Chapter 4 - Adrenal Glands

Abstract

The adrenal glands are complex polyfunctional organs situated above the kidneys, whose secretions are required for maintenance of life. Structurally, they are comprised of an outer region or cortex and an inner region or medulla. This chapter describes the structure of adrenal glands, hormones of adrenal cortex and medulla, and biosynthesis, physiological effects, regulation of secretion, and mechanisms of action of these hormones. The hormones of adrenal cortex are steroids and act at the level of the genome to regulate expression of genes that govern the operation of fundamental processes. There are three major categories of adrenal steroid hormones: mineralocorticoids, glucocorticoids, and androgens. Physiologically, aldosterone is by far the most important mineralocorticoid, while cortisol is the principal glucocorticoid. The starting material for steroid hormone biosynthesis is cholesterol and the key reactions are catalyzed by a particular class of oxidizing enzymes, the cytochromes P450. The normal secretory activity of the adrenal cortex is maintained by adrenocorticotropic hormone (ACTH) secreted by the anterior pituitary gland. While mineralocorticoids mainly influence the ratio of sodium to potassium concentrations in urine, glucocorticoids produce diverse physiological effects on energy metabolism, water balance, and anti-inflammatory responses. Cells of adrenal medulla produce two important hormones, epinephrine and norepinephrine, that play a crucial role in the acute response to stress. These hormones are derivatives of the amino acid tyrosine, and belong to a class of compounds called catecholamines. Physiologically, the catecholamines affect the cardiovascular, respiratory, excretory, and skeletal systems, and equip the body for fright, fight, or flight.