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Fig. 1184 – Suprarenal glands viewed from behind.]] In Mammal s, the adrenal glands (also known as '''suprarenal glands''') are the triangle-shaped Endocrine Gland s that sit atop the Kidney s; their name indicates that position (''ad'', "near" or "at" + ''renes'', "kidneys"). They are chiefly responsible for regulating the Stress response through the Synthesis of Corticosteroid s and Catecholamine s, including Cortisol and Adrenaline . OVERVIEW s.]] 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, the Adrenal Medulla and the Adrenal Cortex , both of which receive regulatory input from the Nervous System . As its name suggests, the adrenal medulla is at the center of the adrenal gland surrounded by the adrenal cortex. The ''adrenal medulla'' is the body's main source of the catecholamine hormones Epinephrine and Norepinephrine . By contrast, some cells of the ''adrenal cortex'' belong to the Hypothalamic-pituitary-adrenal Axis and are the source of Cortisol synthesis. Other cortical cells produce Androgen s such as Testosterone , while some regulate Water and Electrolyte concentrations by secreting Aldosterone . ADRENAL MEDULLA Composed mainly of hormone-producing Chromaffin Cell s, the adrenal medulla is the principal site of the conversion of the Amino Acid Tyrosine into the catecholamines Epinephrine and Norepinephrine (also called adrenaline and noradrenaline, respectively). Medullary cells are derived from the Embryo nic Neural Crest and, as such, are simply modified Neuron s. In particular, they are modified Postganglionic Cell s of the Sympathetic Nervous System that have lost their Axon s and Dendrite s, receiving innervation from corresponding Preganglionic Fiber s. Moreover, as the synapses between pre- and postganglionic fibers are called '' Ganglia '', the adrenal medulla is actually a ganglion of the sympathetic nervous system. In response to , Blood Vessel Constriction , Bronchiole dilation, and increased Metabolism , all of which are characteristic of the Fight-or-flight Response . ADRENAL CORTEX Situated along the perimeter of the adrenal gland, the adrenal cortex mediates the stress response through the production of Mineralocorticoid s and Glucocorticoid s, including Aldosterone and Cortisol respectively. It is also a secondary site of Androgen synthesis. The cortex can be divided into three distinct layers of Tissue based on their organization. The most superficial cortical layer is the Zona Glomerulosa , which produces Mineralocorticoid s (eg, Aldosterone ). Beneath the glomerulosa are the Zonae Fasciculata and Reticularis , which produce Glucocorticoid s (eg, Cortisol ) and weak Androgen s (eg, Dehydroepiandrosterone ). All adrenocortical hormones are synthesised from Cholesterol . Cholesterol is transported into the inner mitochondrial membrane by steroidogenic acute regulatory protein (StAR), where it is converted into Pregnenolone by the enzyme CYP11A. Accordingly, production of hormones in all three layers of the adrenal cortex is limited by the transportation of cholesterol into the mitochondria and by its conversion into pregnenolone. Pregnenolone can be either dehydrogenated to Progesterone or hydroxylated to 17-alpha-hydroxypregnenolone. The steps up to this point occur in many steroid-producing tissues. Subsequent steps, however, only occur in the adrenal cortex.
The layers of the cortex can be remembered with the Mnemonic '' GFR '': G - ''glomerulosa'', '''F''' - ''fasciculata'', '''R''' - ''reticularis''. Another mnemonic that is used is ''Go Find Rex, Make Good Sex'': G - ''glomerulosa'', '''F''' - ''fasciculata'', '''R''' - ''reticularis'', '''M''' - ''mineralcorticoids'', G - ''glucocorticoids'', '''S''' - ''sex hormones''. Zona glomerulosa The zona glomerulosa is the most superficial layer of the adrenal cortex, lying directly beneath the adrenal gland's capsule. Its cells are ovoid in shape and are arranged in clusters or arches (''glomus'' is Latin for "ball"). In response to increased Potassium levels or decreased blood flow to the kidneys, cells of the zona glomerulosa secrete the mineralocorticoid Aldosterone into the blood as part of the Renin-angiotensin System . Aldosterone regulates the body's concentration of Electrolyte s, primarily Sodium and potassium, by acting on the Distal Convoluted Tubule of kidney Nephron s to:
Zona fasciculata Cells of the zona fasciculata sit directly beneath the zona glomerulosa and are organized in bundles (or fascicles). The zona chiefly produces glucocorticoids (eg, Cortisol ) and a small amount of weak Androgen s (eg, Dehydroepiandrosterone ). Cortical cells responsible for the production of glucocorticoids are the primary effectors of Adrenocorticotropic Hormone (ACTH). The Hypothalamus secretes Corticotropin-releasing Hormone which stimulates the Anterior Pituitary Gland to release ACTH; another hypothalamic hormone, Arginine Vasopressin augments ACTH secretion, with the two together stimulating larger release than ACTH in isolation. ACTH acts on the adrenal cortex to stimulate the release of glucocorticoids. This three-organ endocrine system is commonly called the Hypothalamic-pituitary-adrenal Axis . The primary glucocorticoid released by the adrenal gland is Cortisol . Upon binding to its target, cortisol enhances Metabolism in several ways:
In addition to glucocorticoid production, the fasciculata is a secondary source of androgens such as Testosterone , Dihydrotestosterone (DHT), Androstenedione , and Dehydroepiandrosterone (DHEA). These enhance Muscle mass, stimulate Cell Growth , and aid in the development of the Secondary Sex Characteristic s. Zona reticularis The innermost layer of the adrenal cortex, the zona reticularis sits beneath the zona fasciculata and atop the adrenal medulla. Its cells are arranged in a network of cords (a reticulum) and have the same functions as cells of the zona fasciculata. It is thought that the zona reticularis is the principal source of glucocorticoids and adrenal androgens, with the zona fasciculata activating only after prolonged stimulation. PATHOLOGY Adrenal cortex:
Adrenal medulla:
BLOOD SUPPLY 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: the superior, middle and inferior suprarenal (or adrenal) arteries. Each (left and right) superior suprarenal artery is a branch of the Phrenic Artery on that side of the body. The left and right phrenic arteries supply the Diaphragm , and come off the Aorta . The middle suprarenal artery is a branch directly off the aorta, while the inferior suprarenal artery branches from the aorta or the Renal Artery . SEE ALSO REFERENCES
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