Adrenal gland

The two adrenal glands sit on the upper poles of the kidneys, wrapped with them in fat at the back of the abdomen; the right is roughly triangular and the left more crescent-shaped. They have one of the richest blood supplies in the body. Each is really two organs with separate embryonic origins: an outer, glandular cortex and a central medulla that develops from the neural crest, like the sympathetic ganglia.

The cortex has three zones that make steroid hormones from cholesterol. The outer zona glomerulosa produces mineralocorticoids, chiefly aldosterone, which makes the kidneys keep sodium and lose potassium, raising blood volume and pressure. The middle zona fasciculata produces glucocorticoids, chiefly cortisol, in response to ACTH from the pituitary; cortisol raises blood glucose, helps the body withstand long-term stress and damps inflammation. The inner zona reticularis makes small amounts of androgens. The medulla works as part of the sympathetic nervous system: when nerve fibers stimulate its chromaffin cells, they pour epinephrine (adrenaline) and norepinephrine into the blood in the fight-or-flight response.

Excess cortisol causes Cushing syndrome, and destruction of the cortex causes Addison disease, a potentially life-threatening lack of cortisol and aldosterone. The cortex is essential for life, whereas the medulla can be removed.