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Adrenal Glands

Definition

Adrenal glands produce and release several different hormones that maintain internal fluid levels, maintain sodium and potassium levels, and mediate the stress response.

Description

The human body contains two adrenal glands, one on top of each kidney. The gland is triangular in shape and yellowish in appearance. Subdivisions include the adrenal cortex and the medulla. The cortex is the outer layer and is responsible for the production of steroid hormones called glucocorticoids, mineralcorticoids, and androgens. The medulla forms the inner layer of the adrenal glands and releases norepinephrine and epinephrine (known commonly as noradrenaline and adrenaline) directly into the bloodstream. The adrenal cortex and medulla serve very different functions in the human body because of the different hormones that they produce and release.

Function

The adrenal cortex converts cholesterol into glucocorticoids, mineralcorticoids, and androgens. The glucocorticoids are so named due to their role in regulating body glucose levels. The major glucocorticoid produced by the adrenal is cortisol. Cortisol is also known as the stress hormone because the body releases cortisol in order to help cope with stressful situations. The immediate effects of cortisol are to increase levels of fatty acids, proteins, and glucose in the blood. Cortisol causes these increases by taking protein from muscles, fatty acids from fatty tissues, increasing gluconeogenesis (the process of making glucose), and decreasing the body's uses of glucose. Thus, in Cushing's syndrome (hypercortisolism), patients may experience muscle wasting (too much protein is taken from the muscles). Paradoxically, although cortisol increases the levels of free fatty acids, patients with too much cortisol over a long period of time experience an increase in fat on the upper body.

Cortisol and other glucocorticoids are also potent inhibitors of the immune system. Thus, it is not too surprising that synthetic glucocorticoids, like prednisone, are used to treat autoimmune diseases and allergies—diseases in which the immune system is too active. The release of cortisol is under the control of the anterior pituitary. The anterior pituitary accomplishes this by releasing the peptide hormone, adrenocorticotropin (ACTH) onto the adrenal gland.

The mineralcorticoids are so named due to their role in establishing mineral and water levels in the body. The major mineralcorticoid produced by the adrenal gland is aldosterone (although the glucocorticoid cortisol has some mineralcorticoid activity). The primary role of aldosterone is to regulate the balance of potassium, sodium, and water by affecting the absorption of all three in the kidney. Briefly, aldosterone causes the kidney to reabsorb more water and sodium, while causing potassium to be excreted in exchange for the sodium. When too much aldosterone is produced by the adrenal glands, as in primary aldosteronism, the level of potassium in the blood is low (hypokalemia) and the retention of sodium and water is high. Patients can experience hypertension and muscle weakness.

When there is too little aldosterone, potassium levels are high and there are extreme water and sodium losses. The loss of water and increased potassium levels can lead to extremely low blood pressure and death. A disease that might result in a loss of mineralcorticoid function is Addison's disease. In Addison's disease, the adrenals are usually at least 90% decimated before symptoms arise. The peptide, angiotensin II, and potassium levels are the main control for the release of aldosterone from the adrenals.

The adrenal glands also secrete small amounts of androgens such as testosterone. However, in the adult, that amount of androgens produced from a normally functioning adrenal gland is so small that it is unlikely to have a major effect. Nevertheless, pathology of the adrenals may result in abnormally high levels of androgens being secreted. The androgens may cause masculinization in males or females.

The adrenal medulla is really an extension of the sympathetic division of the autonomic nervous system. The sympathetic nervous system mediates "fight or flight" responses to environmental stimuli. Sympathetic nerves that originate in the spinal cord release the neurotransmitter, acetylcholine, onto the adrenal glands. The adrenal glands respond by releasing dopamine, norepinephrine, and epinephrine directly into the bloodstream. Norepinephrine and epinephrine are commonly referred to as noradrenaline and adrenaline, respectively. Epinephrine makes up the majority of the substance secreted by the adrenal medulla. Circulating norepinephrine and epinephrine can increase heart rate, constrict blood vessels, dilate eye pupils, inhibit motility and digestion in the stomach and intestines, increase sweating, increase metabolism, and increase breathing. The adrenal medulla is stimulated to release norepinephrine and epinephrine under stressful situations such as exercise or emotional distress.

Role in human health

Thus, the adrenal glands play a significant role in mental and physical health. Not only are the adrenal glands vital for maintaining a proper balance of sodium and potassium, they mediate the body's response to stress, both short-term and long-term. In response to immediate stress, the adrenal glands produce epinephrine, norepinephrine, and the hormone cortisol. The body is prepared for flight or fight, and energy is made available for instant use. In the long term, the adrenal glands may have a negative impact on human health. Prolonged stress can produce undesirable changes in the body that range from immune system depression to fertility complications.

Diseases and disorders

Addison's disease

Addison's disease arises from a hypoactive adrenal cortex. The adrenal cortex fails to produce adequate amounts of the glucocorticoid cortisol, and sometimes the mineralcorticoid aldosterone. A lack of aldosterone causes the kidneys to excrete excess sodium and water while retaining potassium. This can lead to hyperkalemia (high levels of potassium), hypovolemia (low blood volume), and hypotension (low blood pressure). Hyperkalemia may cause fatal heart arrhythmias, and (severe) hypovolemia can lead to shock and kidney failure. Common symptoms include frequent urination, dehydration, fatigue, dizziness, skin discoloration, nausea, vomiting, weakness, and cold intolerance. Treatment includes oral or intravenous glucocorticoids such as prednisone, and, if necessary, administration of the oral mineralcorticoid fludrocortisone acetate to replace aldosterone. The cause of Addison's disease is not known, but in 80% of the cases there is a wasting or atrophy of the adrenal cortex.

Primary aldosteronism

Conn's syndrome is also known as primary aldosteronism. In this disease, too much of the mineralcorticoid, aldosterone, is made by the adrenal glands. The increased levels of aldosterone cause excessive potassium excretion while promoting excessive sodium and water retention. This leads to hypertension (high blood pressure) and hypokalemia (low serum potassium). Hypokalemia is an important diagnostic clue in the process of differentiating primary aldosteronism from other similar disorders. In some cases, Conn's syndrome is due to an adrenal tumor, in which case it may be surgically removed. In other cases, the diuretic drugs, spironalactone or amiloride, are given to block the effects of aldosterone.

Cushing's syndrome

The adrenal cortex is overactive in Cushing's syndrome. The adrenal cortex overproduces glucocorticoids, which can lead to high blood sugar levels and high blood pressure. Symptoms include obesity, muscle wasting, fatigue, irritability, excessive hair growth in women, irregular menstrual cycles, and decreased male fertility. A tumor of the adrenal gland, or an overproduction of ACTH by the pituitary, may cause Cushing's syndrome. Treatment may include chemotherapy or hormone-inhibiting medications.

Pheochromocytoma

Pheochromocytomas are tumors of the adrenal glands that secrete large quantities of norepinephrine and epinephrine. The most common symptom is extremely high blood pressure. Treatment is usually surgical removal of the tumor.

KEY TERMS

Adrenal cortex—Outer layer of the adrenal gland that produces steroid hormones.

Adrenal medulla—Inner layer of the adrenal gland that releases adrenaline.

Androgens—Male sex hormones.

Hyperkalemia—An abnormally high level of potassium in the blood.

Hypokalemia—An abnormally low level of potassium in the blood.

Hypovolemia—An abnormally low volume of blood.

Glucocorticoid—A steroid hormone, like cortisol, that affects fat, carbohydrate, and protein levels in the blood as well as regulating the immune response.

Mineralcorticoid—A steroid hormone, like aldosterone, that regulates the excretion of salt, potassium, and water.

Resources

BOOKS

Guyton A.C. and J.E. Hall. Medical Physiology, 10th edition, Philadelphia, PA: W.B. Saunders Company, 2000.

Guyton A.C. Medical Physiology. 8th edition. Philadelphia, PA: W.B. Saunders Company, 1991.

PERIODICALS

Hostetter T.H., Rosenber M.E., Ibrahim H.N., and Juknevicius I. "Aldosterone in renal disease." Current Opinion in Nephrology Hypertension (10 January 2001): 105-110.

ORGANIZATIONS

National Adrenal Diseases Foundation. 505 Northern Blvd, Great Neck, NY 11021. (516) 487-4992. 〈http://medhlp.netusa.net/nadf/〉.