Adrenal glands

What Are Adrenal Glands?

Adrenal glands are paired endocrine organs that sit on the superior pole of each kidney and secrete steroid hormones and catecholamines into the bloodstream. Each gland weighs roughly 4 to 5 grams in an adult and consists of two functionally and embryologically distinct tissues packaged in a single capsule: an outer cortex of mesodermal origin that manufactures steroids, and an inner medulla of neuroectodermal origin that behaves as a modified sympathetic ganglion. The cortex accounts for about 90 percent of the mass. Because the two tissues answer to different control systems, the pituitary axis for the cortex and the sympathetic nervous system for the medulla, a single organ mediates both slow metabolic regulation and the rapid fight-or-flight response.

The glands are among the most densely perfused tissues in the body relative to their size, supplied by branches from the inferior phrenic artery, the aorta, and the renal artery, and drained by a single central vein on each side. The anatomical arrangement, including the asymmetry between the shorter right adrenal vein and the longer left one, is set out in the StatPearls description of adrenal anatomy and matters directly to interventional radiologists performing adrenal vein sampling.

The Adrenal Cortex

The cortex is organized into three concentric zones, each with a different enzymatic complement and therefore a different hormonal product. The outermost zona glomerulosa synthesizes the mineralocorticoid aldosterone, which regulates sodium retention and potassium excretion and is controlled chiefly by angiotensin II and extracellular potassium concentration. The middle zona fasciculata produces the glucocorticoid cortisol under the control of adrenocorticotropic hormone released by the anterior pituitary, and cortisol in turn governs gluconeogenesis, immune suppression, and the stress response. The innermost zona reticularis secretes adrenal androgens, principally dehydroepiandrosterone and its sulfate. The zonation, its embryonic origin, and the steroidogenic enzymes involved are described in detail in the Endotext chapter on the adrenal cortex.

The Adrenal Medulla

The medulla contains chromaffin cells, which are innervated directly by preganglionic sympathetic fibers and release epinephrine and norepinephrine into the venous circulation rather than onto a synapse. Roughly 80 percent of the catecholamine output is epinephrine, a proportion that reflects the local availability of cortisol draining from the cortex, which induces the enzyme that converts norepinephrine to epinephrine. Release occurs by calcium-triggered exocytosis of dense-core vesicles, a process studied at single-cell resolution using carbon-fiber amperometry and microelectrode arrays. Work on activity-dependent catecholamine release in adrenal slices has mapped how native splanchnic stimulation patterns shape secretion in space and time.

Regulation and Disease

Cortisol secretion follows a circadian rhythm with a peak shortly after waking, and it feeds back negatively on both the hypothalamus and the pituitary. Failure of that axis produces distinct clinical syndromes: Addison disease from cortical destruction, Cushing syndrome from cortisol excess, primary aldosteronism from autonomous aldosterone production, and pheochromocytoma from a catecholamine-secreting medullary tumor. The physiology underlying these conditions is summarized in the NCBI Bookshelf overview of the adrenal gland. Diagnosis depends on quantitative assays, dynamic suppression and stimulation testing, and cross-sectional imaging.

Applications

Study of the adrenal glands has applications in a range of fields, including: