adrenal gland

An adrenal gland is a small, triangular gland sitting like a cap on top of each kidney. Despite weighing only a few grams, it is really two organs fused into one: an outer rind and an inner core that have completely different jobs and even different embryonic origins. Think of it as a tiny factory complex where one wing makes slow, long-acting chemical signals and the other fires off an instant emergency alarm.

The outer rind is the adrenal cortex, which makes steroid hormones from cholesterol. These steroids fall into three families: glucocorticoids such as cortisol, mineralocorticoids such as aldosterone, and weak androgens such as DHEA. The inner core is the adrenal medulla, which is essentially a piece of modified nervous system that pours out catecholamines, mainly epinephrine, when the body needs a rapid stress response.

Because the cortex and medulla regulate very different things — long-term metabolism, salt and water balance, and the fight-or-flight reaction — losing adrenal function is dangerous. Severe deficiency (as in Addison's disease) can cause a life-threatening crisis, while chronic excess (as in Cushing's syndrome) damages many organ systems over time.