The Adrenal Glands


The adrenals are two small glands located atop the kidneys. Each adrenal gland has two parts that act as separate glands. The inner area is called the medulla, and the outer portion is called the cortex (Fig. 8-7).
Hormones From the Adrenal Medulla The hormones of the adrenal medulla are released in response to stimulation by the sympathetic nervous system. The principal hormone produced by the medulla is epinephrine, also called adrenaline. Another hormone released from the adrenal medulla, norepinephrine (noradrenalin), is closely related chemically and is similar in its actions to epinephrine. These two hormones are referred to as the fight-or-flight hormones because of their effects during emergency situations. We have already learned about these hormones in studying the autonomic nervous system. When released from nerve endings instead of being released directly into the bloodstream, they function as neurotransmitters. Some of their effects are as follows:
* Stimulation of the involuntary muscle in the walls of the arterioles, causing these muscles to contract and blood pressure to rise accordingly;
* Conversion of glycogen stored in the liver into glucose. The glucose pours into the blood and travels throughout the body, allowing the voluntary muscles and other tissues to do an extraordinary amount of work;
* Increase in the heart rate;
* Increase in the metabolic rate of body cells;
* Dilation of the bronchioles, through relaxation of the smooth muscle of their walls.
Figure 8-7 The adrenal gland. The medulla secretes epinephrine and norepinephrine. The cortex secretes steroid hormones.
adrenal gland
Hormones From the Adrenal Cortex There are three main groups of hormones secreted by the adrenal cortex:
* Glucocorticoids maintain the carbohydrate reserve of the body by stimulating the liver to convert amino acids into glucose (sugar) instead of protein. The production of these hormones increases in times of stress to aid the body in responding to unfavorable conditions.
They raise the level of nutrients in the blood, not only glucose, but also amino acids from tissue proteins and fatty acids from fats stored in adipose tissue. Glucocorticoids also have the ability to suppress the inflammatory response and are often administered as medication for this purpose. The major hormone of this group is cortisol, which is also called hydrocortisone.
* Mineralocorticoids are important in the regulation of electrolyte balance. They control sodium reabsorption and potassium secretion by the kidney tubules. The major hormone of this group is aldosterone.
* Sex hormones are secreted in small amounts, having little effect on the body.
Disorders of the Adrenal Cortex Hypofunction of the adrenal cortex gives rise to a condition known as Addison disease, a disease characterized chiefly by muscle atrophy (loss of tissue), weakness, skin pigmentation, and disturbances in salt and water balance. Hypersecretion of cortisol results in a condition known as Cushing syndrome, the symptoms of which include obesity with a round (“moon”) face, thin skin that bruises easily, muscle weakness, bone loss, and elevated blood sugar. Use of steroid drugs also may produce these symptoms. If aldosterone is secreted in excess, as a result of hyperfunction of the adrenal cortex, the condition is termed aldosteronism. Adrenal gland tumors give rise to a wide range of symptoms resulting from an excess or a deficiency of the hormones secreted.
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