Hormones

The two adrenal glands, one on top of each kidney, each have two distinct regions. The outer region (the medulla) produces adrenaline and noradrenaline and is under the control of the sympathetic nervous system. The inner region (the cortex) produces a number of steroid hormones. The cortical steroid hormones include mineralocorticoids (mainly aldosterone), glucocorticoids (mainly cortisol), and gonadocorticoids. These steroids are derived from cholesterol. Although cholesterol receives a lot of bad press, some of it is necessary. Steroid hormones act by regulating gene expression, hence, their presence controls the production of numerous factors with multiple roles. Aldosterone and cortisol are the major human steroids in the cortex. However, testosterone and estrogen are secreted by adults (both male and female) at very low levels.

Aldosterone plays an important role in regulating body fluids. It increases blood levels of sodium and water and lowers blood potassium levels. Low blood sodium levels trigger aldosterone secretion via the renin-angiotensin pathway. Renin is produced by the kidney, and angiotensin originates in the liver. High blood potassium levels also trigger aldosterone release. ACTH has a minor promoting effect on aldosterone. Aldosterone targets the kidney where it promotes sodium uptake and potassium excretion. Since sodium ions influence water retention, the result is a net increase in body fluid volume.

Blood cortisol levels fluctuate dramatically throughout the day and peak around 8 A.M. Presumably, this early peak enables humans to face the varied daily stressors they encounter. Cortisol secretion is stimulated by physical trauma, cold, burns, heavy exercise, and anxiety. Cortisol targets the liver, skeletal muscle, and adi-pose tissue. Its overall effect is to provide amino acids and glucose to meet synthesis and energy requirements for normal metabolism and during periods of stress. Because of its anti-inflammatory action, it is used clinically to reduce swelling. Excessive cortisol secretion leads to Cushing syndrome which is characterized by weak bones, obesity, and a tendency to bruise. Cortisol deficiency can lead to Addison disease which has the symptoms of fatigue, low blood sodium levels, low blood pressure, and excess skin pigmentation.

The adrenal medullary hormones are epinephrine (adrenaline) and nor-epinephrine (nor-adrenaline). Both of these hormones serve to supplement and prolong the fight or flight response initiated in the nervous system. This response includes the neural effects of increased heart rate, peripheral blood vessel constriction, sweating, spleen contraction, glycogen conversion to glucose, dilation of bronchial tubes, decreased digestive activity, and lowered urine output.

The condition of stress presents a model for reviewing one way that multiple systems and hormones interact. During stress, the nervous, endocrine, digestive, urinary, respiratory, circulatory, and immune response are all tied together. For example, the hypothalamus sends nervous impulses to the spinal cord to stimulate the fight or flight response and releases CRH which promotes ACTH secretion by the pituitary. ACTH, in turn, triggers interleukins to respond which promote immune cell functions. ACTH also stimulates cortisol release at the adrenal cortex which helps buffer the person against stress. As part of a negative feedback loop, ACTH and cortisol receptors on the hypothalamus assess when sufficient levels of these hormones are present and then inhibit their further release. De-stressing occurs over a period of time after the stressor is gone. The systems eventually return to normal.