Worlds of David Darling
Encyclopedia of Science
   
Home > Encyclopedia of Science

adrenal gland





adrenal glands
An endocrine gland, also called the suprarenal gland, one of which is located near the upper portion of each kidney. Each adrenal gland is divided into an outer part called the cortex and an inner part called the medulla. The cortex and medulla of the adrenal gland, like the anterior and posterior lobes of the pituitary gland, develop from different embryonic tissues and secrete different hormones. The adrenal cortex is essential to life, but the medulla may be removed with no life-threatening effects.

The hypothalamus of the brain influences both portions of the adrenal gland but by different mechanisms. The adrenal cortex is regulated by negative feedback involving the hypothalamus and adrenocorticotropic hormone (ACTH); the medulla is regulated by nerve impulses from the hypothalamus.


Anatomy of the adrenal glands

Both adrenal glands are yellowish in color. The right adrenal gland is pyramidal in shape and caps the upper pole of the right kidney. It lies behind the right lobe of the liver and extends medially behind the inferior vena cava. It rests posteriorly on the diaphragm.

The left adrenal gland is crescent-shaped and extends along the medial border of the left kidney from the upper pole to the hilus. It lies behind the pancreas, the lesser sac, and the stomach, and rests posteriorly on the diaphragm.

Three arteries supply each gland: (1) the suprarenal branch of the inferior phrenic artery, (2) the suprarenal branch of the aorta, and (3) the suprarenal branch of the renal artery. A single vein emerges from the hilus of each gland and drains into the inferior vena cava on the right and into the renal vein on the left. Lymph vessels pass to the lateral aortic nodes.

The nerves are predominantly preganglionic sympathetic fibers and are derived from the splanchnic nerves; the majority of the nerves end in the medulla of the gland.


Hormones of the adrenal cortex

The adrenal cortex consists of three different regions, each of which produces a different group or type of hormones. Chemically, all the cortical hormones are steroids.

Mineralocorticoids are secreted by the outermost region of the adrenal cortex. The principal mineralocorticoid is aldosterone, which acts to conserve sodium ions and water in the body. Glucocorticoids are secreted by the middle region of the adrenal cortex. The principal glucocorticoid is cortisol, which increases blood glucose levels.

The third group of steroids secreted by the adrenal cortex is the gonadocorticoids, or sex hormones. These are secreted by the innermost region. Male hormones (androgens) and female hormones (estrogens) are secreted in minimal amounts in both sexes by the adrenal cortex, but their effect is usually masked by the hormones from the testes and ovaries. In females, the masculinization effect of androgen secretion may become evident after menopause, when estrogen levels from the ovaries decrease.


Hormones of the adrenal medulla

The adrenal medulla develops from neural tissue and secretes two hormones, adrenaline (epinephrine) and noradrenaline (norepinephrine). These two hormones are secreted in response to stimulation by sympathetic nerves, particularly during stressful situations. A lack of hormones from the adrenal medulla produces no significant effects. Hypersecretion, usually from a tumor, causes prolonged or continual sympathetic responses.


Disorders of the adrenal glands

Excessive or deficient production of hormones by the adrenal glands can happen for a variety of reasons. These disorders are uncommon but can be serious.


Congenital defects

Congenital adrenal hyperplasia (also called adrenogenital syndrome) affects about one newborn baby in 10,000. The adrenal cortex is unable to synthesize enough cortisol and (sometimes) aldosterone, and the baby is ill or fails to thrive. As a side-effect of the reduced cortisol production, the glands are stimulated to produce excessive amounts of androgens, which can cause masculinization of female babies.


Tumors

Malignant or benign tumors of the adrenal glands are rare and generally lead to excess secretion of hormones. Tumors of the adrenal cortex may secrete aldosterone, causing primary aldosteronism (Conn's syndrome), or cortisol, causing Cushing's syndrome. Tumors of the adrenal medulla may cause excess secretion of adrenaline and noradrenaline. Two types of tumor affect the adrenal medulla: neuroblastoma and phaechromocytoma. These cause intermittent hypertension (high blood pressure) and sweating attacks. Surgical removal of a tumor, or even a whole adrenal gland, often cures the conditions caused by excess hormone secretion.


Impaired blood supply

Loss or obstruction of the blood supply to the adrenal glands, sometimes as a result of arterial disease, is another possible cause of Addison's disease or acute adrenal failure.


Other disorders

In many cases disturbed activity of the adrenals is caused not by disease of the glands themselves, but by an increase or decrease in the blood level of hormones that influence the activity of the glands. Cortisol production by the adrenal cortex is controlled by the secretion of ACTH by the pituitary. A tumor or other disorder of the pituitary, or tumors in the lung, breast, and elsewhere, can cause excess ACTH secretion, leading to too much cortisol being produced by the adrenals and, hence, to Cushing's syndrome. Pituitary disease is, in fact, the most common cause of Cushing's syndrome. Destruction or removal of the pituitary has the opposite effect, stopping ACTH secretion, preventing stimulation of the adrenal cortex, and thus leading to adrenal failure.


Related categories

   • ANATOMY AND PHYSIOLOGY
   • HEALTH AND DISEASE