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In mammals, the adrenal gland (also known as suprarenal glands) are the triangle-shaped endocrine glands that sit atop the kidneys; their name indicates that position (ad, "near" or "at" + renes, "kidneys"). They are chiefly responsible for regulating the stress response through the synthesis of corticosteroids and catecholamines, including cortisol and adrenaline.
Anatomically, the adrenal glands are located in the abdomen, situated on the anteriosuperior aspect of the kidneys. In humans, the adrenal glands are found at the level of the 12th thoracic vertebra and receive their blood supply from the adrenal arteries.
It is separated into two distinct structures, both of which receive regulatory input from the nervous system.
- See main article: adrenal medulla
As its name suggests, the adrenal medulla is the central core of the adrenal gland, surrounded by the adrenal cortex. The chromaffin cells of the medulla are the body's main source of the catecholamine hormones adrenaline (epinephrine) and noradrenaline (norepinephrine). These water-soluble hormones, derived from the amino acids phenylalanine and tyrosine, are part of the fight-or-flight response initiated by the sympathetic nervous system. The adrenal medulla can be considered specialized ganglia of the sympathetic nervous system lacking distinct synapses, instead releasing secretions directly into the blood.
By contrast, the adrenal cortex is devoted to synthesis of steroid hormones from cholesterol. Some cells belong to the hypothalamic-pituitary-adrenal axis and are the source of cortisol synthesis. Other cortical cells produce androgens such as testosterone, while some regulate water and electrolyte concentrations by secreting aldosterone. In contrast to the direct innervation of the medulla, the cortex is regulated by neuroendocrine hormones secreted by the pituitary gland and hypothalamus, as well as by the renin-angiotensin system.
Although variations of the blood supply to the adrenal glands (and indeed the kidneys themselves) are common, there are usually three arteries that supply each adrenal gland:
|superior suprarenal artery||inferior phrenic|
|middle suprarenal artery||abdominal aorta|
|Inferior suprarenal artery||renal artery|
The adrenal glands secrete steroids, including some sex hormones, and catecholamines. Steroids are synthesized and secreted by the adrenal cortex, while catecholamines are synthesized and secreted by chromaffin cells of the adrenal medulla.
- The principal steroids are aldosterone (a mineralocorticoid) and cortisol (a glucocorticoid). The zona glomerulosa of the cortex is predominantly responsible for mineralocorticoid production, the zona fasciculata for glucocorticoid production.
- Aldosterone promotes sodium retention and potassium excretion and is therefore important in maintaining fluid balance and blood pressure.
- Cortisol on the other hand has a wide range of metabolic effects such as protein and fat breakdown that aim to elevate blood glucose levels.
- The zona reticularis of the adrenal cortex is responsible for sex corticoid production. Many sex hormones are secreted including testosterone and estrogen. The sex hormone that is secreted by the adrenals that has the most influence is dehydroepiandrosterone (DHEA). It has virilising effects and is important in development and maintenance of pubic hair, axillary hair, pubertal growth spurts, and sex drive. The effects are only significant in females as the effects are masked by high testosterone levels in males.
- Catecholamines that the adrenal glands secrete are adrenaline and noradrenaline. Adrenaline has the more influential effects. The effects of adrenaline and noradrenaline are wide ranging; adrenaline has a more marked effect on the heart and metabolic activities while noradrenaline is involved more in peripheral vasoconstriction. Adrenaline and noradrenaline secretion is stimulated directly by sympathetic neurons in response to stressors.
The adrenal glands secrete other hormones as well.
- Anatomy Atlases - Microscopic Anatomy, plate 15.292 - "Adrenal Gland"
- Histology at Boston University 14501loa
- SUNY Labs 40:03-0105 - "Posterior Abdominal Wall: The Retroperitoneal Fat and Suprarenal Glands"
- Adrenal Gland, from Colorado State University
- Cross section at UV pembody/body8a
Human anatomy, endocrine system: endocrine glands
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