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adrenal glands

adrenal glands 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.

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COLIN BLAKEMORE and SHELIA JENNETT. "adrenal glands." The Oxford Companion to the Body. 2001. Encyclopedia.com. 27 Jun. 2016 <http://www.encyclopedia.com>.

COLIN BLAKEMORE and SHELIA JENNETT. "adrenal glands." The Oxford Companion to the Body. 2001. Encyclopedia.com. (June 27, 2016). http://www.encyclopedia.com/doc/1O128-adrenalglands.html

COLIN BLAKEMORE and SHELIA JENNETT. "adrenal glands." The Oxford Companion to the Body. 2001. Retrieved June 27, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O128-adrenalglands.html

Adrenal Gland

Adrenal Gland

The adrenal glands are located on the upper pole of each kidney. In fact, their name designates their location: the prefix ad means "adjacent," and renal refers to the kidney. In the human body, they are small yellowish glands that weigh about five grams (0.175 ounces) each.

The adrenal gland is actually two organs in one. The outer portion, called the adrenal cortex (cortex means "bark," as in the bark of a tree), is about nine-tenths of the gland's total weight. The inner part, called the adrenal medulla (medulla means "marrow," as found in the inside of a bone), is about one-tenth. They are both endocrine glands, meaning that they secrete chemical messengers called hormones into the bloodstream. However, the adrenal cortex and medulla are different in their embryological development, their tissue structure, the types of hormones they secrete, and the way they are regulated. So why is one located inside the other?

Adrenal Cortex

The adrenal cortex develops from the mesoderm (middle layer) of the embryo. The tissue destined to become the adrenal cortex aggregates near the developing kidney and becomes organized into three zones. The outer zone is called the zona glomerulosa (meaning that the cells are arranged in little balls called glomeruli), the middle zone is the zona fasiculata (the cells are in parallel fascicles or bundles), and the zona reticularis (reticular means network) is innermost.

The hormones secreted from each zone all resemble the molecule cholesterol and are called steroids , but each zone secretes slightly different hormones. The zona glomerulosa secretes hormones that influence the kidneys to excrete or retain sodium and potassium, depending on the needs of the body. These hormones are called mineralocorticoids (sodium and potassium are minerals ). The zona fasiculata secretes hormones called glucocorticoids that influence the metabolism of carbohydrates , including glucose . The glucocorticoids include hydrocortisone, corticosterone, and cortisone.

In addition to regulating metabolism, these steroids provide resistance to stress and suppress the inflammatory response and some allergic reactions. Steroids such as these are often rubbed onto inflamed and itchy skin to make it feel better. The zona reticularis secretes steroids that resemble the sex hormones secreted by the ovary in the female and testes in the male.

The adrenal cortex is regulated by the pituitary gland in the head. The pituitary gland secretes a hormone called adrenocorticotropic hormone (ACTH). Tropic (pronounced with a long o) is from a Greek word meaning "nourishment," so ACTH simply refers to this hormone's ability to produce a change in the adrenal cortex. ACTH is necessary for cell growth and maintenance and stimulates glucocorticoid synthesis.

Adrenal Medulla

The adrenal medulla forms from ectoderm (outer layer) very near the embryonic spinal cord. From its beginnings, the adrenal medulla is part of the nervous system. These cells migrate into the middle of the developing adrenal cortex and form into a solid ball. The cells of the adrenal medulla secrete a class of hormones called catecholamines, adrenaline (or epinephrine) being the best known. Norepinephrine is also secreted.

In times of acute stress, the brain and spinal cord send a signal to the adrenal medulla, and it secretes adrenaline into the bloodstream. This causes the heart to beat faster, opens up the airways, and gets the body ready for physical activity. This "fight or flight" reaction is a survival mechanism, allowing people (and other animals) to escape from a dangerous situation. A person experiences the effects of the adrenal medulla when he or she gets scared or excited.

Why is the adrenal medulla inside the cortex? Steroids in the adrenal cortex activate the enzyme that puts the final atoms onto adrenaline. Therefore, the adrenal cortex helps the adrenal medulla to synthesize adrenaline, allowing the medulla to do its job.

see also Anabolic Steroids; Endocrine System; Homeostasis; Hormones; Pituitary Gland; Stress Response

Stephen W. Carmichael

Bibliography

Carmichael, Stephen W., and Hans Winkler. "The Adrenal Chromaffin Cell." Scientific American 253 (August 1985): 4049.

Ross, Michael H., Lynn J. Rommerell, and Gordon I. Kaye. Histology: A Text and Atlas, 3rd ed. Baltimore: Williams & Wilkins, 1995.

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adrenal gland

adrenal gland (ədrēn´əl) or suprarenal gland (sōōprərēn´əl), endocrine gland (see endocrine system) about 2 in. (5.1 cm) long situated atop each kidney. The outer yellowish layer (cortex) of the adrenal gland secretes about 30 steroid hormones, the most important of which are aldosterone and cortisol. Cortisol regulates carbohydrate, protein, and fat metabolism, and its secretion is controlled by the output of adrenocorticotropic hormone (ACTH) from the pituitary gland. Aldosterone regulates water and salt balance in the body; its secretion is only slightly influenced by the pituitary. Steroid hormones also counteract inflammation and allergies and influence the secondary sex characteristics to a limited degree. The adrenal cortex controls metabolic processes that are essential to life and if it ceases to function death ensues within a few days. Artificial synthesis of the steroid hormones has made it possible to treat many conditions related to underactivity of the adrenal cortex, e.g., Addison's disease. The inner reddish portion (medulla) of the adrenal gland, which is not functionally related to the adrenal cortex, secretes epinephrine (adrenaline) and norepinephrine. The release of these hormones is stimulated when an animal is excited or frightened, causing increased heart rate, increased blood flow to the muscles, elevated blood sugar, dilation of the pupils of the eyes, and other changes that increase the body's ability to meet sudden emergencies.

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adrenal gland

adrenal gland In vertebrates, an organ that secretes certain hormones. Many vertebrates possess multiple adrenal glands, but in mammals there is one gland close to each kidney. In tetrapods, each gland consists of a central medulla and an outer cortex. The medulla secretes adrenalin and noradrenalin, hormones needed when the animal is in an excited state and must engage in strenuous activity (e.g. fighting or fleeing). The cortex secretes sex hormones and other hormones concerned with regulating the water and salt balances of the body.

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MICHAEL ALLABY. "adrenal gland." A Dictionary of Zoology. 1999. Encyclopedia.com. 27 Jun. 2016 <http://www.encyclopedia.com>.

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adrenal glands

adrenal glands A pair of endocrine glands situated immediately above the kidneys (hence they are also known as the suprarenal glands). The inner portion of the adrenals, the medulla, contains chromaffin tissue and secretes the hormones adrenaline and noradrenaline; the outer cortex secretes small amounts of sex hormones (androgens and oestrogens) and various corticosteroids, which have a wide range of effects on the body. See also ACTH.

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adrenal glands

adrenal glands (suprarenal glands) (ă-dree-năl) pl. n. two triangular endocrine glands, each of which covers the superior surface of a kidney. The medulla forms the grey core of the gland and produces adrenaline and noradrenaline. The cortex is a yellowish tissue surrounding the medulla; it produces corticosteroid hormones.

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"adrenal glands." A Dictionary of Nursing. 2008. Encyclopedia.com. 27 Jun. 2016 <http://www.encyclopedia.com>.

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adrenal gland

adrenal gland One of a pair of small endocrine glands situated on top of the kidneys. They produce many steroids that regulate the blood's salt and water balance and are concerned with the metabolism of carbohydrates, proteins, and fats, and the secretion of the hormones adrenaline and noradrenaline. See also endocrine system

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