Endocrine System

One of the two adrenals sit atop each kidney and are divided into two distinct regions, the cortex and the medulla. The outer area makes up about 80% of each adrenal and is called the cortex. And the inner portion is called the medulla. The adrenals provide the body with important buffers against stress while helping it adapt to stressful situations.

The cells of the cortex form three distinct layers, the zona glomerulosa (ZG), the zona fasciculata (ZF), and the zona reticularis (ZR). All three layers secrete steroid hormones. The ZG secretes mineralocorticoids; the ZF secretes glucocorticoids; and the ZR secretes primary androgens. Cholesterol droplets are interspersed throughout the cortex for synthesis of cortical steroids. ACTH released from the anterior pituitary triggers glucocorticoids release from the ZF. The major glucocorticoid in humans is cortisol, which fluctuates in concentration throughout the day: the highest levels exist in the morning around 8 a.m. with the lowest levels around midnight. Both physical and emotional stress can affect cortisol secretion. The major human mileralocorticoid is aldosterone. Aldosterone acts on the kidneys and is important in regulating fluid balance. It triggers increased extracellular fluid that leads to increased blood pressure.

Cells of the adrenal medulla, called chromaffin cells, secrete the hormones epinephrine (adrenaline) and non-epinephrine (nor-adrenaline). Chromaffin cells are neuroendocrine cells which function like some nerve fibers of the sympathetic nervous system. However, these cells are endocrine, because the neurohormones that they release target distant organs. Although the effects of these two medullary hormones are the same whether they originate in the endocrine or the nervous system, endocrine hormonal effects are prolonged, because they are removed more slowly from blood than from a nerve terminal. Both cortical and medullary hormones work together in emergencies, or stressful situations, to meet the physical demands of the moment.