Role of Hormones in Homeostasis
Homeostasis is the constancy of the internal environment of growth and reproduction, body temperature, metabolism of glucose, water and slat balances, etc. Hormones also help to maintain the constancy of blood glucose level, heart rate and blood pressure. Thus by their coordinated regulation of the functions of various target organs, hormones help to maintain the constancy of the internal environment of the body. In other words hormones help to maintain the homeostasis of the body.
To enable the maintenance of homeostasis by hormones their secretions are to be regulated in a coordinated manner so that the right hormone is secreted at the right time in the right quantity. The secretion of hormones is often under feed back control by factors or other hormones. The regulation of secretion of thyroxine from the thyroid gland is an example of such feed back control mechanism. The thyrotropin releasing factor (TRF) from the hypothalamus stimulates the pituitary secretion thyrotropin (TSH) and; thyrotropin in turn, stimulates the thyroid gland to secrete thyroxine. A high amount of thyroxine in blood exerts an inhibitory influence on hypothalamus so that hypothalamus secretes less amounts of TRF. This results in a decrease in the TSH secretion and eventually a decrease in thyroxine secretion. Such inhibitory influence, as exerted by thyroxine in the present example, is known as negative feedback. On the other hand a fall in the blood thyroxine level stimulates the hypothalamus to secrete more of TRH which results in increased secretion of TSH which in turn stimulates increased secretion of thyroxine. Such regulatory mechanisms are called positive feed back mechanisms. There are many examples of such feedback control of hormone secretions.
Sometimes the feedback control is exerted by a metabolite. The output of the pancreatic hormone insulin depends on the level of glucose in the blood. After a meal a sudden release of glucose from the intestine to the blood occurs. The increased level of glucose in blood stimulates the secretion of insulin. Insulin then lowers the level of blood glucose to normal again. A fall in blood sugar, on the other hand, removes this stimulatory influence of glucose on insulin secretion to that the level of blood sugar is prevented from falling further. This helps in the maintenance of blood sugar homeostasis by insulin. Besides the feedback controls by hormones and metabolites, the secretions of certain hormones are under the control of the nervous system. The secretion of epinephrine (adrenalin) by the adrenal medulla is under the control of the sympathetic nervous system.
Hormonal Control of Homeostasis: Negative Feed Back Control