why do the adrenal glands have a good blood supply
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. The adrenal (or suprarenal) glands are paired endocrine glands situated over the medial aspect of the upper poles of eachP. They secretePsteroidPandPcatecholamineP hormones directly into the blood.
In this article, we shall look at the anatomy of the adrenal glands their location, structure and vascular supply. The adrenal glands are located in the posterior abdomen, between the superomedial kidney and the diaphragm. They are retroperitoneal, with parietal peritoneum covering theirPanterior surface only. The right gland is pyramidal in shape, contrasting with the semi-lunar shape of the left gland. Perinephric (or renal) fascia encloses the adrenal glands and the kidneys. This fascia attaches the glands to the crura of the diaphragm. They are separated from the kidneys by thePperirenal fat. The adrenal glands consist of an outer connective tissue capsule, a cortex and a medulla. PVeins and lymphatics leave each gland via the hilum, but arteries and nerves enter the glands at numerous sites. The outer cortex and inner medulla are the functional portions of the gland. They are two separate endocrine glands, with different embryological origins: Cortex derived from the embryonic mesoderm. Medulla derived from the ectodermal neural crest cells. The cortex and medulla synthesise different hormones. Cortex The cortex is yellowish in colour. It secretes two cholesterol derived hormones corticosteroids and androgens. Functionally, the cortex can be divided into three regions (superficial to deep): Zona glomerulosa produces and secretes mineralocorticoids such as aldosterone.
Zona fasciculata Pproduces and secretes corticosteroids such as cortisol. It also secretes a small amount of androgens. Zona reticularis Pproduces and secretes androgens such asPdehydroepiandrosterone (DHES). It also secretes a small amount of corticosteroids. Medulla The medulla lies in the centre of the gland, and is dark brown in colour. It contains chromaffin cells, which secrete catecholamines (such as adrenaline) into the bloodstream in response to stress. These hormones produce a flight-or-fight response. Chromaffin cells also secrete enkephalins which function in pain control. Superior adrenal artery Middle adrenal artery P arises from the abdominal aorta. Inferior adrenal artery P arises from the renal arteries. Right and left adrenal veins drain the glands. The right adrenal vein drains into thePinferior vena cava,Pwhereas the left adrenal vein drains into theP left renal vein. The adrenal glands are innervated by theP coeliac plexus PandP abdominopelvic splanchnic nerves. Sympathetic innervation to the adrenal medulla is via myelinated pre-synaptic fibres, mainly from the T10 to L1 spinal cord segments. Lymph drainage is to the lumbar lymph nodes by adrenalPlymphatic vessels. These vessels originate from two lymphatic plexuses one deep to the capsule, and the other in the medulla.
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