The two adrenal glands, each of which weighs about 4 grams, lie at the superior poles of the two kidneys. As shown in Figure 77–1, each gland is composed of two distinct parts, the adrenal medulla and the adrenal cortex. The adrenal medulla, the central 20per cent of the gland, is functionally related to the sympathetic nervous system; it secretes the hor-mones epinephrine and norepinephrine in response to sympathetic stimulation. In turn, these hormones cause almost the same effects as direct stimulation of the sympathetic nerves in all parts of the body.
The adrenal cortex secretes an entirely different group of hormones, called corticosteroids. These hormones are all synthesized from the steroid cholesterol,and they all have similar chemical formulas. However, slight differences in their molecular structures give them several different but very important functions.
Corticosteroids Mineralocorticoids, Glucocorticoids, and Androgens. Two major types ofadrenocortical hormones, the mineralocorticoidsand the glucocorticoids, are secreted by the adrenal cortex. In addition to these, small amounts of sex hor-mones are secreted, especially androgenic hormones, which exhibit about the same effects in the body as the male sex hormone testosterone. They are nor-mally of only slight importance, although in certain abnormalities of the adrenal cortices, extreme quantities can be secreted and can result in masculinizing effects.
The mineralocorticoids have gained this name because they especially affect the electrolytes (the “minerals”) of the extracellular fluids-sodium and potas-sium, in particular. The glucocorticoids have gained their name because they exhibit important effects that increase blood glucose concentration. They have additional effects on both protein and fat metabolism that are equally as impor-tant to body function as their effects on carbohydrate metabolism.
More than 30 steroids have been isolated from the adrenal cortex, but two are of exceptional importance to the normal endocrine function of the human body: aldosterone, which is the principal mineralocorticoid, and cortisol, which is the principal glucocorticoid.
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