Triangular endocrine gland situated on top of the kidney. (The name derives from the Latin word for kidney – ad-renal and supra-renal meaning next to and above the kidney, respectively). The adrenals are soft and yellow, and consist of two parts: the cortex and medulla. The cortex (outer part) secretes various steroid hormones and other hormones that control salt and water metabolism and regulate the use of carbohydrates, proteins, and fats. The medulla (inner part) secretes the hormones adrenaline and noradrenaline which, during times of stress, cause the heart to beat faster and harder, increase blood flow to the heart and muscle cells, and dilate airways in the lungs, thereby delivering more oxygen to cells throughout the body and in general preparing the body for ‘fight or flight’.
The hormones produced by the cortex can be divided into three main classes according to their function.
1. Adrenal androgens These are basically sex hormones. Their functions include the normal development of male sex organs, the growth of axillary and pubic hair at puberty, the stimulation of sebum (grease) production in skin, and the development of greater muscular power seen in men. This latter property of the adrenal androgens, the so-called ‘anabolic’ effect, has led to searches for synthetic steroids that have the muscle-building power of the natural products without the side effects. Many weightlifters and bodybuilders (and perhaps some female shot-putters) have contributed to their success by careful use of synthetic adrenal androgens.
2. Mineralocorticoids The principal hormone in this group is aldosterone, and its function is to maintain the sodium and potassium ions (the minerals) in the body at a steady level. The hormone does this by influencing the kidney; when aldosterone is at a high concentration in the blood, the kidney retains more sodium and loses more potassium than normal. Removal of the adrenal glands in humans causes death through lack of aldosterone, for without aldosterone the body's sodium rapidly disappears via the urine, and sodium is essential for maintaining the volume of circulating blood. Failure of the adrenal glands is called hypoadrenalism or Addison's disease.
3. Glucocorticoids The most important in this class of hormones is cortisol (hydrocortisone). The glucocorticoids have a wide range of effects which include the maintenance of normal blood pressure, the ability to respond to stress, and the maintenance of normal levels of blood glucose. When present in excess, the glucocorticoids inhibit the body's inflammatory responses, and the successful treatment of chronic inflammatory disease (for example, rheumatoid arthritis) with steroid drugs bears this out. Overproduction of glucocorticoids can occur spontaneously, and is known as Cushing's disease.
Classes 1 and 3 of the adrenal cortical hormones are released from the gland under the influence of another hormone, adrenocorticotrophic hormone (ACTH, ‘adrenal-cortex-growth’ hormone), which is released from the pituitary gland under the influence of the hypothalamus; without ACTH the adrenal cortex shrivels up (atrophies). It is clear that disease of the adrenal cortex can result from disordered function of the adrenal gland itself or of the pituitary gland.
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