The adrenal glands sit atop the kidneys (see Fig. 10.1). Each adrenal gland consists of an inner portion called the adrenal medulla and an outer portion called the adrenal cortex. These portions, like the anterior pituitary and the posterior pituitary, have no physiological connection with one another. The adrenal medulla is under nervous control, and the adrenal cortex is under the control of ACTH, an anterior pituitary hormone. Stress of all types, including emotional and physical trauma, prompts the hypothalamus to stimulate the adrenal glands (Fig. 10.8).
The hypothalamus initiates nerve impulses that travel by way of the brain stem, spinal cord, and sympathetic nerve fibers to the adrenal medulla, which then secretes its hormones.
Epinephrine (adrenaline) and norepinephrine (noradrenaline) produced by the adrenal medulla rapidly bring about all the body changes that occur when an individual reacts to an emergency situation. The effects of these hormones provide a short-term response to stress.
In contrast, the hormones produced by the adrenal cortex provide a long-term response to stress (Fig. 10.8).
The two major types of hormones produced by the adrenal cortex are the mineralocorticoids and the glucocorticoids. The mineralocorticoids regulate salt and water balance, leading to increases in blood volume and blood pressure. The glucocorticoids regulate carbohydrate, protein, and fat metabolism, leading to an increase in blood glucose level. Cortisone, the medication often administered for inflammation of joints, is a glucocorticoid.
The adrenal cortex also secretes a small amount of male sex hormones and a small amount of female sex hormones in both sexes. That is, in the male, both male and female sex hormones are produced by the adrenal cortex, and in the female, both male and female sex hormones are also produced by the adrenal cortex.
Figure 10.8 Adrenal glands. Both the adrenal medulla and the adrenal cortex are under the control of the hypothalamus when they help us respond to stress. Left: The adrenal medulla provides a rapid, but short-term, stress response. Right: The adrenal cortex provides a slower, but long-term, stress response.
Cortisol is a biologically significant glucocorticoid produced by the adrenal cortex. Cortisol raises the blood glucose level in at least two ways: (1) It promotes the breakdown of muscle proteins to amino acids, which are taken up by the liver from the bloodstream. The liver then breaks down these excess amino acids to glucose, which enters the blood. (2) Cortisol promotes the metabolism of fatty acids rather than carbohydrates, and this spares glucose for the brain.
Cortisol also counteracts the inflammatory response that leads to the pain and swelling of joints in arthritis and bursitis. The administration of cortisol aids these conditions because it reduces inflammation. Very high levels of glucocorticoids in the blood can suppress the body’s defense system, including the inflammatory response that occurs at infection sites. Cortisone and other glucocorticoids can relieve swelling and pain from inflammation, but by suppressing pain and immunity, they can also make a person highly susceptible to injury and infection.
Aldosterone is the most important of the mineralocorticoids. Aldosterone primarily targets the kidney where it promotes renal absorption of sodium (Na+) and renal excretion of potassium (K+).
The secretion of mineralocorticoids is not controlled by the anterior pituitary. When the blood sodium level and therefore the blood pressure are low, the kidneys secrete renin (Fig. 10.9). Renin is an enzyme that converts the plasma protein angiotensinogen to angiotensin I, which is changed to angiotensin II by a converting enzyme found in lung capillaries. Angiotensin II stimulates the adrenal cortex to release aldosterone. The effect of this system, called the renin-angiotensin-aldosterone system, is to raise blood pressure in two ways: Angiotensin II constricts arterioles, and aldosterone causes the kidneys to reabsorb sodium. When the blood sodium level rises, water is reabsorbed in part because the hypothalamus secretes ADH. Reabsorption means that water enters kidney capillaries and thus the blood. Then blood pressure increases to normal.
There is an antagonistic hormone to aldosterone, as you might suspect. When the atria of the heart are stretched due to increased blood volume, cardiac cells release a hormone called atrial natriuretic hormone (ANH), which inhibits the secretion of aldosterone from the adrenal cortex. The effect of ANH is the excretion of sodium-that is, natriuresis. When sodium is excreted, so is water, and therefore blood pressure lowers to normal.
Malfunction of the Adrenal Cortex
Malfunction of the adrenal cortex can lead to a syndrome, a set of symptoms that occur together. The syndromes commonly associated with the adrenal cortex are Addison disease and Cushing syndrome.
Addison Disease and Cushing Syndrome
When the level of adrenal cortex hormones is low due to hyposecretion, a person develops Addison disease. The presence of excessive but ineffective ACTH causes a bronzing of the skin because ACTH can lead to a buildup of melanin (Fig. 10.10). Without cortisol, glucose cannot be replenished when a stressful situation arises. Even a mild infection can lead to death. The lack of aldosterone results in a loss of sodium and water, the development of low blood pressure, and possibly severe dehydration. Left untreated, Addison disease can be fatal.
Figure 10.9 Regulation of blood pressure and volume. Bottom: When the blood sodium (Na+) level is low, a low blood pressure causes the kidneys to secrete renin. Renin leads to the secretion of aldosterone from the adrenal cortex. Aldosterone causes the kidneys to reabsorb Na+, and water follows, so that blood volume and pressure return to normal. Top: When the blood Na+ is high, a high blood volume causes the heart to secrete atrial natriuretic hormone (ANH). ANH causes the kidneys to excrete Na+, and water follows. The blood volume and pressure return to normal.
When the level of adrenal cortex hormones is high due to hypersecretion, a person develops Cushing syndrome (Fig. 10.11). The excess cortisol results in a tendency toward diabetes mellitus as muscle protein is metabolized and subcutaneous fat is deposited in the midsection. The trunk is obese, while the arms and legs remain a normal size. An excess of aldosterone and reabsorption of sodium and water by the kidneys leads to a basic blood pH and hypertension. The face is moon-shaped due to edema. Masculinization may occur in women because of excess adrenal male sex hormones.
Figure 10.10 Addison disease. Addison disease is characterized by a peculiar bronzing of the skin, particularly noticeable in these light-skinned individuals. Note the color of (a) the face and (b) the hands compared to the hand of an individual without the disease.
Figure 10.11 Cushing syndrome. Cushing syndrome results from hypersecretion of adrenal cortex hormones. a. Patient first diagnosed with Cushing syndrome. b. Four months later, after therapy.