The Adrenal Gland
The adrenal gland is divided into the cortex and medulla. The adrenal cortex secretes androgens, mineralocorticoids (eg, aldosterone), and glucocor-ticoids (eg, cortisol). The adrenal medulla secretes catecholamines (primarily epinephrine, but also small amounts of norepinephrine and dopamine). The adrenal androgens have almost no relevance for anesthetic management and will not be considered further.
Aldosterone is primarily involved with fluid and electrolyte balance. Aldosterone secretion causes sodium to be reabsorbed in the distal renal tubule in exchange for potassium and hydrogen ions. The net effect is an expansion in extracellular fluid volume caused by fluid retention, a decrease in plasma potassium, and metabolic alkalosis. Aldosterone secretion is stimulated by the renin– angiotensin system (specifically, angiotensin II), pituitary adrenocorticotropic hormone (ACTH), and hyperkalemia. Hypovolemia, hypotension, con-gestive heart failure, and surgery result in an eleva-tion of aldosterone concentrations. Blockade of the renin–angiotensin–aldosterone system with angio-tensin-converting enzyme inhibitors or angiotensin receptor blockers, or both, is a cornerstone of ther-apy (and produces increased survival) in hyperten-sion and chronic heart failure. Aldosterone receptor blockers (spironolactone or eplerenone) added to standard therapy prolong survival in patients with chronic heart failure.
Glucocorticoids are essential for life and have multiple physiological effects, including enhanced gluconeogenesis and inhibition of peripheral glucose utilization. These actions tend to raise blood glucose and worsen diabetic control. Glucocorticoids are required for vascular and bronchial smooth muscle to respond to catecholamines. Because glucocorticoids are structurally related to aldosterone, most tend to promote sodium retention and potassium excretion (a mineralocorticoid effect). ACTH released by the anterior pituitary is the principal regulator of glu-cocorticoid secretion. Basal secretion of ACTH and glucocorticoids exhibits a diurnal rhythm. Stressful conditions promote secretion of ACTH and cortisol, while circulating glucocorticoids inhibit ACTH and cortisol secretion. Endogenous production of corti-sol, the most important endogenous glucocorticoid, averages 20 mg/d.
. Epinephrine constitutes 80% of adrenal catecholamine output in humans. Catecholamine release is regulated mainly by sympathetic cholinergic preganglionic fibers that innervate the adrenal medulla. Stimuli include exercise, hemorrhage, surgery, hypotension, hypo-thermia, hypoglycemia, hypercapnia, hypoxemia, pain, and fear.