Adrenal Anatomy

The two adrenal glands (also called the suprarenal glands) are situated in the abdomen, on either side of the vertebral column, above the kidneys and below the diaphragm. Because of the large size of the liver above it, the right adrenal and kidney tend to lie lower than the left adrenal.

The adrenals 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 pyramidal in shape, while the left 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 (Fig. 1).

Fig.1: Adrenal gland anatomyBlood supply

Like other endocrine glands (that secrete hormones directly into the bloodstream) the adrenals receive a very rich blood supply. This 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. These vessels form a plexus within the capsule of the adrenal, which gives rise to a network of small arteries that descend through the various layers of the gland. 

Once through the cortex (outer layers) and into the central part of the gland, called the medulla, the arteries form an elaborate network of capillaries around the medullary secretory cells.

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 usually single suprarenal (adrenal) veins, which join the inferior vena cava on the right side and the left renal vein on the left.

Nerve supply

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.

Internal structure

When cut in half each gland consists of an outer cortex, yellow in colour and an inner medulla, which is dark red, or grey.

Fig.2: Internal structure of the adrenal gland showing the cortical layers and medullaThe cortex consists of three distinct zones (Fig. 2). They are:

• Zona glomerulosa
• Zona fasciculata
• Zona reticularis  

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 secrete noradrenaline and adrenaline into the sinusoids, which are delivered by the bloodstream to the rest of the body.