood flow to the extremities and surface of the skin is reduced and to prevent heat loss and is circulated to the important organs of the body, preferentially.

Hydraulic functions

The restriction of blood flow can also be used in specialized tissues to cause engorgement, resulting in an erection of that tissue; examples are the erectile tissue in the penis, nipples, and clitoris.

Another example of a hydraulic function is the jumping spider, in which blood forced into the legs under pressure causes them to straighten for a powerful jump, without the need for bulky muscular legs.[23]


In insects, the blood (more properly called hemolymph) is not involved in the transport of oxygen. (Openings called tracheae allow oxygen from the air to diffuse directly to the tissues). Insect blood moves nutrients to the tissues and removes waste products in an open system.

Other invertebrates use respiratory proteins to increase the oxygen-carrying capacity. Hemoglobin is the most common respiratory protein found in nature. Hemocyanin (blue) contains copper and is found in crustaceans and mollusks. It is thought that tunicates (sea squirts) might use vanabins (proteins containing vanadium) for respiratory pigment (bright-green, blue, or orange).

In many invertebrates, these oxygen-carrying proteins are freely soluble in the blood; in vertebrates they are contained in specialized red blood cells, allowing for a higher concentration of respiratory pigments without increasing viscosity or damaging blood filtering organs like the kidneys.

Giant tube worms have unusual hemoglobins that allow them to live in extraordinary environments. These hemoglobins also carry sulfides normally fatal in other animals.



Capillary blood from a bleeding finger
Venous blood collected during blood donation

Hemoglobin is the principal determinant of the color of blood in vertebrates. Each molecule has four heme groups, and their interaction with various molecules alters the exact color. In vertebrates and other hemoglobin-using creatures, arterial blood and capillary blood are bright red, as oxygen imparts a strong red color to the heme group. Deoxygenated blood is a darker shade of red; this is present in veins, and can be seen during blood donation and when venous blood samples are taken. Blood in carbon monoxide poisoning is bright red, because carbon monoxide causes the formation of carboxyhemoglobin. In cyanide poisoning, the body cannot utilize oxygen, so the venous blood remains oxygenated, increasing the redness. While hemoglobin-containing blood is never blue, there are several conditions and diseases wherein the color of the heme groups make the skin appear blue. If the heme is oxidized, methaemoglobin, which is more brownish and cannot transport oxygen, is formed. In the rare condition sulfhemoglobinemia, arterial hemoglobin is partially oxygenated, and appears dark red with a bluish hue (cyanosis).

Veins in the skin appear blue for a variety of reasons only weakly dependent on the color of the blood. Light scattering in the skin, and the visual processing of color play roles as well.[24]

Skinks in the genus Prasinohaema have green blood due to a buildup of the waste product biliverdin.[25]


The blood of most molluscs – including cephalopods and gastropods – as well as some arthropods, such as horseshoe crabs, is blue, as it contains the copper-containing protein hemocyanin at concentrations of about 50 grams per litre.[26] Hemocyanin is colorless when deoxygenated and dark blue when oxygenated. The blood in the circulation of these creatures, which generally live in cold environments with low oxygen tensions, is grey-white to pale yellow,[26] and it turns dark blue when exposed to the oxygen in the air, as seen when they bleed.[26] This is due to change in color of hemocyanin when it is oxidized.[26] Hemocyanin carries oxygen in extracellular fluid, which is in contrast to the intracellular oxygen transport in mammals by hemoglobin in RBCs.[26]


General medical disorders

  • Disorders of volume
    • Injury can cause blood loss through bleeding.[27] A healthy adult can lose almost 20% of blood volume (1 L) before the first symptom, restlessness, begins, and 40% of volume (2 L) before shock sets in. Thrombocytes are important for blood coagulation and the formation of blood clots, which can stop bleeding. Trauma to the internal organs or bones can cause internal bleeding, which can sometimes be severe.
    • Dehydration can reduce the blood volume by reducing the water content of the blood. This would rarely result in shock (apart from the very severe cases) but may result in orthostatic hypotension and fainting.
  • Disorders of circulation
    • Shock is the ineffective perfusion of tissues, and can be caused by a variety of conditions including blood loss, infection, poor cardiac output.
    • Atherosclerosis reduces the flow of blood through arteries, because atheroma lines arteries and narrows them. Atheroma tends to increase with age, and its progression can be compounded by many causes including smoking, high blood pressure, excess circulating lipids (hyperlipidemia), and diabetes mellitus.
    • Coagulation can form a thrombosis, which can obstruct vessels.
    • Problems with blood composition, the pumping action of the heart, or narrowing of blood vessels can have many consequences including hypoxia (lack of oxygen) of the tissues supplied. The term ischemia refers to tissue that is inadequately perfused with blood, and infarction refers to tissue death (necrosis), which can occur when the blood supply has been blocked (or is very inadequate).

Hematological disorders

  • Disorders of coagulation
    • Hemophilia is a genetic illness that causes dysfunction in one of the blood's clotting mechanisms. This can allow otherwise inconsequential wounds to be life-threatening, but more commonly results in hemarthrosis, or bleeding into joint spaces, which can be crippling.
    • Ineffective or insufficient platelets can also result in coagulopathy (bleeding disorders).
    • Hypercoagulable state (thrombophilia) results from defects in regulation of platelet or clotting factor function, and can cause thrombosis.
  • Infectious disorders of blood
    • Blood is an important vector of infection. HIV, the virus, which causes AIDS, is transmitted through contact with blood, semen or other body secretions of an infected person. Hepatitis B and C are transmitted primarily through blood contact. Owing to blood-borne infections, bloodstained objects are treated as a biohazard.
    • Bacterial infection of the blood is bacteremia or sepsis. Viral Infection is viremia. Malaria and trypanosomiasis are blood-borne parasitic infections.

Carbon monoxide poisoning

Substances other than oxygen can bind to hemoglobin; in some cases this can cause irreversible damage to the body. Carbon monoxide, for example, is extremely dangerous when carried to the blood via the lungs by inhalation, because carbon monoxide irreversibly binds to hemoglobin to form carboxyhemoglobin, so that less hemoglobin is free to bind oxygen, and less oxygen can be transported in the blood. This can cause suffocation insidiously. A fire burning in an enclosed room with poor ventilation presents a very dangerous hazard, since it can create a build-up of carbon monoxide in the air. Some carbon monoxide binds to hemoglobin when smoking tobacco.

Medical treatments

Blood products

Blood for transfusion is obtained from human donors by blood donation and stored in a blood bank. There are many different blood types in humans, the ABO blood group system, and the Rhesus blood group system being the most important. Transfusion of blood of an incompatible blood group may cause severe, often fatal, complications, so crossmatching is done to ensure that a compatible blood product is transfused.

Other blood products administered intravenously are platelets, blood plasma, cryoprecipitate, and specific coagulation factor concentrates.

Intravenous administration

Many forms of medication (from antibiotics to chemotherapy) are administered intravenously, as they are not readily or adequately absorbed by the digestive tract.

After severe acute blood loss, liquid preparations, generically known as plasma expanders, can be given intravenously, either solutions of salts (NaCl, KCl, CaCl2 etc...) at physiological concentrations, or colloidal solutions, such as dextrans, human serum albumin, or fresh frozen plasma. In these emergency situations, a plasma expander is a more effective life-saving procedure than a blood transfusion, because the metabolism of transfused red blood cells does not restart immediately after a transfusion.


In modern evidence-based medicine, bloodletting is used in management of a few rare diseases, including hemochromatosis and polycythemia. However, bloodletting and leeching were common unvalidated interventions used until the 19th century, as many diseases were incorrectly thought to be due to an excess of blood, according to Hippocratic medicine.


Classical Greek medicine

In classical Greek medicine, blood was associated with air, with Springtime, and with a merry and gluttonous (sanguine) personality. It was also believed to be produced exclusively by the liver.

Hippocratic medicine

In Hippocratic medicine, blood was considered to be one of the four humors, the others being phlegm, yellow bile, and black bile.

Cultural and religious beliefs

Due to its importance to life, blood is associated with a large number of beliefs. One of the most basic is the use of blood as a symbol for family relationships; to be "related by blood" is to be related by ancestry or descendance, rather than marriage. This bears closely to bloodlines, and sayings such as "blood is thicker than water" and "bad blood", as well as "Blood brother". Blood is given particular emphasis in the Jewish and Christian religions because Leviticus 17:11 says "the life of a creature is in the blood." This phrase is part of the Levitical law forbidding the drinking of blood or eating meat with the blood still intact instead of being poured off.

Mythic references to blood can sometimes be connected to the life-giving nature of blood, seen in such events as childbirth, as contrasted with the blood of injury or death.

Indigenous Australians

In many indigenous Australian Aboriginal peoples' traditions, ochre (particularly red) and blood, both high in iron content and considered Maban, are applied to the bodies of dancers for ritual. As Lawlor states:

In many Aboriginal rituals and ceremonies, red ochre is rubbed all over the naked bodies of the dancers. In secret, sacred male ceremonies, blood extracted from the veins of the participant's arms is exchanged and rubbed on their bodies. Red ochre is used in similar ways in less-secret ceremonies. Blood is also used to fasten the feathers of birds onto people's bodies. Bird feathers contain a protein that is highly magnetically sensitive.[28]

Lawlor comments that blood employed in this fashion is held by these peoples to attune the dancers to the invisible energetic realm of the Dreamtime. Lawlor then connects these invisible energetic realms and magnetic fields, because iron is magnetic.

Indo-European paganism

Among the Germanic tribes (such as the Anglo-Saxons and the Norsemen), blood was used during their sacrifices; the Blóts. The blood was considered to have the power of its originator, and, after the butchering, the blood was sprinkled on the walls, on the statues of the gods, and on the participants themselves. This act of sprinkling blood was called bleodsian in Old English, and the terminology was borrowed by the Roman Catholic Church becoming to bless and blessing. The Hittite word for blood, ishar was a cognate to words for "oath" and "bond", see Ishara. The Ancient Greeks believed that the blood of the gods, ichor, was a mineral that was poisonous to mortals.


In Judaism, blood cannot be consumed even in the smallest quantity (Leviticus 3:17 and elsewhere); this is reflected in Jewish dietary laws (Kashrut). Blood is purged from meat by salting and soaking in water.

Another ritual involving blood involves the covering of the blood of fowl and game after slaughtering (Leviticus 17:13); the reason given by the Torah is: "Because the life of the animal is [in] its blood" (ibid 17:14).

Also if an person of the orthodox Jewish faith suffers a violent death, religious laws order the collection of their blood for burial with them.


Some Christian churches, including Roman Catholicism, Eastern Orthodoxy, Oriental Orthodoxy, and the Assyrian Church of the East teach that, when consecrated, the Eucharistic wine actually becomes the blood of Jesus. Thus in the consecrated wine, Jesus becomes spiritually and physically present. This teaching is rooted in the Last Supper, as written in the four gospels of the Bible, in which Jesus stated to his disciples that the bread that they ate was his body, and the wine was his blood. "This cup is the new testament in my blood, which is shed for you." (Luke 22:20).

Various forms of Protestantism, especially those of a Wesleyan or Presbyterian lineage, teach that the wine is no more than a symbol of the blood of Christ, who is spiritually but not physically present. Lutheran theology teaches that the body and blood is present together "in, with, and under" the bread and wine of the Eucharistic feast.

Christ's blood is also seen as the means for atonement for sins for Christians.

At the Council of Jerusalem, the apostles prohibited Christians from consuming blood, probably because this was a command given to Noah (Genesis 9:4, see Noahide Law). This command continued to be observed by the Eastern Orthodox.


Consumption of food containing blood is forbidden by Islamic dietary laws. This is derived from the statement in the Qur'an, sura Al-Ma'ida (5:3): "Forbidden to you (for food) are: dead meat, blood, the flesh of swine, and that on which hath been invoked the name of other than Allah."

Blood is considered as unclean and in Islam cleanliness is part of the faith, hence there are specific methods to obtain physical and ritual status of cleanliness once bleeding has occurred. Specific rules and prohibitions apply to menstruation, postnatal bleeding and irregular vaginal bleeding.

However it is often said that the soul is made of the blood to distinguish souls from other souls through the ancestry in the blood. The Qalb which is the soul (heart in Arabic) pumps its Ruh through the veins and the Ruh is a life force called blood.

Jehovah's Witnesses

Out of obedience to commands in the Bible, such as: "Keep abstaining...from blood."-Acts 15:28, 29, Jehovah's Witnesses do not partake in the consumption of blood or accept transfusions of whole blood or its four major components namely, red blood cells, white blood cells, platelets (thrombocytes), and whole plasma. Members are instructed to personally decide whether or not to accept fractions and medical procedures that involve their own blood.

Chinese and Japanese culture

In Chinese popular culture, it is often said that, if a man's nose produces a small flow of blood, this signifies that he is experiencing sexual desire. This often appears in Chinese-language and Hong Kong films as well as in Japanese culture parodied in anime and manga. Characters, mostly males, will often be shown with a nosebleed if they have just seen someone nude or in little clothing, or if they have had an erotic thought or fantasy; this is based on the idea that a male's blood pressure will spike dramatically when aroused.[29]

Blood libel

Various religious and other groups have been falsely accused of using human blood in rituals; such accusations are known as blood libel. The most common form of this is blood libel against Jews. Although there is no ritual involving human blood in Jewish law or custom, fabrications of this nature (often involving the murder of children) were widely used during the Middle Ages to justify Antisemitic persecution.

Vampire legends

Vampires are mythical creatures that drink blood directly for sustenance, usually with a preference for human blood. Cultures all over the world have myths of this kind; for example the 'Nosferatu' legend, a human who achieves damnation and immortality by drinking the blood of others, originates from Eastern European folklore. Ticks, leeches, female mosquitoes, vampire bats, and an assortment of other natural creatures do drink blood, but only bats are associated with vampires. This has no relation to vampire bats, which are new world creatures discovered well after the origins of the European myths.


In the applied sciences

Blood residue can help forensic investigators identify weapons, reconstruct a criminal action, and link suspects to the crime. Through bloodstain pattern analysis, forensic information can also be gained from the spatial distribution of bloodstains.

Blood residue analysis is also a technique used in archeology.

In art

Blood is one of the body fluids that has been used in art.[30] In particular, the performances of Viennese Actionist Hermann Nitsch, Franko B, Lennie Lee, Ron Athey, Yang Zhichao, and Kira O' Reilly, along with the photography of Andres Serrano, have incorporated blood as a prominent visual element. Marc Quinn has made sculptures using frozen blood, including a cast of his own head made using his own blood.

In genealogy & family history

The term, blood, is used in genealogical circles to refer to one's ancestry, origins, and ethnic background, as in the word, bloodline. Other terms where blood is used in a family history sense are blue-blood, royal blood, mixed-blood and blood relative.

See also


  1. ^ The Franklin Institute Inc.. "Blood – The Human Heart". Retrieved 19 March 2009. 
  2. ^ Maton, Anthea; Jean Hopkins, Charles William McLaughlin, Susan Johnson, Maryanna Quon Warner, David LaHart, Jill D. Wright (1993). Human Biology and Health. Englewood Cliffs, New Jersey, USA: Prentice Hall. ISBN 0-13-981176-1. 
  3. ^ a b Waugh, Anne; Grant, Allison (2007). "2". Anatomy ans Physiology in Health and Illness (Tenth ed.). Churchill Livingstone Elsevier. pp. 22. ISBN 978 0 443 10102 1. 
  4. ^ Alberts, Bruce (2005). "Leukocyte functions and percentage breakdown". Molecular Biology of the Cell. NCBI Bookshelf. Retrieved 2007-04-14. 
  5. ^ Shmukler, Michael (2004). "Density of Blood". The Physics Factbook. Retrieved 2006-10-04. 
  6. ^ "Medical Encyclopedia: RBC count". Medline Plus. Retrieved 18 November 2007. 
  7. ^ Robert B. Tallitsch; Martini, Frederic; Timmons, Michael J. (2006). Human anatomy (5th ed.). San Francisco: Pearson/Benjamin Cummings. p. 529. ISBN 0-8053-7211-3. 
  8. ^ a b Ganong, William F. (2003). Review of medical physiology (21 ed.). New York: Lange Medical Books/McGraw-Hill. p. 518. ISBN 0-07-121765-7. 
  9. ^ Acid-Base Regulation and Disorders at Merck Manual of Diagnosis and Therapy Professional Edition
  10. ^ Sellmeyer DE, Stone KL, Sebastian A, Cummings SR (January 2001). "A high ratio of dietary animal to vegetable protein increases the rate of bone loss and the risk of fracture in postmenopausal women. Study of Osteoporotic Fractures Research Group". Am. J. Clin. Nutr. 73 (1): 118–22. PMID 11124760. 
  11. ^ Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 404–406. ISBN 0-03-910284-X. 
  12. ^ Harvey, William (1628). "Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus" (in Latin). 
  13. ^ Williams, Peter W.; Gray, Henry David (1989). Gray's anatomy (37th ed.). New York: C. Livingstone. ISBN 0-443-02588-6. 
  14. ^ Dominguez de Villota ED, Ruiz Carmona MT, Rubio JJ, de Andrés S (December 1981). "Equality of the in vivo and in vitro oxygen-binding capacity of haemoglobin in patients with severe respiratory disease". Br J Anaesth 53 (12): 1325–8. doi:10.1093/bja/53.12.1325. PMID 7317251. 
  15. ^ a b Costanzo, Linda S. (2007). Physiology. Hagerstwon, MD: Lippincott Williams & Wilkins. ISBN 0-7817-7311-3. 
  16. ^ Ventilation and Endurance Performance
  17. ^ Transplant Support- Lung, Heart/Lung, Heart MSN groups
  18. ^ Mortensen SP, Dawson EA, Yoshiga CC, et al. (July 2005). "Limitations to systemic and locomotor limb muscle oxygen delivery and uptake during maximal exercise in humans". J. Physiol. (Lond.) 566 (Pt 1): 273–85. doi:10.1113/jphysiol.2005.086025. PMID 15860533. 
  19. ^ The 'St George' Guide To Pulmonary Artery Catheterisation
  20. ^ Oxygen Carriage in Blood - High Altitude
  21. ^ October 2006. Clinical correlates of pH levels: bicarbonate as a buffer.
  22. ^ Acid-Base Regulation and Disorders at Merck Manual of Diagnosis and Therapy Professional Edition
  23. ^ "Spiders: circulatory system". Encyclopedia Britannica online. Retrieved 2007-11-25. 
  24. ^ Kienle, Alwin; Lothar Lilge, I. Alex Vitkin, Michael S. Patterson, Brian C. Wilson, Raimund Hibst, and Rudolf Steiner (March 1, 1996). "Why do veins appear blue? A new look at an old question" (PDF). Applied Optics 35 (7): 1151–60. doi:10.1364/AO.35.001151. 
  25. ^ Austin CC, Perkins SL (2006). "Parasites in a biodiversity hotspot: a survey of hematozoa and a molecular phylogenetic analysis of Plasmodium in New Guinea skinks". J. Parasitol. 92 (4): 770–7. doi:10.1645/GE-693R.1. PMID 16995395. 
  26. ^ a b c d e Shuster, Carl N (2004). "Chapter 11: A blue blood: the circulatory system". in Shuster, Carl N, Jr; Barlow, Robert B; Brockmann, H. Jane. The American Horseshoe Crab. Harvard University Press. pp. 276–7. ISBN 0674011597.,M1. 
  27. ^ "Blood - The Human heart". The Franklin Institute. Retrieved 19 March 2009. 
  28. ^ Lawlor, Robert (1991). Voices of the first day: awakening in the Aboriginal dreamtime. Rochester, Vt: Inner Traditions International. pp. 102–3. ISBN 0-89281-355-5. 
  29. ^ Law of Anime #40 aka Law of Nasal Sanguination at, The Anime Cafe.
  30. ^ "Nostalgia" Artwork in blood

External links

1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

BLOOD, the circulating fluid in the veins and arteries of animals. The word itself is common to Teutonic languages; the O. Eng. is blod, cf. Gothic bloth, Dutch bloed, Ger. Blut. It is probably ultimately connected with the root which appears in "blow," "bloom," meaning flourishing or vigorous. The Gr. word for blood, aiµa, appears as a prefix haemo- in many compound words. As that on which the life depends, as the supposed seat of the passions and emotions, and as that part which a child is believed chiefly to inherit from its parents, the word "blood" is used in many figurative and transferred senses; thus "to have his blood," "to fire the blood," "cold blood," "blood-royal," "half" or "whole blood," &c. The expression "blue blood" is from the Spanish sangre azul. The nobles of Castile claimed to be free from all admixture with the darker blood of Moors or Jews, a proof being supposed to lie in the blue veins that showed in their fairer skins. The common English expletive "bloody," used as an adjective or adverb, has been given many fanciful origins; it has been supposed to be a contraction of "by our Lady," or an adaptation of the oath common during the 17th century, "'sblood," a contraction of "God's blood." The exact origin of the expression is not quite clear, but it is certainly merely an application of the adjective formed from "blood." The New English Dictionary suggests that it refers to the use of "blood" for a young rowdy of aristocratic birth, which was common at the end of the 17th century, and later became synonymous with "dandy," "buck," &c.; "bloody drunk" meant therefore "drunk as a blood," "drunk as a lord." The expression came into common colloquial use as a mere intensive, and was so used till the middle of the 18th century. There can be little doubt that the use of the word has been considerably affected by the idea of blood as the vital principle, and therefore something strong, vigorous, and parallel as an intensive epithet with such expressions as "thundering," "awfully" and the like.

Anatomy And Physiology In all living organisms, except the most minute, only a minimum number of cells can come into immediate contact with the general world, whence is to be drawn the food supply for the whole organism. Hence those cells - and they are by far the most numerous - which do not lie on the food-absorbing surface, must gain their nutriment by some indirect means. Further, each living cell produces waste products whose accumulation would speedily prove injurious to the cell, hence they must be constantly removed from its immediate neighbourhood and indeed from the organism as a whole. In this instance again, only a few cells can lie on a surface whence such materials can be directly discharged to the exterior. Hence the main number of the cells of the organism must depend upon some mechanism by which the waste products can be carried away from them to that group of cells whose duty it is to modify them, or discharge them from the body. These two ends are attained by the aid of a circulating fluid, a fluid which is constantly flowing past every cell of the body. From it the cells extract the food materials they require for their sustenance, and into it they discharge the waste materials resulting from their activity. This circulating medium is the blood.

Whilst undoubtedly the two functions of this circulating fluid above given are the more prominent, there are yet others of great importance. For instance, it is known that many tissues as a result of their activity produce certain chemical substances which are of essential importance to the life of other tissue cells. These substances - internal secretions as they are termed - are carried to the second tissue by the blood stream. Again, many instances are known in which two distant tissues communicate with one another by means of chemical messengers, bodies termed hormones (6p ii u', to stir up), which are produced by one group of cells, and sent to the other group to excite them to activity. Here, also, the path by which such messengers travel is the blood stream. A further and most important manner in which the circulating fluid is utilized in the life of an animal is seen in the way in which it is employed in protecting the body should it be invaded by micro-organisms.

Hence it is clear that the blood is of the most vital importance to the healthy life of the body. But the fact that it is present as a circulating medium exposes the animal to a great danger, viz. that it may be lost should any vessel carrying it become ruptured. This is constantly liable to happen, but to minimize as far as possible any such loss, the blood is endowed with the peculiar property of clotting, i.e. of setting to a solid or stiff jelly by means of which the orifices of the torn vessels become plugged and the bleeding stayed.

The performance of these essential functions depends upon the maintenance of a continuous flow past all tissue cells, and this is attained by the circulatory mechanism, consisting of a central pump, the heart, and a system of ramifying tubes, the arteries, through which the blood is forced from the heart to every tissue '(see' Vascular System). A second set of tubes, the veins, collects the blood and returns it to the heart. In many invertebrates the circulating fluid is actually poured into the tissue spaces from the open terminals of the arteries. From these spaces it is in turn drained away by the veins. Such a system is termed a haemolymph system and the circulating fluid the haemolymph. Here the essential point gained is that the fluid is brought into direct contact with the tissue cells. In all vertebrates, the ends of the arteries are united to the commencements of the veins by a plexus of extremely minute tubes, the capillaries, consequently the blood is always retained within closed tubes and never comes into contact with the tissue cells. It is while passing through the capillaries that the blood performs its work; here the blood stream is at its slowest and is brought nearest to the tissue cell, only being separated from it by the extremely thin wall of the capillary and by an equally thin layer of fluid. Through this narrow barrier the interchanges between cell and blood take place.

The advantage gained in the vertebrate animal by retaining the blood in a closed system of tubes lies in the great diminution of resistance to the flow of blood, and the consequent great increase in rate of flow past the tissue cells. Hence any food stuffs which can travel quickly through the capillary wall to the tissue cell outside can be supplied in proportionately greater quantity within a given time, without requiring any very great increase in the concentration of that substance in the blood. Conversely, any highly diffusible substance may be withdrawn from the tissues by the blood at a similarly increased pace. These conditions are more peculiarly of importance for the supply of oxygen and the removal of carbonic acid - especially for the former, because the amount of it which can be carried by the blood is small. But as the rate at which a tissue lives, i.e. its activity, depends upon the rate of its chemical reactions, and as these are fundamentally oxidative, the more rapidly oxygen is carried to a tissue the more rapidly it can live, and the greater the amount of work it can perform within a given time. The rate of supply is of much less importance in the case of the other food substances because they are far more soluble in water, so that the supply in sufficient quantity can easily be met by a relatively slow blood flow. Hence we find that the gradual evolution of the animal kingdom goes hand in hand with the gradual development of a greater oxygen-carrying capacity of the blood and an increase in the rate of its flow.

In the groundwork of a tissue are a number of spaces - the tissue spaces. They are filled with fluid and intercommunicate freely, finally connecting with a number of fine tubes, the lymphatics, through which excess of fluid or any solid particles present are drained away. The contained fluid acts as an intermediary between the blood and the cell; from it, the cell takes its various food stuffs, these having in the first instance been derived from the blood, and into it the cell discharges its waste products. On the course of the lymphatics a number of typical structures, the lymphatic glands, are placed, and the lymph has to pass through these structures where any deleterious products are retained, and the fluid thus purified is drained away by further lymphatics and finally returned to the blood. Thus there is a second stream of fluid from the tissues, but one vastly slower than that of the blood. The flow is too slow for it to act as the vehicle for the removal of those waste products (carbonic acid, &c.)which must of necessity be removed quickly. These must be removed by the blood. The same is true for the main number of other waste products, which, however, being of small molecular size are readily absorbed into the blood stream.

But in addition to fluid, the tissue spaces may at times be found to contain solid matter in the form of particles, which may represent the debris of destroyed cells, or which are, as is quite commonly the case, micro-organisms. Apparently such material cannot be removed from a tissue by absorption into the blood stream - indeed in the case of living organisms such an absorption would in many instances rapidly prove fatal, and special provision is made to prevent such an accident. These, therefore, are made to travel along the lymphatic channels, and so, before gaining access to the blood stream and thus to the body generally, have to run the gauntlet of the protective mechanism provided by the lymphatic glands, where in the major number of cases they are readily destroyed.

Hence we see that first and foremost we have to regard the blood as a food-carrier to all the cells of the body; in the second place as the vehicle carrying away most if not all the waste products; in a third direction, it is acting as a means for transmitting chemical substances manufactured in one tissue to distant cells of the body for whose nutrition or excitation they may be essential; and in addition to these important functions there is yet another whose value it is almost impossible to overestimate, for it plays the essential role in rendering the animal immune to the attacks of invading organisms. The question of immunity is discussed elsewhere, and it is sufficient merely to indicate the chief means by which the blood subserves this essential protective mechanism. Should living organisms find their way into the surface cells or within the tissue spaces, the body fights them in a number of ways. (I) It may produce one or more chemical substances capable of neutralizing the toxic material produced by the organism. (2) It may produce chemical substances which act as poisons to the micro-organism, either paralysing it or actually killing it. Or (3) the organism may be attacked and taken up into the body of wandering cells, e.g. certain of the leucocytes, and then digested by them. Such cells are therefore called phagocytes (41t yecv, to eat). Thus, by its power of reacting in these ways the body has become capable of withstanding the attacks of many different varieties of microorganisms, of both animal and vegetable origin.

Table of contents

General Properties

Blood is an opaque, viscid liquid of bright red colour possessing a distinct and characteristic odour, especially when warm. Its opacity is due to the presence of a very large number of solid particles, the blood corpuscles, having a higher refractive index than that of the liquid in which they float. The specific gravity in man averages about 1.055. The specific gravity of the liquid portion, the plasma (Gr. 7rA&Qµa, something formed or moulded, irXawvecv, to mould), is about 1.027, whilst that of the corpuscles amounts to 1.088. To litmus it reacts as a weak alkali.

Blood Plasma

The plasma is a solution in water of a varied number of substances, and as a solvent it confers on the blood its power of acting as a carrier of food stuffs and waste products. One important food substance, oxygen, is, however, only partly carried in solution, being mainly combined with haemoglobin in the red corpuscles. The food stuffs carried by the plasma are proteins, carbohydrates, salts and water. The main waste products dissolved in it are ammonium carbonate, urea, urates, xanthin bases, creatin and small amounts of other nitrogenous bodies, carbonic acid as carbonates, other carbon compounds such as cholesterin, lecithin and a number of other substances. Thus, if we take mammalian blood as a type, the plasma would have the following approximate composition: In moo grms. plasma Water. 901.'51 Substances not vaporizing at 120° C.

Fibrin. 8.06 Other proteins and organic substances 81.92 Inorganic substances Chlorine.

3'536 Sulphuric acid. 0.129 Phosphoric acid.. 0'145 Potassium.. 0314 Sodium 3.410 Calcium. 0.298 Magnesium. ... 0.218 Oxygen. 0'455 8.505 98.49 I 000 00 Proteins. - The proteins of the blood plasma belong to the two classes of the albumins and the globulins. The globulins present are named fibrinogen and serum-globulin; as its name implies, the chief physiological property of fibrinogen is that it can give rise to fibrin, the solid substance formed when blood clots. It possesses the typical properties of a globulin, i.e. it coagulates on heating (in this instance at a temperature of 56° C.), and is precipitated by half saturating its solution with ammonium. sulphate. It differs from other globulins in that it is less soluble. It is only present in very small quantities, o 4%. The other globulin, serum-globulin, is not coagulated until 75° C. is reached, and we now know that it is in reality a mixture of several proteins, but so far these have not been completely separated from one another and obtained in a pure form. On dialysing a solution of serum-globulin a part is precipitated, and this portion has been termed the eu-globulin fraction, the remainder being known, in contradistinction, as the pseudo-globulin. Again, on diluting a solution and adding a small amount of acetic acid a precipitate is formed which in some respects differs from the remainder of the globulin present. Whether in these two instances we are dealing with approximately pure substances is extremely doubtful. A further important point in connexion with the chemistry of the globulins is that dextrose may be found among their decomposition products, i.e. that a part of it, or possibly the whole, possesses a glucoside character.

Serum-albumin gives all the typical colour and precipitation reactions of the albumins. If plasma be weakly acidified with sulphuric acid, then treated with crystals of ammonium sulphate until a slight precipitate forms, filtered and the filtrate allowed to evaporate very slowly, typical crystals of serum-albumin may form. According to many it is a uniform and specific substance, but others hold the view that it consists of at least three distinct substances, as shown by the fact that if a solution be gradually heated coagulation will occur at three different temperatures, viz. at 73°, 77° and 84° C. On the other hand the ,close agreement between different analyses of even the amorphous preparations points to there being but one serum-albumin.

When blood clots two new proteins make their appearance in the fluid part of the blood, or serum, as it is now called. The first of these is fibrin ferment (for its origin see section on Clotting below). The other, fibrinoglobulin, possesses all the typical characteristics of the globulins and coagulates at 64° C.


Three several carbohydrates are described as occurring in plasma, viz. glycogen, animal gum and dextrose. If glycogen is present in solution in the plasma it is there in very small quantities only, and has probably arisen from the destruction of the white blood corpuscles, since some leucocytes undoubtedly contain glycogen. A small amount of carbohydrate having the formula for starch and yielding a reducing sugar on hydrolysis with acid has also been described. The constant carbohydrate constituent of plasma, however, is dextrose. This is present to the approximate amount of o-15% in arterial blood. The amount may be much greater in the blood of the portal vein during carbohydrate absorption, and according to some observers there is less in venous than in arterial blood, but the difference is small and falls within the error of observation. The statement that when no absorption is taking place the blood of the hepatic vein is richer in dextrose than that of the portal vein (Bernard) is denied by Pavy.


Plasma or serum is as a rule quite clear, but after a meal xich in fats it may become quite milky owing to the presence of neutral fats in a very fine state of subdivision. This suspended fat rapidly disappears from the blood after fat absorption has ceased. To some extent it varies in composition with that of the fat absorbed, but usually consists of the glycerides of the common fatty acids - palmitic, stearic and oleic. In addition, there is a small amount of fatty acid in solution in the plasma. As to the form in which this occurs there is some uncertainty. It is possibly present as a soap or even as a neutral fat, since a little can be dissolved in plasma, the solvent substance being probably protein or cholesterin. Fatty acids also appear, to be present to some extent combined with cholesterin forming cholesterin esters (about o 06%).

Other Organic Compounds

In addition to the substances above described, belonging to the three main classes of food stuffs, there are still other organic bodies present in plasma in small amounts, which for convenience we may classify as non-nitrogenous and nitrogenous. Among the former may be mentioned lactic acid, glycerin, a lipochrome, and probably many other substances of a similar type whose separation has not yet been effected.

The non-protein nitrogenous constituents consist of the following: ammonia as carbonate or carbamate (0.2 to o 6%), urea (0.02 to 0.05%), creatine, creatinine, uric acid, xanthine, hypoxanthine and occasionally hippuric acid. Three ferments are also described as being present: (1) a glycolytic ferment exerting an action upon dextrose; (2) a lipase or fat-splitting ferment; and (3) a diastase capable of converting starch into sugar.


The saline constituents of plasma comprise chlorides, phosphates, carbonates and possibly sulphates, of sodium, potassium, calcium and magnesium. The most abundant metal is sodium and the most abundant acid is hydrochloric. These two are present in sufficient amount to form about o 65% of sodium chloride. The phosphate is present to about 0-02%. Sulphuric acid is always present if the blood has been calcined for the purposes of the analysis, and may then be present to about 0.013%. This is, however, probably produced during the destruction of the protein, since it has been shown that no sulphate can be removed from normal plasma by dialysis. The amount of potassium present (o-03%) is less than one-tenth of that of the sodium, and the quantities of calcium and magnesium are even less.

Formed Elements

When viewed under the microscope the main number of these are seen to be small yellow bodies of very uniform size, size and shape varying, however, in different animals. When observed in bulk they have a red colour, their presence in fact giving the typical colour to blood. These are the red blood corpuscles or erythrocytes (Gr. EpvOpos, red). Mingled with them in the blood are a smaller number of corpuscles which possess no colour and have therefore been called white blood corpuscles or leucocytes (Gr. %eVKOS, white). Lastly, there are present a large number of small lens-shaped structures, less in number than the red corpuscles, and much more difficult to distinguish. These are known as blood platelets. Red Corpuscles. - These are present in very large numbers and, under normal conditions, all possess exactly the same appearance. With rare exceptions their shape is that of a biconcave disk with bevelled edges, the size varying somewhat in different animals, as is seen in the following table which gives their diameters: 0 0075 mm. 0.007 3 mm. 0 o069 mm. o o065 mm. 0 0041 mm.

Pigeon. .

. 0.0147 mm. long by 0.0065 mm. wide.

Frog.. .

. 0.0223 „ 0.0157

Newt. .

. 0.0293 „


Proteus. .

. 0.0580 „


Amphiuma .

. 0.0770 „

o 0460

Man .

4,000,000 to 5,000,000 per cub. mm.


9,000,000 to 10,000,000


13,000,000 to 14,000,000

Birds .

1,000,000 to 4,000,000


250,000 to 2,000,000

Frog .

500,000 per cub. mm.

Proteus. .


The coloured corpuscles of amphibia as well as of nearly all vertebrates below mammals are biconvex and elliptical. The following are the dimensions of some of the more common: Their number also varies as follows: - In mammals they are apparently homogeneous in structure, have no nucleus, but possess a thin envelope. Their specific gravity is distinctly higher than that of the plasma (1.088), so that if clotting has been prevented, blood on standing yields a large deposit which may form as much as half the total volume of the blood.

Chemical Composition

On destruction the red corpuscles yield two chief proteins, haemoglobin and a nucleo-protein, and a number of other substances similar to those usually obtained On the break-down of any cellular tissue, such for instance as lecithin, cholesterin and inorganic salts. The most important protein is the haemoglobin. To it the corpuscle owes its distinctive property of acting as an oxygen carrier, for it possesses the power of combining chemically with oxygen and of yielding up that same oxygen whenever there is a decrease in the concentration of the oxygen in the solvent. Thus in a given solution of haemoglobin the amount of it which is combined with oxygen depends absolutely on the oxygen concentration. The greatest dissociation of oxyhaemoglobin occurs as the oxygen tension falls from about 40 to 20 mm. of mercury. That the oxygen forms a definite compound with the haemoglobin is proved by the fact that haemoglobin thoroughly saturated with oxygen (oxyhaemoglobin) has a definite absorption spectrum showing two bands between the D and E lines, whilst haemoglobin from which the oxygen has been completely removed only gives one band between those lines. In association with this, oxyhaemoglobin has a typical bright red colour, whereas haemoglobin is dark purple. A further striking characteristic of haemoglobin is that it contains iron in its molecule. The amount present, though small bears a perfectly definite quantitative relation to the amount of oxygen with which the haemoglobin is capable of combining (two atoms of oxygen to one of iron). One gram of haemoglobin crystals can combine with 1.34 cc. of oxygen. On destruction with an acid or alkali, haemoglobin yields a pigment portion, haematin, and a protein portion, globin, the latter belonging to the group of the histones (Gr. ivTOS, web, tissue).

Man. Dog. Rabbit. Cat Goat .

In this cleavage the iron is found in the pigment. By the use of a strong acid, it may be made to yield iron-free pigment, the remainder of the molecule being much further decomposed.

Destruction and Formation

In the performance of their work the corpuscles gradually deteriorate. They are then destroyed, chiefly in the liver, but whether the whole of this process is effected by the liver alone is not decided. It is proved, however, that the destruction of the haemoglobin is entirely effected there. It was for a long time considered to be one of the functions of the spleen to examine the red corpuscles and to destroy or in some way to mark those no longer fitted for the performance of their work. It is proved that the destruction of the haemoglobin is entirely effected in the liver, since both the main cleavage products may be traced to this organ, which discharges the pigmentary portion as the bile pigment, but retains the iron-protein moiety at any rate for a time. The amount of bile pigment eliminated during the day indicates that the destruction must be considerable, and since the number of corpuscles does not vary there must be an equivalent formation of new ones. This takes place in the red bone-marrow, where special cells are provided for their continuous production. In embryonic life their formation is effected in another way. Certain mesodermic cells, resembling those of the connective tissue, collect masses of haemoglobin, and from these elaborate red blood corpuscles which thus come to lie in the fluid part of the cell. By a canalization of the branches of these cells which unite with branches of other cells the precursors of the blood capillaries are formed.

White Blood Corpuscles

These constitute the second important group of formed elements in the blood, and number about 12,000 to 20,000 per cubic mm. They are typical wandering cells carried to all parts of the body by the blood stream, but often leave that stream and gain the tissue spaces by passing through the capillary wall. They exist in many varieties and were first classified according as, under the microscope, they presented a granular appearance or appeared clear. The cells were also distinguished from one another according as they possessed fine or coarse granules. The granules are confined to the protoplasm of the cell, and it has been shown that they differ chemically, because their staining properties vary. Thus, some granules select an acid stain, and the cells containing them are then designated acidophile or eosinophile; 1 other granules select a basic stain and are called basophile, while yet others prefer a neutral stain (neutrophile). In human blood the following varieties of leucocytes may be distinguished: I. The Polymorphonuclear Cell. - This possesses a nucleus of very complicated outline and a fair amount of protoplasm filled with numbers of fine granules which stain with eosin. They vary in size but are usually about o oi mm. in diameter. They are highly amoeboid and phagocytic, and form about 70% of the total number of leucocytes.

2. The Coarsely Granular Eosinophile Cell. - These large cells contain a number of well-defined granules which stain deeply with acid dyes. The nucleus is crescentic. The cells amount to about 2% of the total number of leucocytes, though the proportion varies considerably. They are actively amoeboid.

3. The Lymphocyte

This is the smallest leucocyte, being only about o o065 mm. in diameter. It has a large spherical nucleus with a small rim of clear protoplasm surrounding it. It forms from i 5 to 40% of the number of leucocytes, and is less markedly amoeboid than the other varieties.

4. The Hyaline (Gr. UtXevos, glassy, crystalline, uaAos, glass) cell or macrocyte (Gr. µarcpos, long or large). - This is a cell similar to the last with a spherical, oval or indented nucleus, but it has much more protoplasm. It constitutes about 4% of all the leucocytes and is highly amoeboid and phagocytic.

5. The Basophile Cell. - This possesses a spherical nucleus and the protoplasm contains a small number of granules staining The suffix -phile, Greek eeXeiv, to love, prefer, is in scientific terminology frequently applied to substances that exhibit such preference for particular stains or reagents, the names of which form the first part of the word.

deeply with basic dyes. It is rarely found in the blood of adults except in certain diseases.


These cells act as scavengers or as destroyers of living organisms that may have gained access to the tissue spaces. They play an important part in the chemical processes underlying the phenomena of immunity, and some at least are of importance in starting the process of clotting.

They are constantly suffering destruction in the performance of their work. Many, too, are lost to the body by their passage through the different mucous surfaces. Their origin is still obscure in many points. The lymphocytes are derived from lymphoid tissue, wherever it exists in the different parts of the body. The polymorphonuclear and eosinophile cells are derived from the bone-marrow, each by division of specific mother cells located in that tissue. The macrocyte is believed by many to represent a further stage in the development of the lymphocyte. Their rate of formation may be influenced by a variety of conditions - for instance, they are found to vary in number according to the diet and also, to a considerable extent, in disease.


The platelets or thrombocytes (Gr. 9p6,u[30s, clot) are the third class of formed elements occurring in mammalian blood. There are still, however, many observers who consider that platelets are not present in the normal circulating blood, but only make their appearance after it has been shed or otherwise injured. They are minute lens-shaped structures, and may amount to as many as 800,000 per cubic mm. Under certain conditions, examination has shown that they are protoplasmic and amoeboid, and that each one contains a central body of different staining properties from the remainder of the structure. This has been regarded by some as a nucleus. On being brought into contact with a foreign surface they adhere to it firmly, very rapidly passing through a number of phases resulting ultimately in the formation of granular debris. In shed blood they tend to collect into groups, and during clotting, fibrin filaments may be observed to shoot out from these clumps.

Variations in the Blood of different Animals

If we contrast the blood of different animals of the vertebrate class we find striking differences both in microscopic appearances and in chemical properties. In the first place, the corpuscles vary in amount and in kind. Thus, whilst in a mammal the corpuscles form 40 to 50% of the total volume of the blood, in the lower vertebrates the volume is much less, e.g. in frogs as low as 25% and in fishes even lower. The deficiency is chiefly in the red corpuscles, the ratio of white to red increasing as we examine the blood from animals lower in the scale. The corpuscles themselves are also found to vary, especially the red ones. In the mammal they are biconcave disks with bevelled edges, they do not contain a nucleus so that they are not cells. In the bird they are larger, ellipsoidal in shape and have a large nucleus in the centre of the cell. In reptiles and amphibia the red corpuscles are also nucleated, but the stroma portion containing the haemoglobin is arranged in a thickened annular part encircling the nucleus. When seen from the flat they are oval in section. In fishes the corpuscles show very much the same structure. A further very significant difference to be observed between the bloods of different vertebrates is in the amount of haemoglobin they contain; thus in the lower classes, fishes and amphibia, not only is the number of red corpuscles small but the amount of haemoglobin each corpuscle contains is relatively low. The concentration of the haemoglobin in the corpuscles attains its maximum in the mammal and the bird. Since the haemoglobin is practically the same from whatever animal it is obtained and can only combine with the same amount of oxygen, the oxygen-capacity of the blood of any vertebrate is in direct proportion to the amount of haemoglobin it contains. Therefore we see that as we ascend the scale in the vertebrate series the oxygen-carrying capacity of the blood rises. This increase was a natural preliminary condition for the progress of evolution. In order that a more active animal might be developed the main essential was that the chemical processes of the cell should be carried out more rapidly, and as these processes are fundamentally oxidative, increased activity entails an increased rate of supply of oxygen. This latter has been brought about in the animal kingdom in two ways, first by an increase in the concentration of the haemoglobin of the blood effected by an increase both in the number of corpuscles and in the amount of haemoglobin contained in each, and secondly by an increase in the rate at which the blood has been made to pass through the tissues. In the lower vertebrates the blood pressure is low and the haemoglobin content of the blood is low, consequently both rate of blood-flow and oxygencontent are low. In contrast with this, in higher vertebrates the blood pressure is high and the haemoglobin content of the blood is high, consequently both rate of blood-flow and oxygen-content are high. We must associate with this important step in evolution the means employed for the more rapid absorption of oxygen and for its increased rate of discharge to the tissues, the most important features of which are a diminution in the size of the corpuscle and the attainment of its peculiar shape, both resulting in the production of a relatively enormous corpuscular surface in a unit volume of blood.

Variations are also found in the white corpuscles as well as in the red, but these differences are not so striking and lie chiefly in unimportant details of structure of individual cells. Enormous variations are to be found in different species of mammals, but the cells generally conform to the types of secreting cells or phagocytes.

The platelets also differ in the different species. In the frog, for instance, many are spindle-shaped and contain a nucleus-like structure. Birds' blood is stated to contain no platelets. The variations in number of these bodies have not been satisfactorily ascertained on account of the difficulties involved in any attempt to preserve them and to render them visible under the microscope.

Differences are also found in the chemical composition of the plasma. The chief variation is in the amount of protein present, which attains its maximum concentration in birds and mammals, while in reptiles, amphibia and fishes it is much less. The bloods of the latter two classes are much more watery than that of the mammal. Moreover, it has been proved that there are specific differences in the chemical nature of the various proteins present even between different varieties of mammals. Thus the ratio of the globulin fraction to the albumin fraction may vary considerably, and again, one or other of the proteins may be quite specific for the animal from which it is derived.


If a sample of blood be withdrawn from an animal, within a short time it undergoes a series of changes and becomes converted into a stiff jelly. It is said to clot. If the process is watched it is seen to start first from the surfaces where it is in contact with any foreign body; thence it extends through the blood until the whole mass sets solid. A short time elapses before this process commences - a time dependent upon two chief conditions, viz. the temperature at which the blood is kept and the extent of foreign surface with which it is brought into contact. Thus in a mammal the blood clots most quickly at a temperature a little above body temperature, while if the blood be cooled quickly the clotting is considerably delayed and in the case of some animals altogether prevented. For example, human blood kept at body temperature clots in three minutes, while if allowed to cool to room temperature the first sign of clotting may not make its appearance until eight minutes after its removal from the body. The process of clotting is also considerably accelerated by making the blood flow in a thin stream over a wide surface. The full completion of the process occupies some time if the blood be kept quiet, but ultimately the whole mass of the blood becomes converted into a solid. At this stage the containing vessel may be inverted without any drop of fluid escaping. A short time after this stage has been reached drops of a yellow fluid appear upon the surface and, increasing in size and number, run together to form a layer of fluid separated from the clot. This fluid is termed serum; its appearance is due to the contraction of the clot, which thus squeezes out the fluid from between its solid constituents. Contraction continues for about twenty-four hours, at the end of which time a large quantity (one-third or more of the total volume) of serum may have been separated. The clot contracts uniformly, thus preserving throughout the same general shape as that of the vessel in which the blood has been collected. Finally the clot swims freely in the serum which it has expressed.

The cause of the clot formation has been found to be the precipitation of a solid from the liquid. plasma of the blood. This solid is in the form of very minute threads and hence is termed fibrin. The threads traverse the mass of blood in every possible direction, interlacing and thus confining in their meshes all the solid elements of the blood. Soon after their deposition they begin to contract, and as the meshwork they form is very minute they carry with them all the corpuscles of the blood. These with the fibrin form the shrunken clot.

If the rate at which blood clots be retarded either by cooling or by some other process the corpuscles may have time to settle, partially or completely, in which case distinct layers may form. The lowermost of these contains chiefly the red corpuscles, the second layer may be grey owing to the high percentage of leucocytes present, while a third, marked by opalescence only, may be very rich in platelets. Above these a clear layer of fluid may be found. This is plasma. The formation of these layers depends solely upon the rate of sedimentation of these elements, the rate depending partly upon differences in specific gravity, and partly upon the tendency the corpuscles have to run into clumps. Horse's blood offers one of the best instances of the clumping of red corpuscles, and in this animal sedimentation of the red corpuscles is most rapid.

If now such a sedimented blood is allowed to clot the process is found to start in the middle two layers, i.e. in those containing the white corpuscles and platelets. From these layers it spreads through the rest of the liquid, being most retarded, however, in the red corpuscle layer, and particularly so if the sedimentation has been very complete. Not only does the clotting process start from the layers containing the leucocytes and platelets, but in them it also proceeds more quickly. These observations clearly indicate that the clotting process is initiated by some change starting from these elements.

The object of the clotting of the blood is quite clear. It is to prevent, as far as possible, any loss of blood when there is an injury to an animal's vessels. The shed blood becomes converted into a solid, and this, extending into the interior of the ruptured vessel, forms a plug and thus arrests the bleeding. It is found that clotting is especially accelerated whenever the blood touches a foreign tissue, for instance, the outer layers of a torn blood-vessel wall, muscle tissue, &c., i.e. in exactly those conditions in which rapid clotting becomes of the greatest importance. Yet another very pregnant fact in connexion with clotting is that if an animal be bled rapidly and the blood collected in successive samples it is found that those collected last clot most quickly. Hence the more excessive the haemorrhage in any case, the greater becomes the onset of the natural cure for the bleeding, viz. clotting.

When we begin to inquire into the nature of clotting we have to determine in the first place whence the fibrin is derived. It has long been known that two chemical substances at least are requisite for its production. Thus certain fluids are known, e.g. some samples of hydrocele or pericardial fluid, which will not clot spontaneously, but will clot rapidly when a small quantity of serum or of an old blood-clot is added to it. The constituent substance which is present in the first-named fluids is known as fibrinogen, and that present in the serum or the clot is known as fibrin-ferment or thrombin. Fibrinogen is present in living blood dissolved in the plasma; it is also present in such fluids as hydrocele or pericardial effusions, which, though capable of clotting, do not clot spontaneously. Thrombin, on the other hand, does not exist in living blood, but only makes its appearance there after blood is shed. It is not yet certain what is the nature of the final reaction between fibrinogen and thrombin. The possibilities are, that thrombin may act - (I) by acting upon fibrinogen, which it in some way converts into fibrin, (2) by uniting with fibrinogen to form fibrin, or (3) by yielding part of itself to the fibrinogen which thus becomes converted into fibrin. The experimental study of the rate of fibrin formation, when different strengths of thrombin solutions are allowed to act upon a fibrinogen solution, leads us to the probable conclusion that the first of these three possibilities is the correct one, and that thrombin therefore exerts a true ferment action upon fibrinogen. It is known that in the reaction, in addition to the formation of fibrin, yet another protein makes its appearance. This is known as fibrinoglobulin, and apparently it arises from the fibrinogen, so that the change would be one of cleavage into fibrin and fibrinoglobulin. It is very noteworthy that although the amount of fibrin formed during the clotting appears very bulky, yet the actual weight is extremely small, not more than 0.4 grms. from 100 cc. of blood.

Having ascertained that the clotting is due to the action of thrombin upon fibrinogen, we now see that the next step to be explained is the origin of thrombin. It has been shown that the final step in its formation consists in the combination of another substance, termed prothrombin, with calcium. Any soluble calcium salt is found to be effective in this respect, and conversely the removal of soluble calcium (e.g. by sodium oxalate) will prevent the formation of thrombin and therefore of clotting.

In the next place it can be proved that prothrombin does not exist as such in circulating blood, so that the problem becomes an inquiry as to the origin of prothrombin. Experiment has shown that in its turn prothrombin arises from yet another precursor, which is named thrombogen, and that thrombogen also is not to be found in circulating blood but only makes its appearance after the blood is shed. The conversion of thrombogen into prothrombin has been proved to be due to the action of a second ferment which has been named thrombokinase, and this latter is again absent from living blood. Hence the question arises, whence are derived thrombogen and thrombokinase? In the study of this question it has been found that if the blood of birds be collected direct from an artery through a perfectly clean cannula into a clean and dust-free glass vessel, it does not clot spontaneously. The plasma collected from such blood is found to contain thrombogen but no thrombokinase. A somewhat similar plasma may be prepared from a mammal's blood by collecting samples of blood from an artery into vessels which have been thoroughly coated with paraffin, though in this instance thrombogen may be absent as well as thrombokinase. If plasma containing thrombogen but no thrombokinase be treated with a saline extract of any tissues it will soon clot. The saline extract contains thrombokinase. This ferment can therefore be derived from most tissues, including also the white blood corpuscles and the platelets. Thrombogen is produced from the leucocytes, but it is not yet certain whether it is also formed from the platelets. The discovery of the origin of the thrombokinase from tissue cells explains a fact that has long been known, namely, that if in collecting blood, it is allowed to flow over cut tissues, clotting is most markedly accelerated. The fact that birds' blood if very carefully collected will not clot spontaneously tends to prove that thrombokinase is not derived from the leucocytes, and makes probable its origin from the platelets, for it is known that birds' blood apparently does not contain platelets, at any rate in the form in which they are found in mammalian blood. When examining the general properties of platelets, attention was drawn to the remarkably rapid manner in which they undergo change on coming into contact with a foreign surface. It is apparently the actual contact which initiates these changes, changes which are fundamentally chemical in character, resulting in the production of thrombokinase and possibly also of thrombogen.

Thus as our knowledge at present stands the following statement gives a recapitulated account of the changes which constitute the many phases of clotting. When blood escapes from a blood-vessel it comes into contact with a foreign surface, either a tissue or the damaged walls of the cut vessel. Very speedily this contact results in the discharge of thrombogen and thrombokinase, the former from the white blood corpuscles and also possibly from the platelets, the latter from the platelets or from the tissue with which the blood comes in contact. The interaction of these two bodies next results in the formation of prothrombin, which, combining with the calcium of any soluble lime salt present, forms thrombin or fibrin-ferment. The last step in the change is the action of thrombin upon fibrinogen to form fibrin, and the clot is complete.

The intrinsic value to the animal of these changes is quite plain. The power of clotting and thus stopping haemorrhage is of essential importance, and yet this clotting must not occur within the living blood-vessels, or it would speedily result in death. That the tissues should be able to accelerate the process is of very obvious value. That the inner lining of the bloodvessels does not act as a foreign tissue is possibly due to the extreme smoothness of their surface.

Further, an animal must always be exposed to a possible danger in the absorption of some thrombin from a mass of clotted blood still retained within the body, and we know that if a quantity of active ferment be injected into the blood-stream intravascular clotting does result. Under all usual conditions this is obviated, the protective mechanism being of a twofold character. First, it is found that thrombin becomes converted very quickly into an inactive modification. Serum, for instance, very quickly loses its power of inducing clotting in fibrinogen solutions. Secondly, the body has been found to possess the power of making a substance, antithrombin, which can combine with thrombin forming a substance which is quite inactive as far as clotting is concerned. Finally, there is evidence that normal blood contains a small quantity of this substance, antithrombin, and that under certain conditions the amount present may be enormously increased. (T. G. BR.) Pathology of the Blood. The changes in the blood in disease are probably as numerous and varied as the diseases which attack the body, for the blood is not only the medium of respiration, but also of nutrition, of defence against organisms and of many other functions, none of which can be affected without corresponding alterations occurring in the circulating fluid. The immense majority of these changes are, however, so subtle that they escape detection by our present methods. But in certain directions, notably in regard to the relations with micro-organisms, changes in the blood-plasma can be made out, though they are not associated in all cases with changes in the formed elements which float in it, nor with any obvious microscopical or chemical alterations.

The phenomena of immunity to the attacks of bacteria or their toxins, of agglutinative action, of opsonic action, of the precipitin tests, and of haemolysis, are all largely dependent on the inherent or acquired characters of the blood serum. It is a commonplace that different people vary in their susceptibility to the attacks of different organisms, and different species of animals also vary greatly. This "natural immunity" is due partly to the power possessed by the leucocytes or white blood corpuscles of taking into their bodies and digesting or holding in an inert state organisms which reach the blood - phagocytosis, - partly to certain bodies in the blood serum which have a bactericidal action, or whose presence enables the phagocytes to deal more easily with the organisms. This natural immunity can be heightened when it exists, or an artificial immunity can be produced in various ways. Doses of organisms or their toxins can be injected on one or several occasions, and provided that the lethal dose be not reached, in most cases an increased power of resistance is produced. The organisms may be injected alive in a virulent condition, or with their virulence lessened by heat or cold, by antiseptics, by cultivation in the presence of oxygen, or by passage through other animals, or they may first be killed, or their toxins alone injected. The method chosen in each case depends on the organism dealt with. The result of this treatment is that in the animal treated protective substances appear in the serum, and these substances can be transferred to the serum of another animal or of man; in other words the active immunity of the experimental animal can be translated into the passive immunity of man. According to the nature of the substances injected into the former, its serum may be antitoxic, if it has been immunized against any particular toxin, or antibacterial, if against an organism. Familiar examples of these are, of the former diphtheria antitoxin, of the latter anti-plague and anti-typhoid sera. An antitoxin exerts its effects by actual combination with the respective toxin, the combination being inert. It is probable that the ultimate source of the antitoxin is to be found in the living cells of the tissues and that it passes from them into the blood. The action of an antibacterial serum depends on the presence in it of a substance known as "immunebody," which has a special affinity and power of combining with the bacterium used. In order that it may exert this power it requires the presence of a substance normally present in the serum known as "complement." The development of these "anti-bodies," though it has been studied mainly in connexion with bacteria and their toxins, is not confined to their action, but can be demonstrated in regard to many other substances, such as ferments, tissue cells, red corpuscles, &c. In some animals, for example, the blood serum has the power of dissolving the red corpuscles of an animal of different species; e.g. the guinea-pig's serum is "haemolytic" to the red corpuscles of the ox. This haemolytic power (haemolysis) can be increased by repeated injections of red corpuscles from the other animal, in this case also, as in the bacterial case, by the production and action of immune-body and complement. The antiserum produced in the case of the red corpuscles may sometimes, if injected into the first animal, whose red corpuscles were used, cause extensive destruction of its red corpuscles, with haemoglobinuria, and sometimes a fatal result.

Opsonic action depends on the presence of a substance, the "opsonin," in the serum of an immunized animal, which makes the organism in question more easily taken up by the phagocytes (leucocytes) of the blood. The opsonin becomes fixed to the organisms. It is present to a certain extent in normal serum, but can be greatly increased by the process of immunization; and the "opsonic index," or relation between the number of organisms taken up by leucocytes when treated with the serum of a healthy person or "control," and with the serum of a person affected with any bacterial disease and under treatment by immunization, is regarded by some as representing the degree of immunity produced.

Agglutinative action is evidence of the presence in a serum of a somewhat similar set of substances, known as "agglutinins." When a portion of an antiserum is added to an emulsion of the corresponding organism, the organisms, if they are motile, cease to move, and in any case become gathered together into clumps. In all probability several different bodies are concerned in this process. This reaction, in its practical applications at least, may be regarded as a reaction of infection rather than of immunization as ordinarily understood, for it is found that the blood serum of patients suffering from typhoid, Malta fever, cholera, and many other bacterial diseases, agglutinates the corresponding organisms. This fact has come to be of great importance in diagnosis.

The precipitin test depends on a somewhat analogous reaction. If the serum of an animal be injected repeatedly into another animal of different species, a "precipitin" appears in the serum of the animal treated, which causes a precipitate when added to the serum of the first animal. The special importance of this fact is that it can be utilized as a method of distinguishing between human blood and that of animals, which is often of importance in medical jurisprudence.

In this summary the facts adduced are practically all biological, and are due to the extraordinary activity with which the study of bacteriology has been pursued in recent years. The chemistry of the blood has not hitherto been found to give information of clinical or diagnostic importance, and nothing need here be added to what is said above on the physiology of the blood. Enough has been said, however, to show the extraordinary complexity of the apparently simple blood serum.

The methods at present employed in examining the blood clinically are: the enumeration of the red and white corpuscles per cubic millimetre; the estimation of the percentage of haemoglobin and of the specific gravity of the blood; the microscopic examination of freshly-drawn blood and of blood films made upon cover-glasses, fixed and stained. In special cases the alkalinity and the rapidity of coagulation may be ascertained, or the blood may be examined bacteriologically. We have no universally accepted means of estimating, during life, the total amount of blood in the body, though the method of J. S. Haldane and J. Lorrain Smith, in which the total oxygen capacity of the blood is estimated, and its total volume worked out from that datum, has seemed to promise important results (Journ. of Physiol. vol. xxv. p. 331, 1900). After death the amount of blood sometimes seems to be increased, and sometimes, as in "pernicious anaemia," it is certainly diminished. But the high counts of red corpuscles which are occasionally reported as evidence of plethora or increase of the total blood are really only indications of concentration of the fluid except in certain rare cases. It is necessary, therefore, in examining blood diseases, to confine ourselves to the study of the blood-unit, which is always taken as the cubic millimetre, without reference to the number of units in the body.

Anaemia is often used as a generic term for all blood diseases, for in almost all of them the haemoglobin is diminished, either as a result of diminution in the number of the red corpuscles in which it is contained, or because the individual red corpuscles contain a smaller amount of haemoglobin than the normal. As haemoglobin is the medium of respiratory interchange, its diminution causes obvious symptoms, which are much more easily appreciated by the patient than those caused by alterations in the plasma or the leucocytes. It is customary to divide anaemias into "primary" and "secondary": the primary are those for which no adequate cause has as yet been discovered; the secondary, those whose cause is known. Among the former are usually included chlorosis, pernicious anaemia, and sometimes the leucocythaemias; among the latter, the anaemias due to such agencies as malignant disease, malaria, chronic metallic poisoning, chronic haemorrhage, tubercle, Bright's disease, infective processes, intestinal parasites, &c. As our knowledge advances, however, this distinction will probably be given up, for the causes of several of the primary anaemias have been discovered. For example, the anaemia due to bothriocephalus, an intestinal parasite, is clinically indistinguishable from the other forms of pernicious anaemia with which it used to be included, and leucocythaemia has been declared by Lowit, though probably erroneously, to be due to a blood parasite closely related to that of malaria. In all these conditions there is a considerable similarity in the symptoms produced and in the pathological anatomy. The general symptoms are pallor of the skin and mucous membranes, weakness and lassitude, shortness of breath, palpitation, a tendency to fainting, and usually also gastro-intestinal disturbance, headache and neuralgia. The heart is often dilated, and on auscultation the systolic murmurs associated with that condition are heard. In fatal cases the internal organs are found to be pale, and very often their cells contain an excessive amount of fat. In many anaemias there is a special tendency to haemorrhage. Most of the above symptoms and organic changes are directly due to diminished respiratory interchange from the loss of haemoglobin, and to its effect on the various organs involved. The diagnosis depends ultimately in all cases upon the examination of the blood.

Though the relative proportions of the leucocytes are probably continually undergoing change even in health, especially as the result of taking food, the number of red corpuscles remains much more constant. Through the agency of some unknown mechanism, the supply of fresh red corpuscles from the bone-marrow keeps pace with the destruction of effete corpuscles, and in health each corpuscle contains a definite and constant amount of haemoglobin. The disturbance of this arrangement in anaemia may be due to loss or to increased destruction of corpuscles, to the supply of a smaller number of new ones, to a diminution of the amount of haemoglobin in the individual new corpuscles, or to a combination of these causes. It is most easy to illustrate this by describing what happens after a haemorrhage. If this is small, the loss is replaced by the fully-formed corpuscles held in reserve in the marrow, and there is no disturbance. If it is larger, the amount of fluid lost is first made up by fluid drawn from the tissues, so that the number of corpuscles is apparently diminished by dilution of the blood; the erythroblasts, or formative red corpuscles, of the bone-marrow are stimulated to proliferation, and new corpuscles are quickly thrown into the circulation. These are apt, however, to be small and to contain a subnormal amount of haemoglobin, and it is only after some time that they are destroyed and their place taken by normal corpuscles. If the loss has been very great, nucleated red corpuscles may even be carried into the bloodstream. The blood possesses a great power of recovery, if time be given it, because the organ (bone-marrow) which forms so many of its elements never, in health, works at high pressure. Only a part of the marrow, the so-called red marrow, is normally occupied by erythroblastic tissue, the rest of the medullary cavity of the bones being taken up by fat. If any long-continued demand for red corpuscles is made, the fat is absorbed, and its place gradually taken by red marrow. This compensatory change is found in all chronic anaemias, no matter what their cause may be, except in some rare cases in which the marrow does not react.

It is often very difficult, especially in "secondary" anaemias, to say which of the above processes is mainly at work. In acute anaemias, such as those associated with septicaemia, there is no doubt that blood destruction plays the principal part. But if the cause of anaemia is a chronic one, a gastric cancer, for instance, though there may possibly be an increased amount of destruction of corpuscles in some cases, and though there is often loss by haemorrhage, the cancer interferes with nutrition, the blood is impoverished and does not nourish the erythroblasts in the marrow sufficiently, and the new corpuscles which are turned out are few and poor in haemoglobin. In chronic anaemias, regeneration always goes on side by side with destruction, and it is important to remember that the state of the blood in these conditions gives the measure, not of the amount of destruction which is taking place so much as of the amount of regeneration of which the organism is capable. The evidence of destruction has often to be sought for in other organs, or in secretions or excretions.

Of the so-called primary anaemias the most common is chlorosis, an anaemia which occurs only in the female sex, between the ages of fifteen and twenty-five as a rule. Its symptoms are those caused by a diminution of haemoglobin, and though it is never directly fatal, and is extremely amenable to treatment with iron preparations, its subjects very frequently suffer from relapses at varying intervals after the first attack. Its causation is probably complex. Bad hygienic conditions, over-fatigue, want of proper food, especially of the iron-containing proteids of meat, the strain put upon the blood and bloodforming organs by the accession of puberty and the occurrence of menstruation, all probably play a part in it. It has also been suggested that internal secretions may be concerned in stimulating the bone-marrow, and that in the female sex in particular the genital organs may act in this way. Imperfect assumption of function by these organs at puberty, caused perhaps by some of the above-mentioned conditions, might lead to sluggishness in the bone-marrow, and to the supply to the blood of the poorly-formed corpuscles deficient in haemoglobin which are characteristic of the disease. Chlorosis is the type of anaemias from imperfect blood-formation. Lorrain Smith has produced evidence to show that the total amount of haemoglobin in the body is not diminished in this disease, but that the blood-plasma is greatly increased in amount, so that the haemoglobin is diluted and the amount in each blood-unit greatly lessened.

Pernicious anaemia is a rarer disease than chlorosis, occurs usually later in life, and is distributed nearly equally between the two sexes. But it is of great importance because of its almost uniformly fatal termination, though its downward course is generally broken by temporary improvement on one or more occasions. The symptoms are those of a progressive anaemia, in which gastro-intestinal disturbance usually plays a large part, and nervous symptoms are common, and they become at last much more severe than those of any secondary anaemia. The patient may die in the first attack, but more usually, when things seem to be at their worst, improvement sets in, either spontaneously or as the result of treatment, and the patient slowly regains apparent health. This remission may be followed by a relapse, that again by a remission, and so on, but as a rule the disease is fatal within, at the outside, two or three years.

The prime cause of the disease is not known. It seems probable indeed that the causal factors are numerous. Severe malarial infection, syphilis, pregnancy, chronic gastro-intestinal disease, chronic gas-poisoning, are all, in different cases, known to have been causally associated with it, and it is probable that a congenital weakness of the bone-marrow has often to do with its production, as in many cases a family or hereditary history of the disease can be obtained. The condition is now regarded as a chronic toxaemia, partly because of the clinical symptoms and pathological appearances, partly because analogous conditions can be produced experimentally by such poisons as saponin and toluylendiamin, and partly because of the facts of both y iocephalus anaemia. The site of production of the toxin, or toxins, for it is possible that several may have the same effect on the blood, is possibly not always the same, but must often be the alimentary canal, as bothriocephalus anaemia proves. Not all persons affected with this intestinal tapeworm contract the disease, but only those in whose intestines the worm is dead and decomposing or sometimes only "sick." The expulsion of the worm puts an end to the absorption of the toxin and the patients recover. No adequate explanation of the formation of the toxin in the immense majority of the cases, in which there is no tapeworm, has yet been given. It is certain that no organism as yet known is concerned.

This toxaemia affects the marrow and through it the blood, the gastro-intestinal apparatus and the nervous system, especially the spinal cord, in different proportions in different cases. The effect upon the marrow is to alter the type of red corpuscle formation, causing a reversion to the embryonic condition, in which the nucleated red corpuscles are large (megaloblasts), and the corpuscles in the blood formed from them are also large, are apparently ill suited to the needs of the adult, and easily break down, as the deposits of iron in the liver, spleen, kidneys and marrow prove. Whether this reversion is due to an exhaustion of the normal process or to an inhibition of it is not definitely known. The result is that the circulating red corpuscles are enormously diminished; it is usual to find i,000,000 or less in the cubic millimetre instead of the normal 5,000,000. Though the haemoglobin is of course absolutely diminished, it is always, in severe cases, present in relatively higher percentage than the red corpuscles, because the average red corpuscle is larger and contains more haemoglobin than the normal. The large nucleated red corpuscles (megaloblasts) with which the marrow is crowded, often appear in the blood.

Other anaemias, such as those known as lymphadenoma, or Hodgkin's disease, splenic anaemia, chloroma, leucanaemia and the anaemia pseudo-leucaemica of children, need not be described here, as they are either rare or their occurrence or nature is still too much under discussion.

The number and nature of the leucocytes in the blood bears no constant or necessary relation to the number or condition of the red corpuscles, and their variations depend on entirely different conditions. The number in the cubic millimetre is usually about 7000, but may vary in health from s000 to io,000. A diminution in their number is known as leucopenia, and is found in starvation, in some infective diseases, as for example in typhoid fever, in malaria and Malta fever, and in pernicious anaemia. An increase is very much more frequent, and is known as leucocytosis, though in this term is usually connoted a relative increase in the proportion of the polymorphonuclear neutrophile leucocytes.

'Leuco- ' cyfosis. Leucocytosis occurs under a great variety of conditions, normally to a slight extent during digestion, during pregnancy, and after violent exercise, and abnormally after haemorrhage, in the course of inflammations and many infective diseases, in malignant disease, in such toxic states as uraemia, and after the ingestion of nuclein and other substances. It does not occur in some infective diseases, the most important of which are typhoid fever, malaria, influenza, measles and uncomplicated tuberculosis. In all cases where it is sufficiently severe and long continued, the reserve space in the bone-marrow is filled up by the active proliferation of the leucocytes normally found there, and is used as a nursery for the leucocytes required in the blood. In many cases leucocytosis is known to be associated with the defence of the organism from injurious influences, and its amount depends on the relation between the severity of the attack and the power of resistance. There may be an increase in the proportions present in the blood of lymphocytes (lymphocytosis), and of eosinophile cells (eosinophilia). This latter change is associated specially with some forms of asthma, with certain skin diseases, and with the presence of animal parasites in the body, such as ankylostoma and filaria.

The disease in which the number of leucocytes in the blood is greatest is leucocythaemia or leucaemia. There are two main forms of this disease, in both of which there are anaemia, enlargement of the spleen and lymphatic glands, or of either of them, leucocytic hypertrophy of the bone-marrow, and deposits of leucocytes in the liver, kidney and other organs. The difference lies in the kind of leucocytes present in excess in the blood, blood-forming organs and deposits in the tissues. In the one form these are lymphocytes, which are found in health mainly in the marrow, the blood itself, the lymph glands and in the lymphatic tissue round the alimentary canal; in the other they are the kinds of leucocytes normally found in the bone-marrow - myelocytes, neutrophile, basophile and eosinophile, and polymorphonuclear cells, also neutrophile, basophile and eosinophile. The clinical course of the two forms may differ. The first, known as lymphatic leucaemia or lymphaemia, may be acute, and prove fatal in a few weeks or even days with rapidly advancing anaemia, or may be chronic and last for one or two years or longer. The second, known as spleno-myelogenous leucaemia or myelaemia, is almost always chronic, and may last for several years. Recovery does not take place, though remissions may occur. The use of the X-rays has been found to influence the course of this disease very favourably. The most recent view of the pathology of the disease is that it is due to an overgrowth of the bonemarrow leucocytes, analogous in some respects to tumour growth and caused by the removal of some controlling mechanism rather than by stimulation. The anaemia accompanying the disease is due partly to the leucocyte overgrowth, which takes up the space in the marrow belonging of right to red corpuscle formation and interferes with it. (G.L.G.)

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Up to date as of January 15, 2010

Definition from Wiktionary, a free dictionary

See also blood





Blood (plural Bloods)

  1. A member of the LA gang The Bloods.



Up to date as of January 23, 2010

From Wikibooks, the open-content textbooks collection



After completing this section, you should know:

  • the main functions of blood
  • what the term haematocrit or packed cell volume (PCV) means
  • what is in blood
  • what plasma is and what is in it
  • the appearance and function of red blood cells (RBCs)
  • the appearance and function of white blood cells particularly granulocytes, lymphocytes and
  • the function of platelets and fibrinogen in blood clotting
  • how oxygen and carbon dioxide are transported in the blood
  • the names of some anticoagulants and their function in the body and in the vet clinic


Blood is a unique fluid containing cells that is pumped by the heart around the body of animals in a system of pipes known as the circulatory system. It carries oxygen and nutrients to the cells of the body and removes waste products like carbon dioxide from them. Blood is also important for :keeping conditions in the body constant, in other words for maintaining homeostasis. It helps keep the acidity or pH stable and helps maintain a constant temperature in the body. Blood also has an important role in defending the body against disease.

A simple way to find out what is in blood is to remove a small amount from an animal and place it in a tube with a substance that prevents it from clotting (an anticoagulant). If you leave the tube to stand for a few hours you will find that it settles out into two layers. The top layer consists of a light yellow fluid, the plasma, and the bottom layer consists of red blood cells (RBCs). If you look very carefully you can also see a thin beige-coloured layer in between these two layers. This consists of the white blood cells (WBCs) (see diagram 8.1).

The above procedure is usually done more rapidly by placing the blood sample in a centrifuge for a few minutes. This machine acts like a super spin drier rotating about 10,000 times a minute and packing the heavier particles (red blood cells) at the bottom of the tube. The sample that results is called the packed cell volume (P.C.V.) or haematocrit. It is a very useful measurement of the concentration of red blood cells in the blood. For most animals the packed cell volume is in the range 30-45%. If it is lower than this it means that the concentration of red blood cells is low and the animal is anaemic. If the reading is above this range it may mean the animal is dehydrated. Animals that live at high altitudes also have high P.C.V.s to compensate for the low oxygen concentration there.

Anatomy and physiology of animals Packed cell volume of blood.jpg Diagram 8.1 - Packed cell volume of blood


Plasma consists of water (91%) in which many substances are dissolved. These dissolved substances include:

  • salts (or electrolytes)
  • proteins
  • nutrients
  • waste products
  • dissolved gases (mainly carbon dioxide)
  • and other chemicals like hormones

Salts in Plasma

Salts in the plasma are in the form of ions or electrolytes which include sodium, potassium, calcium, chloride, phosphate and bicarbonate. Plasma transports these ions to where they are needed e.g. calcium required by the bones, they also help keep the osmotic pressure and acid-base balance (pH) of the blood within the required levels.

Blood Proteins

The proteins in the blood plasma are large molecules with important functions. Some contribute to the osmotic pressure (see chapter 3) and the viscosity (thickness) of the blood, and so help keep the blood volume and pressure stable. Others act as antibodies that attack bacteria and viruses, and yet others are important in blood clotting. Nutrients that are absorbed from the gut and transported to the cells in the plasma include amino acids, glucose, fatty acids and vitamins. Waste products include urea from the breakdown of proteins.

Red Blood Cells

Red blood cells are also known as RBCs or erythrocytes. They are what make blood red. When you look at a blood smear through a microscope, as you will in one of the practical classes, you will see that RBCs are by far the most common cells in the blood. (In fact there are about 5 million per millilitre). If you focus on an individual RBC you will see that they are shaped like discs or doughnuts with a thin central portion surrounded by a fatter margin. This shape has all sorts of advantages, one being that enables the cells to fold up and pass along the narrowest blood capillaries (See diagram 8.2).

Anatomy and physiology of animals Red blood cells or erythrocytes.jpg

Diagram 8.2 - Red blood cells or erythrocytes

The mature RBCs of mammals have neither nucleus nor other organelles and can be thought of as sacks of haemoglobin. Haemoglobin is a red coloured protein containing iron, which joins with oxygen so the blood can transport it to body cells. RBCs are made continuously in the bone marrow and live about 120 days. They are then destroyed in the liver and spleen and the molecules they are made from recycled to make new RBCs. Anaemia results if the rate at which they are made doesn't keep up with the rate at which they are destroyed.

Note that if you happen to look at bird’s, reptiles, frogs or fishes blood down the microscope you will see that these vertebrates all have RBCs with a central nucleus.

White Blood Cells

White blood cells or leucocytes are far less numerous than red blood cells. In fact there is only about one white cell for every 1000 red blood cells. Rather than being white, they are actually colourless as they contain no haemoglobin although unlike RBCs they do have a nucleus. If you make a blood smear and look at it under the microscope it is difficult to see the white blood cells at all. To make them visible you need to stain them with special dyes or stains. There are a variety of stains that can be used, but most dye the nucleus a dark purple or pink colour. The stains may also show up the granules present in the cytoplasm of some white blood cells. White blood cells are divided into two major groups depending on the shape of the nucleus and whether or not there are granules in the cytoplasm.

1.Granulocytes or polymorphonuclear leucocytes (“polymorphs” or “polys”) have granules in the cytoplasm and a purple lobed nucleus (see diagram 8.3). The most common (neutrophils) can squeeze out of capillaries and are involved in engulfing and destroying foreign invaders like bacteria (see diagram 8.4). Some (eosinophils) combat allergies and increase in numbers during parasitic worm infections. Others (basophils) produce heparin that prevents the blood from clotting.

Anatomy and physiology of animals A granulocyte.jpg

Diagram 8.3 - A granulocyte

Anatomy and physiology of animals Neutrophils escaping from a capillary.jpg

Diagram 8.4 - Neutrophils escaping from a capillary

2. Agranulocytes or monomorphonuclear leucocytes have a large unlobed nucleus and no granules in the cytoplasm. There are two types of agranulocytes. The most numerous are lymphocytes that are concerned with immune responses. The second type is the monocyte that is the largest blood cell and is involved in engulfing bacteria etc. by phagocytosis (see diagram 8.5).

Anatomy and physiology of animals A granular leucocytes.jpg

Diagram 8.5 - Agranular leucocytes


As well as red and white blood cells, the blood also contains small irregular shaped fragments of cells known as platelets. They are involved in the clotting of the blood (see later).

Transport Of Oxygen

The purpose of the haemoglobin in red blood cells is to carry oxygen from the lungs to the tissues. In fact it allows the blood to carry about 25 times more oxygen than it would be able to without any haemoglobin.

When oxygen concentrations are high, as in the blood capillaries in the lungs, haemoglobin combines with oxygen to form a compound called oxyhaemoglobin. This compound is bright red and makes the oxygenated blood that spurts from a damaged artery its characteristic bright red colour. When the blood reaches the tissues where the oxygen concentrations are low, the oxygen separates from the haemoglobin and diffuses into the tissues. The haemoglobin in most veins has given up its oxygen and the blood is called deoxygenated blood. It is a purple-red colour.

Carbon Monoxide Poisoning

Carbon monoxide is a colourless, odourless gas found in car exhaust fumes and tobacco smoke. It combines with haemoglobin just like oxygen but does not let go. This means the haemoglobin molecules are not available to carry oxygen to the tissues and the animal or human suffocates. Carbon monoxide poisoning is often fatal but can be treated by giving the patient pure oxygen that slowly replaces the carbon monoxide.

Transport Of Carbon Dioxide

Carbon dioxide is a waste gas produced by cells. It diffuses into the blood capillaries where it is carried to the lungs in the blood. Most is carried in the plasma as bicarbonate ions but a small amount is dissolved directly in the plasma and some combines with haemoglobin.

Transport Of Other Substances

The blood carries water to the cells and organs as well as soluble food substances (sugars, amino acids, fatty acids and vitamins) and hormones dissolved in the plasma. These are delivered to the cells via the tissue fluid (see later in this chapter) that surrounds them. Blood also picks up the waste products like carbon dioxide and urea from the cells and is important in distributing the heat produced in the liver and muscles all over the body.

Blood Clotting

The mechanism that causes the blood to clot is easily seen when you or your animals are injured. However, minor injuries occur all the time in areas that experience wear and tear like the intestine, the lungs and the skin. Without the clotting mechanism, animals would quickly bleed to death from minor injury and internal haemorrhage. This is what happens in animals and people with clotting disorders like haemophilia, as well as animals that are poisoned with rat poisons like warfarin.

Platelets are important in blood clotting. When blood vessels are damaged, substances released cause the blood platelets to disintegrate. This stimulates a complex chain of reactions, which causes the protein fibrinogen to be converted to fibrin. Fibrin forms a dense fibrous network over the wound preventing the escape of further blood. Calcium and vitamin K are essential for the clotting process and any deficiency of these may also lead to clotting problems.

Serum And Plasma

When blood clots it separates into the clot that contains most of the cells and platelets leaving behind a straw coloured fluid. This fluid is called serum. It looks just like plasma and is similar in composition except for one big difference. It doesn’t contain fibrinogen, the protein that forms the clot.


Anticoagulants are substances that interfere with the clotting process. When blood is collected for transfusion or testing it is often important to prevent it clotting and there are a number of different anticoagulants you can use for this. Tubes containing the different anticoagulants are coded with different colours for easy recognition.

  1. Heparin (colour code - green) is a natural anticoagulant produced by the white blood cells but it is also used routinely in the laboratory with samples to be tested for heavy metals like lead.
  2. EDTA (colour code – lavender) is used for routine blood counts.
  3. Fluoroxylate (colour code – grey) is used for biochemical tests for glucose.
  4. Citrate (colour code – light blue) is used for the storage of large quantities of blood, such as used in transfusions.


Haemolysis is the breakdown of the plasma membrane of red blood cells to release the haemoglobin. We have already met this process when discussing osmosis, for haemolysis often occurs when red blood cells are placed in a hypotonic solution and water flows in through the semi permeable plasma membrane to swell and eventually burst the cell. It is therefore important when collecting blood from an animal to make sure there is no water in the syringe or tube. Too much movement due to shaking the tube or sucking up the blood too vigorously can also break down the plasma membrane and cause haemolysis.

Blood Groups

If you have given blood recently you may know your blood group. It may be blood group O, A or B or even AB, the rarest group. Blood groups are the result of different molecules called antigens on the outside of red blood cells. These cause antibodies to be formed that attack viruses and bacteria. Knowledge of a person’s blood group is important when giving transfusions because if blood of another incompatible blood group is given to a patient the red blood cells stick together and block the blood vessels and may lead to death.

Blood groups also exist in many animals. There are three blood groups in cats and great care has to be taken that the groups are compatible when transfusing exotic breeds. The situation is slightly different in dogs. They have a number of blood groups but there is usually no problem with the first blood transfusion a dog receives. However, this first transfusion sensitises the immune system so that a problem may arise with the second and subsequent transfusions.

Haemolysis can occur in the living animal when it is exposed to various poisons and toxins. This may happen when, for example, it eats a poisonous plant, is bitten by a snake or infected with bacteria that destroy red blood cells (haemolytic bacteria).

Blood Volume

Blood accounts for between 6-10% of the body weight of animals, varying with the species and the stage of life. Animals can not tolerate losses of greater than 3% of the total volume when the condition known as shock occurs.

Summary | Blood

  • The main functions of blood are transport of oxygen, food, waste products etc., the maintenance
of homeostasis and defending the body from disease.
  • Blood consists of fluid, plasma, in which red and white blood cells are suspended. The blood
cells typically make up 30-45% of the blood volume.
  • Plasma consists of water containing dissolved substances like proteins, nutrients and carbon
  • Red Blood Cells contain haemoglobin to transport oxygen.
  • White Blood Cells defend the body from invasion. There are 2 kinds:
  • Granular white cells include neutrophils, basophils and eosinophils. Neutrophils :which destroy
bacteria are the most numerous. Eosinophils are involved with allergies and parasitic infections.
  • Non-granular white cells include lymphocytes that produce antibodies to attach :bacteria and
viruses and monocytes that engulf and destroy bacteria and viruses.
  • Platelets are involved in blood clotting.


The exercises in the Blood Worksheet will help you learn how to identify the different types of blood cell and what their functions are.

Test Yourself

1. The liquid part of blood is known as:

2. There are two main types of cells in blood. They are:


3. The most numerous cells in blood are:

4. The main function of the red blood cells is:

5. How would you tell a white cell from a red cell when looking at them through a microscope? (Give at least 2 differences)

6. How does the blood help fight invasion by bacteria and viruses?

7. What would happen to blood if there were no platelets?

Test Yourself Answers


Shows constituents of blood including RBCs, white cells, platelets and plasma. Even shows how to make a blood smear and identify the white cells on it as well as make and read a haematocrit. Some parts are a little too advanced.

  • Wikipedia has good information of red and white blood cells.


Bible wiki

Up to date as of January 23, 2010

From BibleWiki

  1. As food, prohibited in Gen 9:4, where the use of animal food is first allowed. Comp. Deut 12:23; Lev 3:17; Lev 7:26; Lev 17:10ff. The injunction to abstain from blood is renewed in the decree of the council of Jerusalem (Acts 15:29). It has been held by some, and we think correctly, that this law of prohibition was only ceremonial and temporary; while others regard it as still binding on all. Blood was eaten by the Israelites after the battle of Gilboa (1Sam 14:32ff).
  2. The blood of sacrifices was caught by the priest in a basin, and then sprinkled seven times on the altar; that of the passover on the doorposts and lintels of the houses (Ex. 12; Lev 4:5ff; Lev 16:14ff). At the giving of the law (Ex 24:8) the blood of the sacrifices was sprinkled on the people as well as on the altar, and thus the people were consecrated to God, or entered into covenant with him, hence the blood of the covenant (Mt 26:28; Heb 9:19f; Heb 10:29; Heb 13:20).
  3. Human blood. The murderer was to be punished (Gen 9:5). The blood of the murdered "crieth for vengeance" (Gen 4:10). The "avenger of blood" was the nearest relative of the murdered, and he was required to avenge his death (Num 35:24ff). No satisfaction could be made for the guilt of murder (Num 35:31).
  4. Blood used metaphorically to denote race (Acts 17:26), and as a symbol of slaughter (Isa 34:3). To "wash the feet in blood" means to gain a great victory (Ps 5810). Wine, from its red colour, is called "the blood of the grape" (Gen 49:11). Blood and water issued from Jesus' side when it was pierced by the Roman soldier (Jn 19:34). This has led pathologists to the conclusion that the proper cause of Jesus' death was rupture of the heart. (Comp. Ps 6920.)
This article needs to be merged with BLOOD (Jewish Encyclopedia).
This entry includes text from Easton's Bible Dictionary, 1897.

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