ABSTRACT
The hypothalamic-pituitary-adrenal gland (HPA) axis is a critical feedback system for maintaining homeostatic control of hormone synthesis and secretion. In the dog, as in human beings, the stress response is one physiological function that is dependent on proper functioning of the HPA axis. The hormonal connection with the hypothalamic-pituitary system is via production and/or secretion of glucocorticoids and mineralcorticoids by unique zones that comprise the cortex of the adrenal glands. The cortex is of mesodermal origin and is comprised of three functionally distinct but not morphologically distinct layers (zones) which surround the medulla. The outer layer (zona glomerulosa) produces the mineralcorticoid aldosterone, the principal hormone that orchestrates sodium and potassium concentrations in the maintenance of normal fluid balance and circulatory volume. The middle layer (zona fasciculata) secretes glucocorticoids and the inner layer of the cortex (zona reticularis) secretes sex steroids.
