What are adrenal glands?

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A pair of small hormone-producing glands located on the top of each kidney. Each adrenal is made up of a complex outer layer of cells (the cortex), which produces steroid hormones, and an inner layer (the medulla), which produces epinephrine and norepinephrine.
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Structure and Functions

The adrenal glands are perched on top of each kidney; they are often referred to as “caps” or “hats” on the kidneys. The name in Latin describes this position well: “adrenal” means “toward the kidney” and “suprarenal” means “on top of the kidney.” These ductless glands are triangular or pyramidal in shape and measure roughly 1 to 1.5 centimeters in height and 7 to 8 centimeters in length. Each gland consists of two parts: a medulla (center portion) and the surrounding cortex. Together they are responsible for releasing three different classes of hormones that are critical for normal survival. These hormones control such vital functions as daily metabolism (such as managing blood sugar levels), regulating salt and water balance, regulating inflammation, controlling basic stress responses (the fight-or-flight response), and initiating and controlling the onset of puberty.

The adrenal glands work interactively with both the hypothalamus and the pituitary in the brain. The hypothalamus releases corticotropin-releasing factor (CRF), which stimulates the anterior pituitary to secrete adrenocorticotropic hormone (ACTH). ACTH in turn stimulates the adrenal glands to produce cortisol and other hormones, which then causes the hypothalamus to stop producing CRF.

The cortex (80 to 90 percent of the adrenal gland) is the complex outer layer of cells in the adrenal gland, and it secretes a variety of hormones into the bloodstream. The outermost layer of the cortex is known as the zona glomerulosa; it secretes aldosterone, the primary mineralocorticoid, which regulates salt (sodium) excretion from the body. This in turn causes water to be retained, increasing blood volume and therefore blood pressure. The middle layer is the zona fasciculata, which secretes glucocorticoids such as cortisol and corticosterone. They help control the body’s use of carbohydrates, lipids, and proteins for the production of energy needs in the body. They also suppress inflammatory responses in the body and thus have a significant effect on the function of the immune system. The innermost layer (next to the medulla) is the zona reticularis, which secretes androgens that affect the development and maintenance of male characteristics as well as muscle development in both males and females.

The medulla is the inner portion of the adrenal gland, surrounded completely by the cortical layers. Unlike the cortex, it is not essential for life. However, the hormones it secretes are used each day as the body deals with various stressors. The principal hormones of the adrenal medulla are epinephrine and norepinephrine. Epinephrine (also known as adrenaline) causes an increase in heart rate and in the force of heart contractions. It also facilitates increased blood flow to the muscles and brain and causes the relaxation of smooth muscles in the gastrointestinal tract, the reproductive tract, and surrounding small blood vessels. Thus, its net effect is to prepare the body for the sympathetic nervous system response during times of physical activity or stress of various types. Its partner, norepinephrine, also causes contraction of blood vessels and elevation of heart rate and triggers the release of stored glucose into the bloodstream.

Disorders and Diseases

A variety of adrenal disorders are known, including those caused by lack of individual enzymes in the hormone-producing pathways. The three most common disorders are Cushing’s syndrome, Addison’s disease, and congenital adrenal hyperplasia.

Cushing’s syndrome occurs when excess amounts of cortisol are secreted from the adrenal glands. This may be a by-product of adrenal tumors, may be the result of adrenal hyperplasia, or may be caused by various medications. Treatment can include surgery, irradiation, or drugs, depending on the etiology. Adrenalectomy (adrenal gland removal) may be performed if a tumor is present.

Addison’s disease (also called adrenal insufficiency or hypocortisolism) occurs when at least 90 percent of the adrenal cortex has been destroyed, and it results in the inability of the body to control stress responses. It is characterized by weight loss, muscle weakness, fatigue, low blood pressure, and occasional darkening of the skin, and it can be fatal. Several conditions, including autoimmunity, polyendocrine deficiency syndrome, and tuberculosis, can lead to this condition. As with insulin-dependent diabetes, Addison’s disease requires lifelong hormone replacement therapy.

Congenital adrenal hyperplasia (CAH) is an inborn deficiency in the ability of the adrenal glands to make cortisol. This is usually caused by an enzyme deficiency or malfunction, resulting in some hormone pathways (such as cortisol) being turned off and others (such as androgens) being turned on more than normal. Treatment usually involves cortisol injections.

Perspective and Prospects

The adrenal glands were first described in humans in 1563 by Italian physiologist Bartolomeo Eustachio. The first studies of their function in the human were published in 1855 by Thomas Addison, who also later described their dysfunction and for whom Addison’s disease is named. In 1929 Walter Cannon popularized the concept of the “fight-or-flight” response as the basic function of the adrenal medulla, and in 1936 Hans Selye put forth the idea of the “alarm” reaction for the cortex.

There are a variety of disorders associated with the adrenal glands, and treatment options, from medicine to surgery, vary and depend on the specific adrenal problem being addressed. .

Bibliography

Frisch, Lawrence. "Adrenalectomy—Open Surgery." HealthLibrary, May 18, 2013.

Garnier, Linda. My Battle with Cushing’s Disease. Coral Springs, Fla.: Llumina Press, 2003.

Lam, Michael, and Dorine Lam. Adrenal Fatigue Syndrome. Loma Linda, Calif.: Adrenal Institute Press, 2012.

Loechner, Karen. "Adrenal Insufficiency and Addison's Disease." National Endocrine and Metabolic Diseases Information Service. April 6, 2012.

Hsu, C. Y., and Scott Rivkees. Congenital Adrenal Hyperplasia: A Parents’ Guide. Bloomington, Ind.: AuthorHouse, 2005.

Pozzilli, Paolo, et al. Atlas of Endocrine and Metabolic Disease. Chicester, England: John Wiley and Sons, 2014.

Stewart, P. M. “The Adrenal Cortex.” In Williams Textbook of Endocrinology, edited by Shlomo Melmed. 12th ed. Philadelphia: Elsevier/Saunders, 2012.

Wilson, James L. Adrenal Fatigue: The Twenty-first Century Stress Syndrome. Petaluma, Calif.: Smart Publications, 2001.

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