The two adrenal glands (also called the suprarenal glands) are situated in the abdomen, above the kidneys and below the diaphragm.
Click to enlarge
They have a high cholesterol content giving them a yellowish colour. They are contained within the same membrane as the kidney but separated from them by a fibrous layer of tissue. The right gland is tetrahedral in shape and lies lower than the left, which is semilunar in shape and usually the larger of the two. Each gland weighs approximately 5 grams and measures approximately 50mm vertically, 30mm across and 10mm thick.
When cut in half each gland consists of an outer cortex, yellow in colour and an inner medulla, which is dark red, or grey.
The cortex consists of three distinct zones. They are:
Diagram illustrating zones of adrenal cortex - click to enlarge
Each zone has a characteristic histology and secretes different types of hormones. The zona glomerulosa secretes a mineralocorticoid (aldosterone) which is responsible for the regulation of salt and water balance in the body. The zona fasciculata secretes a glucocorticoid (cortisol) which regulates the level of carbohydrate in the body. The zona reticularis secretes sex hormones (progesterone, oestrogen precursors and androgens) which have a role in the development of sexual characteristics. The presence of chromaffin cells in each layer suggests that they have a function, as yet unclear, in the regulation of the glands' activity.
The adrenal medulla has a simple make up. It contains chromaffin cells (also called phaeochromocytes) which are surrounded by a meshwork of blood vessels called venous sinusoids. The chromaffin cells, when stimulated by the sympathetic nervous system (see physiology section), secrete noradrenaline and adrenaline into the sinusoids, which are delivered by the bloodstream to the rest of the body.
Diagram illustrating anatomy of adrenal glands - click to enlarge
A rich blood supply is essential for the optimal function of the adrenal glands. Each gland is supplied by the superior, middle and inferior suprarenal arteries, which arise from the inferior phrenic artery, abdominal aorta and renal artery respectively. The blood reaches the outer surface of the gland before entering and supplying each layer. When the blood reaches the adrenal's centre, it flows into the medullary vein. The medullary veins emerge from the hilum of each gland before forming the suprarenal veins, which join the inferior vena cava on the right side and the left renal vein on the left.
The adrenal glands have a rich nerve supply. These nerves are derived from the coeliac plexus and the thoracic splanchnic nerves. The nerves supply the chromaffin cells of the medulla, but careful microscopy has shown that nerve trunks and plexuses may also appear in the cortical layers.