There are two adrenal glands; one is located on top of each kidney. Each adrenal gland consists of two portions that are distinct endocrine glands: the deep adrenal medulla and the superficial adrenal cortex.
Hormones of The Adrenal Glands
|Epinephrine||Sympathetic division of||Prepare body to meet emergencies;||Hypersecretion causes prolonged|
|and||the autonomic nervous||increase heart rate, cardiac output, blood||responses.|
|norepinephrine||system||pressure, and metabolic rate; increase blood sugar by converting glycogen to glucose; dilate respiratory passages||Hyposecretion causes no major disorders.|
|Aldosterone||Blood electrolyte levels, angiotensin II||Increases blood levels of sodium and water, which decreases blood levels of potassium; increases blood pressure||Hypersecretion inhibits neural and muscular activity, and also causes edema.|
|Cortisol||ACTH from anterior lobe of the pituitary gland||Promotes formation of glucose from noncarbohydrate nutrients; provides resistance to stress and inhibits inflammation||Hyposecretion causes Addison disease.
Hypersecretion causes Cushing syndrome.
|Androgens||ACTH from anterior lobe of the pituitary gland||Effects are insignificant in normal adult males; contribute to the sex drive in females.||Hypersecretion as a result of tumors; causes masculinization in females.|
The adrenal medulla secretes epinephrine (adrenaline) and norepinephrine (noradrenaline), two closely related hormones that have very similar actions on target cells. Epinephrine forms about 80% of the secretions.
The sympathetic division of the autonomic nervous system regulates the secretion of adrenal medullary hormones. They are secreted whenever the body is under stress, and they duplicate the action of the sympathetic division on a bodywide scale. The medullary hormones have a stronger and longer-lasting effect in preparing the body for “fight or flight.” The effects of epinephrine and norepinephrine include (1) a decrease in blood flow to the viscera and skin; (2) an increase in blood flow to the skeletal muscles, lungs, and nervous system; (3) conversion of glycogen to glucose to raise the glucose level in the blood; and (4) an increase in the rate of cellular respiration. Epinephrine and norepinephrine are particularly important in short-term stress situations. In times of chronic stress the adrenal cortex makes further adjustment.
|Parathyroid hormone (PTH)||Blood Ca2+ level||Increases blood Ca2+ level by promoting Ca2+ removal from bones and Ca2+ reabsorption by kidneys||Hyposecretion causes tetany, which may result in death. Hypersecretion causes weak, deformed bones that may fracture spontaneously.|
Hormones of The Adrenal Cortex
Several different steroid hormones are produced by the adrenal cortex, but the most important ones are aldosterone, cortisol, and the sex hormones.
Aldosterone (al-do-ster’-on) is the most important mineralocorticoid secreted by the adrenal cortex. Mineralocorticoids regulate the concentration of electrolytes (mineral ions) in body fluids. Aldosterone stimulates the kidneys to retain sodium ions (Na+) and to excrete potassium ions (K+). This action not only maintains the normal balance of Na+ and K+ in body fluids but also maintains blood volume and blood pressure. The reabsorption of Na+ into the blood causes anions, such as chloride (Cl–) and bicarbonate (HCO3–), to be reabsorbed due to their opposing charges. And it causes water to be reabsorbed by osmosis, which maintains blood volume and blood pressure. Aldosterone secretion is stimulated by several factors, including (1) a decrease in blood level of Na+, (2) an increase in blood level of K+, or (3) a decrease in blood pressure, which leads to angiotensin II production.
Glucocorticoids are so named because they affect glucose metabolism. There are three major actions of glucocorticoids. (1) In response to chronic stress, glucocorticoids ensure a constant fuel supply by promoting the conversion of noncarbohydrate nutrients into glucose. This is important because carbohydrate sources, such as glycogen, may be exhausted after several hours without food or strenuous exercise. (2) They facilitate the utilization of glucose by cells. (3) They reduce inflammation.
Cortisol is the most important of several glucocorticoids that are secreted by the adrenal cortex under the stimulation of ACTH. The blood levels of glucocorticoids are kept in balance because they exert a negative-feedback control on the secretion of CRH and ACTH.
The adrenal cortex also secretes small amounts of androgens (male sex hormones) and estrogens in response to ACTH from the anterior lobe of the pituitary gland. The estrogens have little significant function. The androgens promote the early development of male reproductive organs, but in adult males their effects are masked by sex hormones produced by testes. In females, adrenal androgens contribute to the female sex drive. In both sexes, excessive production results in exaggerated male characteristics.
Disorders Cushing syndrome results from hypersecretion by the adrenal cortex. It may be caused by an adrenal tumor or by excessive production of ACTH by the anterior lobe of the pituitary gland. This syndrome is characterized by high blood pressure, an abnormally high blood glucose level, protein loss, osteoporosis, fat accumulation on the trunk, fatigue, edema, and decreased immunity. A person with this condition tends to have a full, round face and an enlarged abdomen.
Addison disease results from a severe hyposecre- tion by the adrenal cortex. It is characterized by low blood pressure, low blood glucose and sodium levels, an increase in the blood potassium level, dehydration, muscle weakness, and increased skin pigmentation. Without treatment to control blood electrolytes, death may occur in a few days.