Adrenal Gland

The adrenal glands are a pair of small, roughly triangular or pyramid-shaped endocrine glands, one situated capping the upper pole of each kidney (hence their alternative name, the suprarenal glands). Although each adrenal gland appears, from the outside, to be a single compact organ, it is in fact composed of two quite distinct tissues of entirely separate embryonic origin, function and hormonal secretion, fused together within a common connective-tissue capsule: an outer adrenal cortex and an inner adrenal medulla.

The adrenal cortex develops from mesodermal tissue, in the same way that many other steroid-hormone-secreting glands do, and is itself organised into three concentric histological zones, each specialised to secrete a different class of steroid hormone. The outermost zone, the zona glomerulosa, secretes mineralocorticoids, chiefly aldosterone, whose principal role is to regulate the body's sodium and potassium balance: aldosterone acts on the kidney tubules to promote the reabsorption of sodium (and, with it, water) from the urine back into the blood, while promoting the excretion of potassium, helping to maintain both blood volume/pressure and electrolyte balance. The middle zone, the zona fasciculata, the thickest of the three, secretes glucocorticoids, chiefly cortisol; cortisol has wide-ranging metabolic effects, raising blood glucose level by promoting gluconeogenesis (glucose synthesis from non-carbohydrate sources) in the liver and by reducing glucose uptake in some peripheral tissues, mobilising fat and protein reserves, suppressing inflammatory and immune responses, and playing a central role in the body's physiological response to prolonged physical or psychological stress. The innermost zone, the zona reticularis, secretes comparatively small amounts of adrenal sex corticoids (androgens), whose contribution to normal secondary sexual characters is modest compared with that of the gonadal hormones described elsewhere in this chapter, though their overproduction in certain adrenal disorders can have marked effects. …