Adrenal Glands

Adrenal Glands

The pyramid-shaped adrenal glands sit on top of each kidney like caps and are embedded in the adipose tissue enclosing each kidney (FIGURE 16-15). The adre-nal glands are also known as the suprarenal glands because of their position.

The adrenal glands have a central adrenalmedulla and an outer adrenal cortex, each secret-ing different hormones. The adrenal medulla is closely connected with the sympathetic division of the autonomic nervous system and appears more like a mass of nervous tissue than a gland. The adrenal cortex consists of layers of cells, including an outer zone or zona glomerulosa, a middle zone or zonafasciculate , and an inner zone or zona reticularis. Itis derived from the embryonic mesoderm. Both the adrenal medulla and cortex are well supplied with blood vessels.

Adrenal Cortex

The adrenal cortex synthesizes more than 30 ­corticosteroids. This synthesis begins with choles-terol, using a variety of intermediate substances in relation to the hormone being made. Steroid hor-mones are not stored in cells and their rate of release is based on their rate of synthesis. Some of these hormones are vital for survival, especially aldoste-rone, cortisol, and some sex hormones.

Mineralocorticoids

Mineralocorticoids mainly function in the regula-tion of mineral salt or electrolyte concentrations in the extracellular fluids, mostly regulating sodium and potassium. The regulation of sodium is connected to the regulation of potassium, hydrogen, bicarbonate, and chloride. The regulation by mineralocorticoids of sodium and potassium is vital for overall homeo-stasis. Aldosterone accounts for more than 95% of the mineralocorticoids produced.

The outer adrenal cortex or zona glomerulosa synthesizes aldosterone, a mineralocorticoid that helps regulate mineral electrolyte concentrations. Aldosterone helps the kidneys to balance sodium and potassium and stimulates water retention via the process of osmosis. If blood sodium decreases or blood potassium increases, the adrenal cortex secretes aldosterone. The kidneys also stimulate aldosterone secretion if blood pressure falls. Aldosterone also enhances the absorption of sodium from gastric juice, perspiration, and saliva. It has regulatory effects that occur within 20 minutes, allowing precise control of plasma electrolyte balance. The activity of aldosterone involves synthesis and activation of proteins needed for sodium transport. Aldosterone secretion is stimulated by decreased blood volume and pressure and raised blood levels of potassium. Its secretion is inhibited­ by the opposite conditions. The two most important mechanisms that regulate aldosterone secretion are the renin–angiotensin–aldosterone mechanism and the plasma concentrations of potassium.

The renin–angiotensin–aldosterone mechanism regulates aldosterone release, helping to control blood volume, blood pressure, and the reabsorption of sodium and water by the kidneys. When blood pressure or vol-ume falls, certain cells of the juxtaglomerular complex of the kidneys are excited, which respond by releas-ing renin into the blood. The renin cleaves off part of the plasma protein known as angiotensinogen­. This causes an enzymatic cascade to occur forming angiotensin II. This substance stimulates cells of theglomerulosa to release aldosterone. All the effects of this mechanism ultimately raise blood pressure.

The cells of the zona glomerulosa in the adrenal cortex are directly influenced by fluctuating blood levels of potassium. When increased, potassium stimu-lates aldosterone release, and the opposite is also true. ACTH normally has very little effect on aldosterone release. However, when stressors are prevalent, the hypothalamus secretes more CRH. ACTH rises in the blood, increasing aldosterone secretion slightly. This helps to deliver nutrients and respiratory gases in an attempt to cope with the stressors.

Atrial natriuretic peptide is a hormone fromthe heart that is secreted when blood pressure rises. It regulates blood pressure and sodium–water balance and greatly inhibits the renin–angiotensin–­aldosterone mechanism. Renin and aldosterone secretion are blocked, and atrial natriuretic peptide also inhibits other mechanisms that enhance sodium and water reabsorp-tion. Overall, it decreases blood pressure by allowing sodium and water to leave the body in the urine.

Glucocorticoids

In general, the glucocorticoids help us to resist stressors and influence energy metabolism. Cortisol is a glucocorticoid produced in the middle adrenal cortex or zona fasiculata that also influences protein and fat metabolism. Cortisol inhibits protein synthe-sis, promotes the conversion of lipids to glucose, and the formation­ of glycogen in the liver. Cortisol, which is also known as hydrocortisone, helps balance blood glucose and is controlled by negative feedback. ACTH stimulates the adrenal cortex to release cortisol, and stress plays an important part in triggering cortisol release. FIGURE 16-16 shows how negative feedback regulates cortisol secretion. Other glucocorticoid hor-mones include cortisone and corticosterone, but these are relatively insignificant compared with cortisol. Acute stress interrupts normal cortisol rhythm, result-ing in increased ACTH blood levels. Cortisol responds to stress by causing a large rise in blood glucose, amino acids, and fatty acids. Its metabolic effect known as glu-coneogenesis is defined as the formation of glucose fromfats and proteins. Cortisol, in an attempt to conserve glucose for the brain, mobilizes fatty acids from adi-pose tissue so they can be used for energy. Stored pro-teins are broken down, vasoconstriction is enhanced, and nutrients are dispersed to the cells more quickly than normal. Excessive cortisol, however, causes anti-inflammatory and anti-immune effects to a large degree. When excessive, cortisol depresses cartilage formation, bone formation, and the immune system. It disrupts normal cardiovascular, gastrointestinal, and neural function and inhibits inflammation via the decrease in the release of inflammatory chemicals.

Gonadocorticoids

The inner adrenal cortex or zona reticularis pro-duces sex hormones. Under stimulation by ACTH, this part of the adrenal cortex produces smallamounts of androgens­, the sex hormones produced in larger quantities by the testes in males. Some androgens from the zona reticularis are converted to estrogens, which are the dominant sex hormones in females. Adrenal androgens stimulated development of pubic hair in both sexes prior to puberty. Adrenal sex hormones are also called gonadocorticoids. Their release is linked to ACTH. TABLE 16-5 discusses the adrenal cortex hormones.

Adrenal Medulla

The adrenal medulla contains large, round cells that are similar to the cells of the sympathetic ganglia,­ which are innervated by preganglionic sympa-thetic fibers. The secretory activities of the adrenal medulla are controlled by the sympathetic divi-sion of the autonomic nervous system. The adre-nal medulla secretes epinephrine or adrenaline and ­norepinephrine or noradrenaline . Epineph-rine makes up 80% of adrenal medulla secretions, the rest being norepinephrine. These hormones aid in coping with stressors and participate in the fight-or-flight response. Epinephrine is stronger in its stimulation of bronchial dilation and increased blood flow to the heart and skeletal muscles. How-ever, epinephrine decreases peristalsis. It is used clinically as a bronchodilator and heart stimulant, since it increases heart rate and blood pressure. Nor-epinephrine more greatly influences blood pressure and peripheral vasoconstriction.

One group of chemicals produced in nervous tissue, called catecholamines, regulate many dif-ferent functions, including thought processes, hor-mone secretions, blood pressure, and heart rate. The most common catecholamines are epineph-rine, norepinephrine, dopamine, and serotonin. The adrenal medulla is a modified sympathetic ganglion.

The adrenal medulla’s secretions have long-­lasting effects. They increase heart rate, cardiac muscle con-traction force, breathing rate, and blood glucose level while elevating blood pressure and decreasing diges-tive activity. In response to stress, the hypothalamus releases impulses to control the adrenal medulla. These impulses prepare the body for the “fight-­or-flight response.” TABLE 16-6 discusses adrenal medullary ­hormones and their effects.

1. What are the names of the hormones released from the adrenal glands?

2. Explain the effects of aldosterone in the kidneys.

3. Which pituitary hormone stimulates the adrenal cortex?

4. What are the three zonae of the adrenal cortex?