Hormones are ductless endocrine glands which are transported by the blood stream to other parts of the body. All endocrine structures develop from epithelia or nervous tissue or from both. 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.

Hormones have both positive and negative feedback.


Hormone regulation is mostly done by negative feedback
In negative feedback, a hormone causes an effect. The cells that make the hormone detect this effect. Upon detection of the hormone, its production ceases.
A good example of negative feedback is with the hormone, insulin. Insulin is produced by the pancreas. Insulin is released by the pancreas in response to consumption of glucose. The amount of glucose in the blood rises and the pancreas detects this increase. It then secretes insulin into the blood. Insulin increases glucose uptake in target cells. Some glucose is used by the cells but some is also converted to and stored in the form of glycogen. Glucose uptake by cells decreases blood glucose levels - this decrease is detected by the pancreas and in response, it stops secreting insulin in to the bloodstream. As insulin levels in the blood decrease, as does glucose uptake by cells. This negative feedback therefore helps to maintain normal blood glucose levels and prevents extreme changes.

Types of Hormones
There are three main types of hormones: steroid hormones - these are non-polar and do not need a receptor. The other is peptide hormones. The last is Tyrosine derivative hormones, produced by the thyroid.


Counter regulatory 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.


Positive feedback

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 uterus.) 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 uterus 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 uterus force the baby out.

They are paired thickening in the thalamencephalon and thus nervous in origin.They control most of the endocrine structures,acting through the pituitary and autonomic system.Groups of cells produced neurosecretion;droplets produced in the cell traverse the axons into the pars narvosa,to be stored and later released into the blood’Other neurosecretions that is releasing factors pass in the blood or the hypothalamo-hypophyseal portal system into the pars anterior where the stimulate production of other hormones.
Two hormones plus a number of releasing factors are produced.Oxytocin stimulates contraction of the uterus during birth,and promote secretion of milk during suckling.Vasopressin cause a rise in blood pressure by contaction of arteriols,and is also antidiuretic,favourind resorption of water and sodiun ions from the renal tubules.Specific releasing factors affect the secretion of all hormones produced in the pars anterior and pars intermidia.

THE PITUITARY: The pituitary body lies beneath the thalamencephalom. It is formed by fusion of a hollow down-growth from the brain,the infundibulum,with a dorsal invagination from stomodaeum,the hypopysis and it thus nervous and epithelia in origin.The infundibulun becomes the pars navosa;the endocrine parts hypophysis are the pars anterior and pars imtermedia.The former secrets six hormones.Somatotropin STH or growth hormones stimulates rate of protein synthesis, excess produces gigantism, and deficiency, dwarfness; the hormones also promotes production of glucagon in pancreas.Thyrotropic hormones  stimulates develpoment of the thyroid and promotes production of thyrosin.adrenocorticotropic hormones induce production of corticoids from the adrenal.Follicle stimulating hormone promotes develpoment of Grafian follicles in females,and of sperm in males.Luteinizing hormone LTH stimulate ovulation and formation of corpora lutea. Prolactin affect the development of the mammary glands and the maintenance of milk scretion. The pars produces two hormone both melanocyte-stimulating; they affect formation and distribution of melanin granules in the chromatophores

THYRIOD GLAND: Is a mass of soft, red vesicular tissue on each side of the laryns. It develops from epithelium in the floor of the pharyns and I homologous with the endostyle of lower chordates. The hormone tyroxin regulate the basic rate of metabolism i.e BRM; deficiency lower the BRM and lead to hypotyriodism. In the young, this causes retardation of development, a condition known as cretinism or mentally retarded dwarfness; in adult deficiency lead to sluggish metabolism, atrophy of gland and swelling of neck; the condition of mysoedema. Excess of thyrocin causes hyperthyroidism; there is a high BRM, restless activity, rapid heart beat and general wasting of the body.The gland becomes enlargeg and the eye balls protrude;a condition known as exophthalmic goitre. Deficiency of iodine may cause simple goitre.The effects of thyroxin in promoting metamorphosisin amphibian and some fishes are notable.

THE SUPRARENAL GLANDS are small, yellow, ovoid structures, one anterior to each kidney. The cortex is derived from neural crests and the medulla from coelomic epithelium. The medulla secretes adrenalin and nor –adrenalin after sympathetic stimulation.  This produce dramatic changes in the body in response to fear, stress and shock. The three zones of the cortex secrets different hormones ;the outer zone produce mineralocorcorticoids called aldosterone and dexycorticosterone which play a vital part in osmoregulator; the middle zone secrete the glucocortcoids, cortisone and cortisol, which affect carbohydrate metabolism; the inner zone produce androgen and oestrogen which promotes development of the secondary sexual characteristics.

THE PARATHYROID; are two pairs of small glandular masses, lateral to the thyroid, and derived from epithelium the third and forth visceral and clefts. The secrtion, parathormone affect calcium metabolism, playing a large part in exchange of calcium and potassium sulphate ion between bone and blood. It also affect excretion and resorption of these ions.    

THE THYMUS GLAND: derived from epithelium dorsal to the visceral clefts, is a large, soft, pink mass above the heart; it degenerate considerably with age. In the young, it is the main source of lymphocytes and is important in immunological reactions. The hormone, thyroxin, stimulate lymphocyte production in the lymphatic glands.

THE PINEAL BODY: Is a hollow dorsal outgrowth of the thalamencephalon in lamprey larvae and some reptiles, it has a complete eye-like structure, and I the formal, it has an undoubted effect on skin colour. In amphibians, the hormone melatonin cause blanching of the skin; the same effect may occur in mammals. Melatonin certainly stimulates maturation of the gonads, probably by stimulating secretion of gonadotropic releasing factors from the hypothalamus.
THE PANCREAS; is derived from the endodermal epithelium. The islets of lngerhans conyains two types of cells; the large cells which secretes insulin and the smaller cells which secretes glucagons. The former promotes synthesis of glycogen in the liver from blood hexose; the latter  favours breakdown of liver glycogen to glucose. The bigger cells are sensitive o to blood sugar level and smaller cells are stimulated by STH. Deficiency of insulin results in diabetes mellitus.
The ovaries, like the testes are derived from coelomic epithelium.The follicular phase of oestrogen cycle is activated by FSH; this  induces development of the follicle stimulating them to produce oestrogen, the most potent being oestradiol. These activate the reproductive tract in preparation for pregnancy and stimulate development of mammary glands and secondary sexual characteristics. The luteal phase is activated by LH which induce pair of ruptured follicles and growth of corpora lutea;the latter activated by prolactin ,secretes progesterone which inhibits further ovulation, prepare the uterus for implantation and induces further development of the mammae. Relaxin, from the corpora lutea, promotes dilataion of the cervix and relaxation of pubic symphysis during birth

The placenta is epithelial in origin ,part from the uterine lining, part from the trophoblast. Three hormones are secreted; a gonadotropins, oestroegn and progesterone; influence growth of the uterus and mammae after ovarian secretions have waned, and assist  in maintenance of corpora lutea until birth
The testes, after stimulation by FSH, produces at least two androgens, androsterone and testosterone,which play a part in spermatogenesis, control activity of seminal vesticle and prostrate gland and helps in the formation of the male sexual characteristics.
In the alimentary canal, the gastric mucosa secretes gastrin, which induced continued flow of gastric juice; entrocrinin, which starts enzymes secretion in the duodenum, and gastric secretin, which promotes secretion of salt solution from the pancreas. The duodenal mucosa produces secretin, which stimulates further flow of pancreatic salt solution; pancreomysin induces secretion of pancreatic enzymes; cholecystokinin causes contraction of the gall bladder and entrogastrone inhibits acid secretion by the stomach.

NERVE-ENDING: Secrete hormones which excite adjacent cells, sympathetic fibres secrete sympathin, identical in effect with adrenalin; parasympathetic and somatic motor fibres produces acetycholine, which also facilitates transmission of impulses across synapses.
Tissues hormones; histamine and acetycholine, are both powerful depressor, which are probably present in inactive form in all cells. They are activated when cells are damaged and play important part in the process of healing.

Hormones are ductless glands that transfer information and instructions between cells in animals, often described as body’s chemical messengers and these chemicals are transported to target organs or tissues. Hormones regulates growth and development ,control the functions of various tissue ,support reproductive functions and regulate metabolism[the process used to breakdown food to create energy]

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