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The physiologic effects of the glucocorticoids promote the metabolic breakdown or anabolism of carbohydrates, proteins, and fats. Cortisol increases the rate of gluconeogenesis by the liver, decreases the utilization of glucose by the cells, reduces cellular protein and enhances utilization of amino acids by the liver, and promotes mobilization of fatty acids from adipose tissue into the plasma. The net effect of these actions is to make these noncarbohydrate nutritive elements readily available for energy.
The regulation of cortisol secretion involves a complex closed-loop negative feedback system. Initially, the hypothalamus reacts to physical or psychogenic stress by secreting corticotropin-releasing hormone (CRH), which is carried to the anterior pituitary gland (adenohypophysis) via the hypothalamic-hypophyseal portal system. In response to the presence of CRH, the adenohypophysis secretes adrenocorticotropic hormone (ACTH), which stimulates the adrenal cortex to release cortisol. Cortisol then initiates a series of metabolic activities which help to relieve the physiologic effects of stress. Cortisol inhibits both release of CRH from the hypothalamus and of ACTH from the adenohypophysis. This exerts a negative feedback effect; high serum cortisol levels inhibit further production of cortisol. Thus, during times of relative calm when the body is not experiencing abnormal stress, the cortisol level returns to normal.
Another factor that influences the secretory rates of CRH, ACTH, and cortisol is a biologic clock mechanism that establishes a cyclic pattern of signals from the hypothalamus. This is a 24-hour cycle that has its peak right after completion of the major portion of a night's sleep, usually around 4 or 5 AM. About 12 hours later, the blood level of cortisol is at its lowest. This cycle is dependent on sleeping patterns; therefore, if a person changes the pattern and sleeps in the daytime, the cycle of hormonal levels changes accordingly. This information is significant in testing for cortisol levels as a means of diagnosing a disorder of the endocrine system. When blood is drawn for testing, the specimen should be clearly labeled as to the precise time it was taken.
The endocrine glands, or ductless glands, discharge their secretions (hormones) directly into the blood; they include the adrenal, pituitary, thyroid, and parathyroid glands, the islands of Langerhans in the pancreas, the gonads, the thymus, and the pineal body. The exocrine glands discharge through ducts opening on an external or internal surface of the body; they include the salivary, sebaceous, and sweat glands, the liver, the gastric glands, the pancreas, the intestinal, mammary, and lacrimal glands, and the prostate. The lymph nodes are sometimes called lymph glands but are not glands in the usual sense.
Adrenal GlandEither of a pair of small flat triangular glands which lie atop or adjacent to each kidney and which are composed of
(1) The outer adrenal cortex, an endocrine gland composed of three layers, from the outermost zona glomerulosa, which produces mineralocorticoids, primarily aldosterone that regulates blood pressure, the middle zona fasciculata, which produces glucocorticoids, primarily cortisol, and the inner zona reticularis, which produces androgens, primarily DHEA and DHEA-S, and
(2) The central adrenal medulla, a neuroendocrine "organ" which produces catecholmines (epinephrine/adrenaline & norepinephrine/noradrenaline) in response to stress signals from the peripheral nervous system.
• Superior suprarenal artery from the inferior phrenic artery
• Middle suprarenal artery from the abdominal aorta
• Inferior suprarenal artery from the renal artery
• Right suprarenal vein to the inferior vena cava
• Left suprarenal vein to the left renal vein or the left inferior phrenic vein
su·pra·re·nal gland(sū'pră-rē'năl gland) [TA]
Each adrenal gland is a two-part organ composed of an outer cortex and an inner medulla. The cortex arises in the embryo from a region of the mesoderm that also gives rise to the gonads. The medulla arises from ectoderm, which also gives rise to the sympathetic nervous system.
The entire gland is enclosed in a tough connective tissue capsule from which trabeculae extend into the cortex. The cortex consists of cells arranged into three zones: the outer zona glomerulosa, the middle zona fasciculata, and the inner zona reticularis. The cells are arranged in cords. The medulla consists of chromaffin cells arranged in groups or in anastomosing cords. The two adrenal glands are retroperitoneal, each embedded in perirenal fat above its respective kidney. In an adult, the average weight of an adrenal gland is 5 g (range: 4 to 14 g).
The adrenal medulla synthesizes and stores three catecholamines: dopamine, norepinephrine, and epinephrine. The chief effects of dopamine are the dilation of systemic arteries, increased cardiac output, and increased flow of blood to the kidneys. The primary action of norepinephrine is constriction of the arterioles and venules, resulting in increased resistance to blood flow, elevated blood pressure, and slowing of the heart. Epinephrine constricts vessels in the skin and viscera, dilates vessels in skeletal muscle, increases heart rate, dilates the bronchi by relaxing bronchial smooth muscle, increases the conversion of glycogen to glucose in the liver to increase the blood glucose level, and diminishes activity of the gastrointestinal system. The three catecholamines are also produced in other parts of the body.
The adrenal medulla is controlled by the sympathetic nervous system and functions in conjunction with it. It is intimately related to adjustments of the body in response to stress and emotional changes. Anticipatory states tend to bring about the release of norepinephrine. More intense emotional reactions, esp. those in response to extreme stress, tend to increase the secretion of both norepinephrine and epinephrine; epinephrine is important in mobilizing the physiological changes that occur in the “fight or flight” response to emergency situations.
The cortex synthesizes three groups of steroid hormones from cholesterol. These are 1) glucocorticoids (cortisol, corticosterone), which regulate the metabolism of organic nutrients and have an anti-inflammatory effect; 2) mineralocorticoids (aldosterone, dehydroepiandrosterone), which affect metabolism of the electrolytes sodium and potassium; and 3) androgens and estrogens (estradiol), which contribute to body changes at puberty. See: aldosterone; cortisol; steroid
Hypersecretion of adrenal cortical hormones results in Cushing syndrome. Hypersecretion of aldosterone results in a surgically correctable form of hypertension (aldosteronism). Adrenocortical insufficiency may be acute or chronic; acute insufficiency of adrenal hormones produces circulatory shock, while chronic insufficiency results in Addison disease. See: Addison disease; aldosteronism; Cushing syndrome; pheochromocytoma