In biology, a chemical secretion of the ductless endocrine glands and specialized nerve cells (see neurohormones) concerned with control of body functions. Hormones act as chemical messengers and are transported to all parts of the body by the bloodstream where they affect target organs. The major glands are the thyroid, parathyroid, pituitary, adrenal, pancreas, ovary, and testis. There are also hormone-secreting cells in the kidney, liver, gastrointestinal tract, thymus (in the neck), pineal (in the brain), and placenta. Hormones bring about changes in the functions of various organs according to the body's requirements. The hypothalamus, which adjoins the pituitary gland at the base of the brain, is a control centre for overall coordination of hormone secretion; the thyroid hormones determine the rate of general body chemistry; the adrenal hormones prepare the organism during stress for ‘fight or flight’; and the sexual hormones such as oestrogen and testosterone govern reproductive functions. Plants produce chemicals that affect growth and development. These chemicals can also be called hormones (see plant hormone).
Hormones often bring about a slower response than messages sent by the nerve cells, but the response to hormones may be longer lasting.
Examples of hormones include: insulin and glucagon (produced in the pancreas), which are involved in glucose regulation; ADH (produced in the pituitary gland), which regulates the concentration of urine produced by the kidney; oestrogen and progesterone (produced in the ovaries), which regulate the growth and functioning of sex organs for sexual reproduction; testosterone (produced in the testes), which regulates male sexual development; follicle-stimulating hormone (FSH) and luteinizing hormone (LH) (from the pituitary gland), which regulate the menstrual cycle in females; and adrenaline (released from the adrenal glands), which prepares the body for increased activity.
The endocrine system The endocrine system, together with the nervous system, forms the neuroendocrine system. Thus, chemical messages can be relayed to the appropriate part, or parts, of the body in response to stimulants either through nervous impulses via the nerves or through hormones secreted in the blood, or by both together. Hormones regulate homeostasis (a constant state within the body) and the body's responses to external and internal stimuli, and also control tissue development, morphogenesis (the development of an organism's form and structure), and reproduction. Many human diseases that are caused by hormone deficiency can be treated with hormone preparations.
The endocrine glands are ductless glands that have capillaries running through them which provide direct access to the bloodstream. So as a hormone is produced by a gland, it is released directly into the bloodstream and carried to the organ or organs that it affects. Normal functioning of the endocrine glands enables normal functioning of the body cells and results in general well-being.
Regulation of blood glucose Blood glucose concentration is controlled by the hormones insulin and glucagon, which are released by the pancreas. The blood glucose concentration is monitored and controlled by the pancreas. If the blood glucose concentration is too high, the pancreas releases insulin into the blood. This causes the glucose concentration in the blood to drop, because the insulin increases the uptake of glucose into most body cells. The liver converts some of the glucose into insoluble glycogen and stores it rather than making glucose. If the blood glucose concentration is too low, the pancreas releases glucagon, which causes the liver to convert glycogen into glucose, which then enters the blood.
Diabetes is a disease in which a person's blood glucose may rise to a high concentration that can kill. This may be because the pancreas does not make enough of the hormone insulin. Or it may be because the liver, muscle, and fat cells respond less to insulin.
Hormones in reproduction The reproductive organs contain endocrine glands: ovaries in females and testes in males. Male sex hormones are called androgens, the most important of which is testosterone. The androgens regulate the development of the male sex organs and, at puberty, secondary sexual characteristics such as the growth of facial, body, and pubic hair, breaking of the voice, muscular development, and stronger body odour. The female sex hormones, oestrogens, regulate the development of female sex organs and, at puberty, breast development, widening of the hips, and the growth of pubic hair. They also regulate menstruation and pregnancy. Changes in concentrations of sex hormones also cause the emotional changes that occur at puberty and the maturation of the reproductive organs.
The monthly release of an egg from a woman's ovaries and the changes in the thickness of the lining of her womb (uterus), known as the menstrual cycle, are controlled by hormones produced by the pituitary gland and by the ovaries. Several hormones are involved in the menstrual cycle of a woman. The hormones involved in promoting the release of an egg include: FSH, which is secreted by the pituitary gland and causes an egg to mature in one of the ovaries, and also stimulates the ovaries to produce hormones including oestrogen; oestrogen, which is secreted by the ovaries and inhibits the further production of FSH as well as stimulating the pituitary gland to produce a hormone called LH; and LH, which is secreted by the pituitary gland and stimulates the release of the egg around the middle of the menstrual cycle.
Hormones and sport Hormones have been used illegally by sports people to enhance sporting performances. This may have long term damaging effects on the body.
Hormones in agriculture In agriculture, hormones are used to stimulate the production of ova in genetically suitable cows, which can then be collected. The eggs are then fertilized in a laboratory (‘in vitro’) and the resulting embryos are implanted into surrogate mothers to produce calves.
Adrenalin Adrenalin is produced by the adrenal glands in response to extreme emotional disturbances – fright, fear, anger, joy – breaks down very quickly after release and so its effects are not long-lasting. As long as the disturbance continues, however, the glands will be stimulated into secreting further supplies of the hormone. The secretion activates the liver into discharging more sugar into the blood for rapid oxidation in the muscles and brain cells. This, in turn, speeds up thinking and muscular responses. Adrenalin also aids in blood clotting and asthmatic conditions due to constriction of blood vessels (vasoconstriction) and relaxation of the muscles of air tubes. It can be synthesized from animal secretions for injection in cases of heart failure or to control bleeding during surgery.
Thyroxine The secretion of thyroxine by the thyroid gland for the regulation of metabolism and growth is an almost continuous process, although the hormone is secreted in quite small amounts. Thyroxine helps to control metabolism mainly by regulating the speed at which mitochondria break down glucose in respiration. A deficiency of thyroxine in children will result in abnormal physical growth and retarded mental growth – a condition known as cretinism. This can be cured by injections of thyroxine. In adults, thyroxine deficiency leads to sluggishness and overweight; an overactive thyroid has the opposite effect, resulting in hyperactivity and underweight. Thyroxine contains iodine, and a lack of this in the diet limits production of the hormone, resulting in enlargement of the gland in the form of a swelling, or goitre. This can be treated by the addition of iodine to the diet.
The pancreas The pancreas is a double gland – one part, which produces pancreatic juice, has a duct leading to the duodenum. The other part, which is ductless, consists of groups of specialized cells known as the islets of Langerhans. These cells produce insulin which is secreted directly into the blood and carried to the liver. Here the insulin regulates the uptake of sugar, or glucose, from the blood by the liver, some of which is used in respiration. The remainder is stored in the liver and muscle cells as glycogen and quickly converted back to glucose as and when needed. The control of blood glucose is an important part of homeostasis; very high blood glucose levels can damage brain cells, causing coma, and possibly death. Insufficient insulin results in an excess of glucose in the bloodstream, or hyperglycaemia. This can lead to a number of diseases, such as in diabetes mellitus, where overstimulation by an excessive intake of sugar and other carbohydrates in the diet causes the pancreas to become exhausted to a point where insulin secretion diminishes. On the other hand, an excess of insulin production will cause blood sugar levels to drop, resulting in hypoglycaemia – among the symptoms of which are dizziness, fatigue, and irritability.
Discovery of hormones The first hormone to be discovered was secretin, a substance released by the small intestine when food reaches it from the stomach. In 1905 two English physiologists, William Bayliss and Ernest Starling, showed that secretin passed into the circulation and, after travelling round the whole body, reached the pancreas; secretin then stimulated the pancreas to produce juice, enabling food to be broken down and absorbed in the intestine. Thus, digestive juices are only produced when the appropriate stimulation is triggered by the intake of food. Bayliss and Starling were able to show that if blood is taken from an animal which is digesting a meal (whose blood therefore contains secretin) and is injected into the circulation of an animal which has not recently eaten, the second animal's pancreas will in turn be stimulated into secreting digestive juices.
Since 1905 many more hormones have been discovered – one of the most important discoveries was made in the early 1920s by two Canadian scientists, Frederick Banting and Charles Best, who succeeded in isolating insulin from an animal pancreas.
Interaction of hormones The relationships between hormones can be very complex in that some endocrine glands produce hormones that in turn stimulate other glands to produce hormones. In this way, the anterior pituitary gland controls the thyroid gland, the adrenal cortex, the testis, and the ovary. When stimulated by the relevant pituitary hormone (known as a trophic hormone), each one of these glands produces its own hormones which have their own effects. But these hormones do something else that is highly ingenious; they act on the pituitary gland itself. They actually decrease (inhibit) the pituitary's secretion of the trophic hormone, and in this way the amount of circulating hormone is kept constant. This is known as ‘negative feedback’. Homeostatis, or feedback systems, which allow information to pass by negative or positive feedback are a feature of many control systems in biology and engineering. Feedback is used by the body to keep many of its constituents, such as body temperature, at a constant level; it is analogous to the thermostat system used for keeping the temperature of a house within narrow limits.
Like all systems in nature, the endocrine system is a finely balanced one, and the use of hormones is not simple. It is easy to upset hormonal balance, but very difficult to restore it. The use of a synthetic oestrogen in the 1950s to treat infertile women resulted in many of their offspring later developing cancer – cervical and vaginal cancers in females and testicular cancer in males. Adrenocorticotrophic hormone (ACTH), which is injected into arthritic children, often results in hormonal imbalances with symptoms such as fat accumulation, hair loss, and oedema (water retention). The use of oestrogen and progesterone in birth control pills has produced similar symptoms in some women.
Disease has played an important role in our understanding of hormones. Historically, the first evidence of a hormone's existence has often been an unexplained disease which subsequently turns out to be caused by over- or under-secretion of the hormone. In recent years, hormonal imbalances – especially those triggered by the use of anabolic steroids – have been implicated in the production of some forms of cancer, particularly cancer of the breast and uterus in women, and the prostate gland in men.
Functions of homomes and control of diabetes
Function of hormones in human body
Roles of hormones in sexual development
Tropisms and plant hormones
Discovery of Insulin
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