The adrenal glands are a pair of endocrine glands that sit atop the kidneys and that release their hormones directly into the bloodstream. The adrenals are flattened, somewhat triangular bodies that, like other endocrine glands, receive a rich blood supply. The phrenic (from the diaphragm) and renal (from the kidney) arteries send many small branches to the adrenals, while a single large adrenal vein drains blood from the gland.

Each adrenal gland is actually two organs in one. The inner portion of the adrenal gland, the adrenal medulla, releases substances called catecholamines, specifically epinephrine, adrenaline, norepinephrine, noradrenaline, and dopamine. The outer portion of the adrenal gland, the adrenal cortex, releases steroids, which are hormones derived from cholesterol.

There are three somewhat distinct zones in the adrenal cortex: the outer part, the zona glomerulosa (15% of cortical mass) made up of whorls of cells...