Functions of the Adrenal Medulla

The cells of the adrenal medulla secrete epinephrine and nor-epinephrine in an approximate ratio of 4 to 1, respectively. The effects of these catecholamine hormones are similar to those caused by stimulation of the sympathetic nervous system, except that the hormonal effect lasts about ten times longer. The hormones from the adrenal medulla increase the cardiac output and heart rate, dilate coronary blood vessels, increase mental alertness, increase the respiratory rate, and elevate the metabolic rate.

The adrenal medulla is innervated by preganglionic sympathetic axons, and secretes its hormones whenever the sympathetic nervous system is activated during "fight or flight" (chapter 9, fig. 9.7). These sympathoadrenal effects are supported by the metabolic actions of epinephrine and norepi-nephrine: a rise in blood glucose due to stimulation of hepatic glycogenolysis (breakdown of glycogen) and a rise in blood fatty acids due to stimulation of lipolysis (breakdown of fat). The endocrine regulation of metabolism is described more fully in chapter 19.

Wj A tumor of the adrenal medulla is referred to as a pheochromocytoma. This tumor causes hyper__Jj secretion of epinephrine and norepinephrine, which produces an effect similar to continuous sympathetic nerve stimulation. The symptoms of this condition are hypertension, elevated metabolism, hyperglycemia and sugar in the urine, nervousness, digestive problems, and sweating. It does not take long for the body to become totally fatigued under these conditions, making the patient susceptible to other diseases.

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