A hormone (from Greek ὁρμή - "impetus") is a chemical released by a cell in one part of the body, that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. It is essentially a chemical messenger that transports a signal from one cell to another. All multicellular organisms produce hormones; plant hormones are also called phytohormones. Hormones in animals are often transported in the blood. Cells respond to a hormone when they express a specific receptor for that hormone. The hormone binds to the receptor protein, resulting in the activation of a signal transduction mechanism that ultimately leads to cell type-specific responses.
Endocrine hormone molecules are secreted (released) directly into the bloodstream, while exocrine hormones (or ectohormones) are secreted directly into a duct, and from the duct they either flow into the bloodstream or they flow from cell to cell by diffusion in a process known as paracrine signalling.
Hormonal signaling across this hierarchy involves the following:
As can be inferred from the hierarchical diagram, hormone biosynthetic cells are typically of a specialized cell type, residing within a particular endocrine gland, such as thyroid gland, ovaries, and testes. Hormones exit their cell of origin via exocytosis or another means of membrane transport. The hierarchical model is an oversimplification of the hormonal signaling process. Cellular recipients of a particular hormonal signal may be one of several cell types that reside within a number of different tissues, as is the case for insulin, which triggers a diverse range of systemic physiological affects. Different tissue types may also respond differently to the same hormonal signal. Because of this, hormonal signaling is elaborate and hard to dissect.
Most hormones initiate a cellular response by initially combining with either a specific intracellular or cell membrane associated receptor protein. A cell may have several different receptors that recognize the same hormone and activate different signal transduction pathways, or alternatively different hormones and their receptors may invoke the same biochemical pathway.
For many hormones, including most protein hormones, the receptor is membrane associated and embedded in the plasma membrane at the surface of the cell. The interaction of hormone and receptor typically triggers a cascade of secondary effects within the cytoplasm of the cell, often involving phosphorylation or dephosphorylation of various other cytoplasmic proteins, changes in ion channel permeability, or increased concentrations of intracellular molecules that may act as secondary messengers (e.g. cyclic AMP). Some protein hormones also interact with intracellular receptors located in the cytoplasm or nucleus by an intracrine mechanism.
For hormones such as steroid or thyroid hormones, their receptors are located intracellularly within the cytoplasm of their target cell. To bind their receptors these hormones must cross the cell membrane. The combined hormone-receptor complex then moves across the nuclear membrane into the nucleus of the cell, where it binds to specific DNA sequences, effectively amplifying or suppressing the action of certain genes, and affecting protein synthesis. However, it has been shown that not all steroid receptors are located intracellularly, some are plasma membrane associated.
An important consideration, dictating the level at which cellular signal transduction pathways are activated in response to a hormonal signal is the effective concentration of hormone-receptor complexes that are formed. Hormone-receptor complex concentrations are effectively determined by three factors:
The number of hormone molecules available for complex formation is usually the key factor in determining the level at which signal transduction pathways are activated. The number of hormone molecules available being determined by the concentration of circulating hormone, which is in turn influenced by the level and rate at which they are secreted by biosynthetic cells. The number of receptors at the cell surface of the receiving cell can also be varied as can the affinity between the hormone and its receptor.
Most cells are capable of producing one or more molecules, which act as signaling molecules to other cells, altering their growth, function, or metabolism. The classical hormones produced by cells in the endocrine glands mentioned so far in this article are cellular products, specialized to serve as regulators at the overall organism level. However they may also exert their effects solely within the tissue in which they are produced and originally released.
The rate of hormone biosynthesis and secretion is often regulated by a homeostatic negative feedback control mechanism. Such a mechanism depends on factors that influence the metabolism and excretion of hormones. Thus, higher hormone concentration alone cannot trigger the negative feedback mechanism. Negative feedback must be triggered by overproduction of an "effect" of the hormone.
Hormone secretion can be stimulated and inhibited by:
One special group of hormones is the tropic hormones that stimulate the hormone production of other endocrine glands. For example, thyroid-stimulating hormone (TSH) causes growth and increased activity of another endocrine gland, the thyroid, which increases output of thyroid hormones.
To release active hormones quickly into the circulation, hormone biosynthetic cells may produce and store biologically inactive hormones in the form of pre- or prohormones. These can then be quickly converted into their active hormone form in response to a particular stimulus.
Hormones have the following effects on the body:
A hormone may also regulate the production and release of other hormones. Hormone signals control the internal environment of the body through homeostasis.
Vertebrate hormones fall into three chemical classes:
Many hormones and their analogues are used as medication. The most commonly prescribed hormones are estrogens and progestagens (as methods of hormonal contraception and as HRT), thyroxine (as levothyroxine, for hypothyroidism) and steroids (for autoimmune diseases and several respiratory disorders). Insulin is used by many diabetics. Local preparations for use in otolaryngology often contain pharmacologic equivalents of adrenaline, while steroid and vitamin D creams are used extensively in dermatological practice.
A "pharmacologic dose" of a hormone is a medical usage referring to an amount of a hormone far greater than naturally occurs in a healthy body. The effects of pharmacologic doses of hormones may be different from responses to naturally-occurring amounts and may be therapeutically useful. An example is the ability of pharmacologic doses of glucocorticoid to suppress inflammation.
Welcome to the Wikiversity learning project about hormones. Wikiversity learning resources about hormones are under development.
Hormones are molecules that are
Hormones help coordinate the behaviors of distant parts of animals. One of the most familiar hormones is insulin. Insulin is made in the pancreas and is transported through the blood stream to its many target tissues. The pancreas is positioned so as to efficiently sense when food molecules are available in the digestive system. Insulin is released when food molecules are available and it triggers many cells in the body to adjust their metabolic processes so as to help store the available food molecules.
The fact that hormones function as control signals that are transported through the blood makes them an important class of drugs. For example, insulin can be used as a drug by some people with diabetes. Some drugs target the production of hormones or their receptors. We can also often influence the actions of hormones by our daily behaviors. Thus, there are many circumstances when people become interested in hormones for health reasons.
Hormones are chemicals that are used for messaging in multicellular organisms. Every multicellular organism produces hormones. The cells that react to a given hormone have special receptors for that hormone. When a hormone attaches to the receptor protein a mechanism for signalling is activated.
The messages can be sent to nearby cells or to far-away cells. If a cell wants to send a message to a nearby cell, it puts the hormone into the tissue around it. If an animal's cell wants to send a message to a far-away cell, it puts the hormone into the blood. When a hormone is put in the blood it goes to all parts of the animal's body. Sometimes the cell that gets the message can even be the same cell that made the hormone (and sent the message.)
The cell or tissue that gets the message is called the target cell.
Many different kinds of cells can send a message. There are some kinds of cells whose main job is to make hormones. When many of these cells are together in one place, it is called a gland. Glands are groups of cells that make something and release it (put it outside the cell). Some glands make hormones.
Endocrine means something that is made by cells and released into the blood or tissue. So endocrine glands form hormones and release them into the blood or tissue. The opposite word is exocrine and means released outside of the body. An example of exocrine is sweat glands or saliva glands. When people say endocrine they usually mean glands that make hormones.
Hormones do many things. They regulate metabolism. Metabolism is all of the chemical and energy reactions that happen in a living thing. Hormones cause the growth and death of cells and of whole organisms. Hormones also start and control sexual development. For example, the hormones estrogen and progesterone make girls puberty. Hormones help keep homeostasis in an organism. Homeostasis means to keep a constant state inside the body like temperature, amount of water and salts, and amount of sugar. Hormones released by one gland can also tell other glands to make different hormones.
There are four types of hormones in most animals. They are grouped by the chemicals from which they are made. When scientists say hormones are derived from it means they are made from something by changing it. These changes are chemical changes.
In biology regulation means to control something. So regulating hormones means controlling how much hormones are made and released from cells.
Hormone regulation is mostly done by negative feedback.
In negative feedback a hormone makes an effect. The cells that make the hormone see that effect happen. When they see it happen, they stop making more hormone.
A good example of negative feedback is the hormone insulin. Insulin is a hormone that is made by the pancreas. Insulin is released by the pancreas when you eat glucose (a kind of sugar). The amount of glucose in the blood goes up. The pancreas sees this high glucose level. It makes insulin and releases it into the blood. Then the insulin goes through the whole body and tells cells to take glucose out of the blood. Cells use some of this for energy. But some extra is also saved in the cells to use later. When cells take up glucose from the blood this makes the glucose level go down. The pancreas sees this and stops making insulin. When the pancreas stops sending this message (insulin), the cells in the body stop taking extra glucose out of the blood.
So the negative feedback works to keep the blood glucose level normal. If glucose is high, the pancreas makes insulin. The insulin causes the glucose to fall. Then this lower level of glucose tells the pancreas to stop making insulin.
There are two main types of hormones. One is steroid hormones. They are nonpolar and do not need a receptor. The other is peptide hormones.
Sometimes two or more hormones control the same thing. For example, blood glucose is very important to an organism. So it is not controlled by just one hormone. Other hormones also make the glucose level go up or down. If the glucose level gets too low, the body releases hormones that do the opposite of insulin. They do not tell the cells in the body to take up glucose from the blood. They tell the cells to put glucose back into the blood. These kind of hormones that work opposite of other hormones are called counter-regulatory hormones. Counter-regulatory hormones for insulin are glucagon and epinephrine.
Most important things in an organism are kept in homeostasis by negative feedback and counter-regulatory hormones. However a few things are controlled in different ways. One rare way is positive feedback. In negative feedback, the hormone's effect makes a gland stop making hormones. In positive feedback the opposite happens. The effect of the hormone tells the gland to make even more hormones.
An example of positive feedback is the hormone that causes childbirth (when babies are born.) The hormone that causes this is oxytocin. This hormone is made by the pituitary gland. When the baby starts coming out, it stretches the muscle in the cervix (the bottom of the womb.) Nerves in the cervix send a message to the pituitary. This message makes the pituitary release more oxytocin. The oxytocin then causes the muscles of the womb to contract, or squeeze. This causes more stretching in the cervix. This stretching then tells the pituitary to make even more oxytocin. So levels of oxytocin keep rising until the squeezing or contractions of the womb force the baby out. (The womb is also called the uterus.)
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