adrenal gland

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A complex endocrine organ in proximity to the kidney. Adrenal gland tissue is present in all vertebrates. The adrenal consists of two functionally distinct tissues: steroidogenic cells and catecholamine-secreting cells. While “adrenal” refers to the gland's proximity to the kidney, significant variation exists among vertebrates in its anatomic location as well as the relationship of the two endocrine tissues which make up the gland. In mammals, steroidogenic cells are separated into distinct zones that together form a cortex. This cortical tissue surrounds the catecholamine-secreting cells, constituting the medulla. In most other vertebrates, this unique anatomic cortical-medullary relationship is not present. In species of amphibians and fish, adrenal cells are found intermingling with kidney tissue, and the steroidogenic cells are often termed interrenal tissue.

Development

The adrenal gland forms from two primordia: cells of mesodermal origin which give rise to the steroid-secreting cells, and neural cells of ectodermal origin which develop into the catecholamine-secreting tissue (also known as chromaffin tissue). In higher vertebrates, mesenchymal cells originating from the coelomic cavity near the genital ridge proliferate to form a cluster of cells destined to be the adrenal cortex. During the second month of human development, cells of the neural crest migrate to the region of the developing adrenal and begin to proliferate on its surface. The expanding cortical tissue encapsulates the neural cells forming the cortex and medulla. In mammals, three distinct zones form within the cortex: the outermost zona glomerulosa, the middle zona fasiculata, and the inner zona reticularis. The glomerulosa cells contain an enzyme, aldosterone synthase, which converts corticosterone to aldosterone, the principal steroid (mineralocorticoid) secreted from this zone. The inner zones (fasiculata and reticularis) primarily secrete glucocorticoids and large amounts of sex steroid precursors. In many lower vertebrates, the two tissues form from similar primordia but migrate and associate in different ways to the extent that in some cases the two tissues develop in isolation from each other.

Comparative anatomy

While the paired adrenals in mammals have a characteristic cortical-medullary arrangement with distinct zonation present in the cortex, such distinctions are lacking in nonmammalian species. In more primitive fishes, chromaffin cells form in isolation from steroidogenic tissue. A general trend is present, however, throughout vertebrates for a closer association of chromaffin and steroidogenic tissues. Zonation in steroidogenic tissue is largely confined to mammals, although suggestions of separate cell types have been postulated in birds and in some other species.

Comparative endocrinology

Hormones are secreted from the cells of both the medulla and the cortex.

Chromaffin cells

In all vertebrates, chromaffin cells secrete catecholamines into circulation. In most species, the major catecholamine secreted is epinephrine, although significant amounts of norepinephrine are released by many animals. Some dopamine is also secreted. No phylogenetic trend is obvious to explain or predict the ratio of epinephrine to norepinephrine secreted in a given species. A given species may release the two catecholamines in different ratios, depending on the nature of the stimulus. The great majority of the norepinephrine in circulation actually originates from that which is released from non-adrenal sympathetic nerve endings and leaks into the bloodstream. In addition to catecholamines, chromaffin cells secrete an array of other substances, including proteins such as chromogranin A and opioid peptides. See also Epinephrine.

Biologic effects of catecholamines are mediated through their binding to two receptor classes, α- and β-adrenergic receptors. Further examination of these receptors has revealed that subclasses of each type exist and likely account for the responses on different target tissues. In general, biologic responses to catecholamines include mobilization of glucose from liver and muscle, increased alertness, increased heart rate, and stimulation of metabolic rate.

Steroid hormones

In broad terms, most steroids secreted by adrenal steroidogenic cells are glucocorticoids, mineralocorticoids, or sex hormone precursors. However, these classes have been established largely on the basis of differential actions in mammals. The principal glucocorticoids are cortisol and corticosterone, while the main mineralocorticoid is aldosterone. This division of action holds for mammalian species and likely for reptiles and birds. In other vertebrates, such as fish and amphibians, steroids from the interrenal tissue do not show such specialized actions; instead, most show activities of both glucocorticoid and mineralocorticoid type. Mammals, birds, reptiles, and amphibians secrete cortisol, corticosterone, and aldosterone. The ratios of the two glucocorticoids vary across species; in general, corticosterone is the more important product in nonmammalian species. Even within mammals, a large variation exists across species, due to the relative ratio of cortisol to corticosterone from the adrenal cortex.

Effects of adrenal-derived steroids in lower vertebrates involve a diverse array of actions, including control of distribution and availability of metabolic fuels such as glucose, and regulation of sodium and extracellular fluid volume. In nonmammalian vertebrates, corticosterone, cortisol, and aldosterone possess mineralocorticoid effects. Other areas where adrenal steroids likely contribute to biologic processes include control of protein, fat, and carbohydrate balance; reproduction; and growth and development. See also Steroid.

There are two adrenal glands, one sitting on top of each of the kidneys. They are pyramidal in shape and weigh about 4 g each. Their presence was recognized as early as the late sixteenth century, but it was not until 1805 that Cuvier reported that the adrenal was made up of two regions, the cortex on the outside and an inner medulla. Fifty years later, a Guy's Hospital physician, Thomas Addison, showed that the adrenal glands were necessary for life, by identifying them as the site of damage in a previously mysterious and ultimately fatal illness, which became known as Addison's disease.

The adrenal cortex

is known now to have three distinct regions: the zona glomerulosa, zone fasciculata, and zona reticularis. The first of these regions produces the steroid aldosterone, while another steroid hormone, cortisol, is produced by the other two regions. The cells which make up all of these regions are full of lipid droplets containing cholesterol, which can be converted into the steroid hormones.

Aldosterone, by acting on the kidneys, controls the salt content of the body — by which means it also indirectly controls the blood pressure. The amount of aldosterone produced is controlled by other substances, including a protein from the kidney known as renin. Specialized cells in the kidney, which form the juxtaglomerular apparatus, are very sensitive to changes in blood pressure — well placed for this function by being wrapped around arterioles. If there is a fall in blood pressure, for example when getting out of bed in the morning, this is sensed by these cells and they respond by increasing the amount of renin put out into the bloodstream. Renin is an enzyme that converts the protein angiotensinogen to angiotensin I which is converted to angiotensin II. This in turn stimulates more aldosterone to be produced by the adrenal cortex; the aldosterone acts on the kidneys to retain more salt, and the salt is followed by water; both the salt and the water are reabsorbed into the blood and the resulting increase in the volume of the blood helps to restore the blood pressure to normal. Abnormally high production of aldosterone (hyperaldosteronism) causes excessive retention of salt and water in the body. This results in oedema and high blood pressure. If insufficient aldosterone is produced (hypoaldosteronism) there is a loss of water and salt which causes a fall in blood pressure, heart and kidney abnormalities, and general weakness.

Cortisol acts on cells in many tissues in the body and influences general metabolism, blood pressure, and appetite. The amount of cortisol produced is controlled by another hormone, adrenocorticotrophic hormone (ACTH), secreted by the pituitary gland. This secretion in turn is controlled by corticotrophin-releasing hormone (CRH) from the hypothalamus. CRH secretion responds to signals from elsewhere in the brain, but both CRH and ACTH secretion are also influenced by the amount of cortisol in the blood. A major stimulus to this whole sequence of hormone secretions is stress. The biggest increase in the amount of cortisol produced by the adrenal glands is seen during surgery, although modern anaesthetics minimize the increase. Anxiety such as waiting for the beginning of a race or examinations also causes an increase in cortisol production. Cortisol is therefore a key component of the ‘fight or flight’ reaction of the individual in moments of crisis. The condition of cortisol excess is known as Cushing's syndrome after Harvey Cushing, the American neurosurgeon who, in 1932, described a condition associated with obesity and stretch marks (striae) around the abdomen, a round rosy face, hypertension, muscle weakness, diabetes, and increased hair growth on the face and body. These changes are attributable mainly to the action of cortisol on fat and protein in the body, although the growth of hair is due to an excess of the weak androgenic steroids also produced by the adrenal cortex. The features of this condition are associated with the presence of high levels of cortisol in the blood over a long period; it can be due either to overstimulation of the adrenal cortex by an excessive secretion of ACTH from a tumour of the anterior pituitary (the context in which Cushing encountered it), or to an abnormal growth of cortisol-secreting tissue in the adrenals themselves. Prolonged medication with corticosteroids can also mimic the syndrome.

Abnormally low levels of cortisol (hypocortisolism), result in a general feeling of being unwell, with tiredness, vomiting, nausea, and weight loss. A person in this condition is unable to cope with stress and liable to collapse with relatively minor injury or insult. Because there is insufficient cortisol in the blood to inhibit the secretion of ACTH, this hormone is produced in very high amounts and causes the skin to become dark or ‘bronzed’.

There can be loss of secretion of both cortisol and aldosterone if there is destruction of the adrenal glands by tumour or infection. This condition is known as Addison's disease, following its elegant description by Thomas Addison in 1855.

The adrenal medulla

makes up about 10% of the substance of the adrenal glands and is essentially and developmentally a part of the sympathetic division of the autonomic nervous system. It consists of ‘chromaffin cells’ (so named because of their affinity for chromium) and their main product is adrenaline (also known as epinephrine), which is involved in the fight or flight reaction along with cortisol. More adrenaline is produced in times of stress, by the stimulating action of sympathetic nerves directly upon the chromaffin cells. Adrenaline was the first hormone to be discovered, in 1894 — an event which encouraged the search for similar chemical mediators in the body, and led to the creation of the specialty of endocrinology. Unlike cortisol, which is produced exclusively in the adrenal cortex, adrenaline is produced in other parts of the body, including the brain, as well as in the adrenal medulla. Like cortisol, adrenaline has widespread actions at many sites in the body, including the heart, lungs, and blood vessels, facilitating an increase in the supply of nutrients and oxygen. It also redeploys necessary fuels very rapidly, in readiness for immediate action if required: acting for example in the liver to enhance the release of glucose into the blood. However, because adrenaline is produced in other areas of the body, removing the medulla does not seem to be a critical threat to life, though there does seem to be benefit in having adrenaline produced from the medulla at times of acute stress. Noradrenaline (norepinephrine), better known and most important as a neurotransmitter at sympathetic nerve endings, is also secreted by the medulla, along with adrenaline, but in much smaller amounts.

None of the adrenal hormones are released at a constant rate, but in amounts which change in response to various stimuli throughout the day. In addition, in the case of cortisol and to a certain extent aldosterone, there is a gradual change of background levels in the blood over each 24-hour period. This pattern of release is called a circadian rhythm, and is linked to the sleep-wake cycle of the individual — the ‘body clock’. In the normal individual the greatest amounts of cortisol are released at about 8 o'clock in the morning; the level in the blood gradually falls during the day so that the lowest levels are found at about midnight. ACTH also shows a circadian rhythm reaching maximum levels in the blood just before those of cortisol. The circadian rhythm of aldosterone is of much smaller amplitude than that of cortisol. Changing the times a person is asleep or awake will change the pattern of secretion; if shift workers sleep during the day and are awake at night then the circadian rhythm will be displaced by about 12 hours, with the highest blood levels of cortisol occurring in the early evening and the lowest levels about mid-day. Similar changes occur when travelling across time zones. The shift in the circadian rhythm occurs gradually over a period of several days

Corticosteroid therapy

Treatment of a variety of conditions by synthetic corticosteroids became common from the latter half of the twentieth century. They have been invaluable in suppressing adverse reactions to curative drugs, such as in the treatment of tuberculosis and other life-threatening illnesses; also in controlling inflammatory and allergic conditions, notably rheumatoid arthritis, asthma, and some skin diseases. It follows, however, from the normal control of cortisol secretion, that when the level of corticosteroids in the blood is deliberately raised by medication, the secretion of ACTH from the pituitary is suppressed. This becomes a problem if treatment is suddenly withdrawn, leaving the person liable to collapse under stress because there is no ACTH to stimulate the adrenal glands to produce their own cortisol.

— M. Wheeler

See endocrine. See also autonomic nervous system; body clock; body fluids; steroids.

Endocrine gland located at the end of each kidney. The inner part of the adrenal gland, the medulla, secretes epinephrine (adrenaline) and norepinephrine (noradrenaline); its activity is controlled by the sympathetic nervous system. Regular endurance training increases the secretory capacity of the adrenal medulla causing ‘sports adrenal medulla’, characterized by an exaggerated response to various stimuli (e.g. hypoglycaemia, glucagon, and caffeine), and, possibly, an increase in the size of the adrenal medulla. The outer part of each adrenal gland (cortex) secretes adrenocorticoid hormones; its activity is controlled by adrenocorticotrophic hormone (see also adrenal shock).

Two small glands, one located near the upper part of each kidney, that function in the endocrine system. Part of each adrenal gland secretes adrenaline; another part secretes other important hormones.

One of the pair of endocrine organs located near the cranial pole of the kidneys. Each is composed of two parts, an outer cortex and an inner medulla, that are anatomically, embryologically and functionally distinct. See also adrenal cortex, adrenal medulla.

• a. g. fetal cortex — the first adrenal cortex in the fetus; it is subsequently surrounded by a permanent cortex and has disappeared by the time of birth; the function is unknown.
• a. g. hormones — includes epinephrine, norepinephrine from the adrenal medulla and cortisol, corticosterone, cortisone, 11-dehydroxycortisone, desoxycorticosterone, 17-hydroxy-11-desoxycorticosterone, aldosterone, the adrenal corticoids from the adrenal cortex.
• a. g. medulla — see adrenal medulla.
• a. g. tumors — includes myelolipoma and cortical adenomas and carcinomas. Cause local tissue compression and the adenomas and carcinomas can cause hypersecretion of cortisol. Tumors specific to the medulla include neuroblastoma and ganglioneuroma, both of which may cause local tissue compression.

• Endocrine System and Glands - suprarenal gland: adrenal gland

Adrenal gland
Endocrine system
Adrenal gland
Latin	glandula suprarenalis
Gray's	subject #277 1278
System	endocrine
Artery	superior suprarenal artery, middle suprarenal artery, Inferior suprarenal artery
Nerve	celiac plexus, renal plexus
Lymph	lumbar glands
Precursor	mesoderm, neural crest
MeSH	Adrenal+Glands
Dorlands/Elsevier	Adrenal gland

In mammals, the adrenal glands (also known as suprarenal glands) are endocrine glands that sit atop the kidneys; in humans, the right suprarenal gland is triangular shaped, while the left suprarenal gland is semilunar shaped. They are chiefly responsible for releasing hormones in response to stress through the synthesis of corticosteroids such as cortisol and catecholamines such as epinephrine. The adrenal glands affect kidney function through the secretion of aldosterone, a hormone involved in regulating the osmolarity of blood plasma.

Contents 1 Anatomy and Physiology 1.1 Cortex 1.2 Medulla 1.3 Blood supply 2 Terminology 3 References 4 External links

Anatomy and Physiology

Anatomically, the adrenal glands are located in the retroperitoneum superior to the kidneys, bilaterally. They are surrounded by an adipose capsule and renal fascia. In humans, the adrenal glands are found at the level of the 12th thoracic vertebra. Each adrenal gland has two distinct structures, the outer adrenal cortex and the inner medulla, both of which produce hormones. The cortex mainly produces cortisol, aldosterone and androgens, while the medulla chiefly produces epinephrine and norepinephrine. The combined weight of the adrenal glands in an adult human ranges from 7 to 10 grams.^[1]

A CT scan in which the Adrenals are shown as the triangular-shaped organs on top of the kidneys

Cortex

The adrenal cortex is devoted to the synthesis of corticosteroid and androgen hormones. Specific cortical cells produce particular hormones including aldosterone, cortisol, and androgens such as androstenedione. Under normal unstressed conditions, the human adrenal glands produce the equivalent of 35–40 mg of cortisone acetate per day.^[2] In contrast to the direct innervation of the medulla, the cortex is regulated by neuroendocrine hormones secreted from the pituitary gland which are under the control of the hypothalamus, as well as by the renin-angiotensin system.

The adrenal cortex comprises three zones, or layers. This anatomic zonation can be appreciated at the microscopic level, where each zone can be recognized and distinguished from one another based on structural and anatomic characteristics.^[3] The adrenal cortex exhibits functional zonation as well: by virtue of the characteristic enzymes present in each zone, the zones produce and secrete distinct hormones.^[3]

Zona glomerulosa (outer)

The outermost layer, the zona glomerulosa is the main site for production of mineralocorticoids, mainly aldosterone, which is largely responsible for the long-term regulation of blood pressure. Aldosterone's effects are on the distal convoluted tubule and collecting duct of the kidney where it causes increased reabsorption of sodium and increased excretion of both potassium (by principal cells) and hydrogen ions (by intercalated cells of the collecting duct). Sodium retention is also a response of the salivary ducts, distal colon, and sweat glands to aldosterone receptor stimulation. The major stimulus to produce aldosterone is angiotensin II while ACTH from the pituitary only produces a transient effect. Angiotensin is stimulated by the juxtaglomerular cells when renal blood pressure drops below 90 mmHg.^[4]

Zona fasciculata

Situated between the glomerulosa and reticularis, the zona fasciculata is responsible for producing glucocorticoids, such as 11-deoxycorticosterone, corticosterone, and cortisol in humans. Cortisol is the main glucocorticoid under normal conditions and its actions include mobilization of fats, proteins, and carbohydrates, but it does not increase under starvation conditions.^[4] Additionally, cortisol enhances the activity of other hormones including glucagon and catecholamines. The zona fasciculata secretes a basal level of cortisol but can also produce bursts of the hormone in response to adrenocorticotropic hormone (ACTH) from the anterior pituitary.

Zona reticularis

The inner most cortical layer, the zona reticularis produces androgens, mainly dehydroepiandrosterone (DHEA) DHEA sulfate (DHEA-S), and androstenedione (the precursor to testosterone) in humans.^[4]

Medulla

The adrenal medulla is the core of the adrenal gland, and is surrounded by the adrenal cortex. It secretes approximately 20% norepinephrine and 80% epinephrine.^[4] The chromaffin cells of the medulla, named for their characteristic brown staining with chromic acid salts, are the body's main source of the circulating catecholamines adrenaline (epinephrine) and noradrenaline (norepinephrine). Catecholamines are derived from the amino acid tyrosine and these water-soluble hormones are the major hormones underlying the fight-or-flight response.

To carry out its part of this response, the adrenal medulla receives input from the sympathetic nervous system through preganglionic fibers originating in the thoracic spinal cord from T5–T11.^[5] Because it is innervated by preganglionic nerve fibers, the adrenal medulla can be considered as a specialized sympathetic ganglion.^[5] Unlike other sympathetic ganglia, however, the adrenal medulla lacks distinct synapses and releases its secretions directly into the blood.

Cortisol also promotes epinephrine synthesis in the medulla. Produced in the cortex, cortisol reaches the adrenal medulla and at high levels, the hormone can promote the upregulation of phenylethanolamine N-methyltransferase (PNMT), thereby increasing epinephrine synthesis and secretion.^[3]

Adrenal anatomy and physiology

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 suprarenal artery is provided by the inferior phrenic artery
• The middle suprarenal artery is provided by the abdominal aorta
• The inferior suprarenal artery is provided by the renal artery

Venous drainage of the adrenal glands is achieved via the suprarenal veins:

• The right suprarenal vein drains into the inferior vena cava
• The left suprarenal vein drains into the left renal vein or the left inferior phrenic vein.

The suprarenal veins may form anastomoses with the inferior phrenic veins. Since the right supra-renal vein is short and drains directly into the inferior vena cava it is likely to injure the latter during removal of right adrenal for various reasons.

The adrenal glands and the thyroid gland are the organs that have the greatest blood supply per gram of tissue. Up to 60 arterioles may enter each adrenal gland.^[6] This may be one of the reasons lung cancer commonly metastasizes to the adrenals.

Terminology

The adrenal glands are named for their location relative to the kidneys. The term "adrenal" comes from ad- (Latin, "near") and renes (Latin, "kidney"). Similarly, "suprarenal" is derived from supra- (Latin, "above") and renes.

References

1. ^ Page 18 in: Boué A, Nicolas A, Montagnon B (June 1971). "Reinfection with rubella in pregnant women". Lancet 297 (7712): 1251–3. doi:10.1016/S0140-6736(71)91775-2. PMID 4104713. 
2. ^ Jefferies, William McK (2004). Safe uses of cortisol. Springfield, Ill: Charles C. Thomas. ISBN 0-398-07500-X. 
3. ^ ^{a b c} Whitehead, Saffron A.; Nussey, Stephen (2001). Endocrinology: an integrated approach. Oxford: BIOS. pp. 122. ISBN 1-85996-252-1. 
4. ^ ^{a b c d} Dunn R. B.; Kudrath W., Passo S.S., Wilson L.B. (2011). "10" (in English). Kaplan USMLE Step 1 Physiology Lecture Notes. p. 263-289. 
5. ^ ^{a b} Sapru, Hreday N.; Siegel, Allan (2007). Essential Neuroscience. Hagerstown, MD: Lippincott Williams & Wilkins. ISBN 0-7817-9121-9. 
6. ^ Mirilas P, Skandalakis JE, Colborn GL, Weidman TA, Foster RS, Kingsnorth A, Skandalakis LJ, Skandalakis PN (2004). Surgical Anatomy: The Embryologic And Anatomic Basis Of Modern Surgery. McGraw-Hill Professional Publishing. ISBN 960-399-074-4. 

External links

• MedlinePlus Encyclopedia 002219
• Virtual Slidebox at Univ. Iowa Slide 272
• Anatomy Atlases - Microscopic Anatomy, plate 15.292 - "Adrenal Gland"
• Histology at BU 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

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