The cells of the adrenal medulla secrete epinephrine and norepinephrine, which collectively are called cat-echolamines and aresympathomimetic. The secretion of both hormones is stimulated by sympathetic impulses from the hypothalamus, and their functions duplicate and prolong those of the sympathetic divi-sion of the autonomic nervous system (mimetic means “to mimic”).
Epinephrine (Adrenalin) and norepinephrine (nor-adrenalin) are both secreted in stress situations and help prepare the body for “fight or flight.” Norepi-nephrine is secreted in small amounts, and its most significant function is to cause vasoconstriction in the skin, viscera, and skeletal muscles (that is, throughout the body), which raises blood pressure.
Epinephrine, secreted in larger amounts, increases the heart rate and force of contraction and stimulates vasoconstriction in skin and viscera and vasodilation in skeletal muscles. It also dilates the bronchioles, decreases peristalsis, stimulates the liver to change glycogen to glucose, increases the use of fats for energy, and increases the rate of cell respiration. Many of these effects do indeed seem to be an echo of sym-pathetic responses, don’t they? Responding to stress is so important that the body acts redundantly (that is, exceeds what is necessary, or repeats itself) and has both a nervous mechanism and a hormonal mecha-nism. Epinephrine is actually more effective than sym-pathetic stimulation, however, because the hormone increases energy production and cardiac output to a greater extent. The hormones of the adrenal medulla are summarized in Table 10–6, and their functions are shown in Fig. 10–9.