The concentration of a hormone in the plasma depends on two factors:
i) Its rate of secretion, and
ii) the rate at which it is inactivated and removed from the circulation.
A feedback control system monitors changes in the animal or in the external environment and sends information to a central control unit (such as the central nervous system), which makes adjustments. A feedback system that produces a response that counteracts the initiating stimulus is called a negative feedback system. In contrast, a positive feedback system reinforces the initial stimulus. Positive feedback systems are relatively rare in animals because they usually lead to instability or pathological states. A feedback pattern is one in which output is constantly compared with example (1) CRH (corticotropin-releasing hormone), secreted by the hypothalamus, stimulates the pituitary (the target cells) to release ACTH.
2) ACTH stimulates the adrenal gland (the target cells) to secrete cartisol.
3) As the level of ACTH rises in the plasma, it acts on, or feeds back on, the hypothalamus to inhibit release of CRH.
4) Similarly, as the cortisol level rises in the plasma, it “feeds back” on the hypothalamus and pituitary to inhibit release of both CRH and ACTH (adenocorticotropic hormone), respectively.
5) Thus any deviation from the set point (a specific plasma level of each hormone) leads to corrective action in the opposite direction. Such a negative feedback system is highly effective in preventing extreme oscillations in the hormonal output. Fig. 3.4.
Extreme oscillations in hormone output do sometimes occur under natural conditions. However, because they have the potential to disrupt finely tuned homeostatic mechanisms, such extreme oscillations, as a result of positive feedback, are highly regulated and possess an obvious shutoff mechanism. For example, hormones controlling parturition (child birth) are shut off by birth of the young from the uterus; hormones controlling ovulation are shut off by release of an ovum from a follicle.