A hormone controls its particular aspect of the body’s internal environment in a way similar to that of a thermostat regulating the temperature of a room. By switching the heat supply on when room temperature falls, and switching it off when the correct temperature is reached, a thermostat maintains a steady room temperature despite wide variation in temperature outside the room and the amount of heat lost. This type of control mechanism is known as a ‘negative feedback control system’.
Manymaintain a steady state by the same mechanism. For example, the hormone largely responsible for the amount of urine produced, antidiuretic hormone (ADH) – secreted by the posterior, or rear, lobe of the pituitary gland – controls the loss of water from the kidneys, in order to maintain the amount of water in the body fluids constant. On the one hand, if fluid intake is reduced the extracellular fluid becomes more concentrated. This concentration is detected and the pituitary gland in the brain secretes ADH. This hormone then acts on its target organs, the kidneys, to reduce water loss in the urine and so redress the balance. On the other hand, if fluid intake is excessive, the extracellular fluid becomes diluted. Again this is detected and ADH secretion falls. The kidneys, no longer under the control of high ADH levels, allow the excess water to be excreted in the urine until the extracellular fluid concentration returns to normal.
Other hormones, aldosterone and parathormone -secreted by the adrenal glands and parathyroid glands respectively – control the balance of sodium and calcium by a similar mechanism. Some hormones, such as thyroid hormones and Corti- sol from the adrenal glands, have more general effects on nearly all theof the body rather than controlling the balance of a specific chemical constituent. The secretion of these hormones is controlled by a negative feedback mechanism involving regulatory hormones from the pituitary gland. The secretion of thyroid hormones, for instance, is controlled by thyroid stimulating hormone (TSH) produced by the pituitary. TSH stimulates the thyroid to secrete its hormones and these in turn inhibit TSH secretion, thereby maintaining the concentration of both hormones at a constant level.