Contents
Anatomy
The adrenal glands are small, paired endocrine glands that are situated very close to the upper poles of the kidneys in the retroperitoneum. Each adrenal gland weighs 4-6g and measures around 50x30x10mm. However, while the left adrenal gland is a flat crescent, the right adrenal has a more pyramidal or trefoil shape. The adrenal gland possesses a thin capsule.
The adrenal gland is actually a composite organ that is composed of two independent components, the
adrenal cortex and the adrenal medulla. The cortex forms the outer layer of the gland and is yellow due to its high lipid content. The medulla is the central portion and is brown; it constitutes around 10% of the adrenal gland. The yellow outer layer and brown inner region give the cut surface of the adrenal gland a characteristic appearance. The external surface is also often quite distinctive.
The blood supply of the adrenal gland is derived from the inferior phrenic artery, the abdominal aort and the renal artery which give rise to the superior, middle and inferior renal adrenal arteries respectively. Variations deviations from this basic scheme exist. The venous drainage tends to be as a single adrenal vein.
The adrenal medulla is derived from postganglionic neurones of the sympathetic nervous system. These cells are sometimes referred to as chromaffin cells. The cells are arranged in small nests. Spindle-shaped cells known as sustentacular cells are situated at the edges of the nests. The sustenacular cells support the medullary cells and unlike the medullary cells are positive for S100.
Physiology
The adrenal medulla is part of the sympathetic nervous system. It synthesises noradrenaline and adrenaline, which are collectively known as catecholamines. Both adrenaline and noradrenaline can stimulate the adrenoreceptors used by postganglionic sympathetic fibres to communicate with their target cells. The medulla of the adrenal gland releases adrenaline and noradrenaline into the blood and therefore provides a systemic boost to the sympathetic nervous system across multiple organs; direct stimulation of organs by postganglionic sympathetic nerves can be targeted to specific organs. Adrenaline release dominates over noradrenaline by a ratio of around 6 to 1.
Noradrenaline and adrenaline are derived from the amino acid tyrosine. This synthetic pathway also generates the central nervous system neurotransmitter dopamine.
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Diagram of the synthesis of noradrenaline and adrenaline
Diagram courtesy of Wikipedia
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The effect of the release of adrenaline and noradrenaline by the adrenal medulla is to augment the effects of the sympathetic nervous system across the body. The adrenal medulla is thus an integral part of the fight or flight response.
Increased stimulation of the adrenal medulla by the sympathetic nervous system results in an elevation of the heart rate and the strength of myocardial contraction. The actions on blood flow are organ specific due to different balances between alpha and beta adrenoreceptors in different organs, but the overall effect is to elevate the blood pressure while increasing blood flow to skeletal muscle.
Bronchodilatation occurs to improve airflow in and out of the lungs and thus facilitate gas exchange.
The activity of the GI tract is reduced and this includes a suppression of the secretion of saliva. The GI tract is placed into a state of rest in order to divert resources to other organs that are more directly involved in the fight or flight response.
Metabolism is switched to a catabolic state in order to make glucose and lipids available for use; insulin release is reduced.
Dilatation of the pupils occurs.