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suprarenal glands (glandulae suprarenales); paired endocrine organs in higher vertebrates and man.
Each adrenal gland is made up of two parts: a surface portion (the cortex, or cortical layer), which consists of steroidogenic tissue and secretes steroid hormones; and an internal portion (the medulla), which consists of chromaffin, or adrenal, tissue and secretes catecholamine hormones. The adrenal glands are enclosed in a connective-tissue capsule. In humans they cover the top of the kidney like small caps.
In vertebrate evolution the adrenal glands developed gradually as separate organs with two distinct types of glandular tissue. In fish the steroidogenic and chromaffin tissues form independent aggregates of cells near the kidney. In amphibians these aggregates are adjacent but remain independent; in some reptiles they intermingle. The steroidogenic tissue of the adrenal glands in all vertebrates is of mesodermal origin. The chromaffin tissue originates from the same ectodermal rudiment that gives rise to the sympathetic neurons. Several arteries provide the cortex and the medulla with separate blood supplies, and a rich venous network merges into the central adrenal vein. The lymphatic vessels form two plexuses located under the capsule and in the medullary layer. The adrenal glands are innervated by splanchnic nerve fibers, which form the suprarenal plexus, which is connected with the renal and solar plexuses.
In mammals and humans the cortical layer, or cortex, of the adrenal glands is divided into three zones: the zona glomerulosa, zona fasciculata, and zona reticularis. Each has a different function. The cells of the zona glomerulosa are gathered into glomeruli and lie under the capsule. The cells of the zona fasciculata, the most extensive part of the cortex, are arranged in strands or fascicles and are threaded with capillaries that widen in that zone. The cells of the zona reticularis surround the medulla. The hormones secreted by the adrenal cortex are hydrocortisone (cortisol), aldosterone, and corticosterone, which are found in the blood that flows out of the adrenals. More than 40 steroid compounds (adrenocortical steroids) are secreted by the adrenal cortex, but only five of them are biologically active (corticosterone, hydrocortisone, cortisone, aldosterone, and deoxycorticosterone).
Depending on their physiological effect, the steroid compounds of the adrenal cortex are classified as mineralocorticoids (aldosterone and deoxycorticosterone), glucocorticoids (corticosterone, hydrocortisone, and cortisone), androgens (androstenedione, dehydroepiandrosterone, and testosterone, for example), or estrogens (estrone and equilenin, for example). The principal mineralocorticoid—aldosterone, the physiological regulator of mineral metabolism—is formed in the zona glomerulosa. In human blood plasma its concentration is about 0.08 micrograms (μg) per 100 milliliters. About 12–14 μg of the hormone are excreted in the urine every 24 hours. The source of glucocorticoids is the zona fasciculata. The two principal glucocorticoids (hydrocortisone and corticosterone) exhibit a high degree of species specificity. For example, in primates the principal glucocorticoid is hydrocortisone, and in rats and rabbits, corticosterone. Cattle, dogs, and cats have equal parts of both hormones.
The functions of the adrenal cortex are controlled by the adrenocorticotropic hormone (ACTH) of the pituitary and fluctuate daily, chiefly because of the effect of light. The hormones of the adrenal cortex play an important role in the body’s adaptation to unfavorable conditions. The pituitary-adrenal cortex system reacts stereotypically to unfavorable conditions (cold, infection, emotional excitement, and muscular exertion, for example) by releasing ACTH and corticosteroids. The central nervous system (the hypothalamus and the cerebral cortex) activates the pituitary-adrenal cortex system. Disruptions of adrenocortical function caused by tumors or infections and changes in the biosynthesis of steroid hormones may lead to a number of diseases, including Itsenko-Cushing disease (Cushing’s syndrome), Addison’s disease, and the adrenogenital syndrome.
In addition to chromaffin tissue, the adrenal medulla in mammals and humans contains a few neurons. The medulla secretes two hormones—adrenaline (epinephrine) and noradrenaline (norepinephrine). The blood flowing out of the adrenal glands also contains a small quantity of the precursor of the medullary hormones—dioxyphenylethylamine (dopamine), which may be an independent hormone. In humans the adrenal glands contain 0.5 mg of adrenaline and 0.1 mg of noradrenaline per g of tissue. The content of the two hormones in the adrenals varies, depending on the animal species. Adrenaline intensifies the absorption of oxygen by the tissues, stimulates metabolism, increases systolic arterial pressure, increases cardiac output, and accelerates the heartbeat. Noradrenaline increases both the systolic and diastolic arterial pressure, decreases cardiac output, and slows the heartbeat. Both hormones stimulate glycogenolysis in the liver, which raises the blood sugar. Disruption of adrenomedullary function may be caused by tumors, such as pheochromocytoma.
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