Homeostasis

The nervous system and the endocrine system share major responsibility for maintaining homeostasis, a stable internal environment, in the face of external changes. Homeostasis requires that a number of vital factors be controlled within a proper range. Some of these vital factors include body temperature, blood glucose concentration, the concentration of individual electrolytes (sodium, potassium, calcium) in the blood, and blood pressure and volume.

Homeostatic control is accomplished mostly by the use of negative feedback, in which the movement of a condition outside the vital range stimulates an effect that tends to move it in the opposite direction. For example, most adults have a resting body temperature between 36.7 and 37.2°C (98.1 to 99.0°F). The "normal" value for a person's vital factor is termed the set point. If the temperature rises about 0.2°C above the set-point

Figure 9.5 Some of the most important endocrine glands. (From Van de Graaff and Rhees, 1997. © The McGraw-Hill Companies, Inc. Used with permission.)

temperature, the hypothalamus stimulates blood vessels in the skin to dilate and sweat glands to increase their secretion. Both of these tend to increase the loss of heat from the body, limiting or reversing the temperature increase. A body temperature decrease causes the reverse effects, plus other activities, such as shivering.

Body temperature also involves feedforward control, in which events that would alter a vital factor are detected and the body responds before the alteration actually occurs. The hypothalamus also receives signals from temperature sensors in the skin. These stimulate the temperature control effects before the sensors within the hypothalamus actually detect an internal change.

A few mechanisms involve positive feedback, in which a stimulus produces an effect that increases the stimulus. This usually involves processes that need to be completed quickly, such as blood clotting. When childbirth begins, stretch receptors in the wall of the uterus stimulate the brain to cause the pituitary gland to release stored oxytocin, a hormone that stimulates uterine contractions. This increases the rate of contractions, expelling the baby faster. Once the baby has left the birth canal, the uterine receptors relax, leading to a drop in oxytocin levels, breaking the positive feedback loop.

Factors that cause deviation from homeostasis are termed stresses. Stress can be physical, emotional, environmental, or metabolic. When stress initiates, the body enters what is called the alarm phase and responds with the fight or flight response described in Section 9.4.3. If the stress continues for the long term, the body switches to the resistance phase, in which glucocorticoid hormones are released, especially cortisol, plus lesser amounts of epinephrine, growth hormones, and thyroid hormones. These maintain the rate of energy supply at elevated levels by mobilizing lipid and protein reserves and saving glucose for nervous tissue. If the stress is starvation, the resistance phase ends when reserves run out. Otherwise, it may end due to the side effects of the hormones, such as the slowing of wound healing due to the anti-inflammatory effects of glucocorti-coids, elevated blood pressure and volume, and altered mineral balance (especially loss of potassium in the blood) due to aldosterone and ADH, or exhaustion of the ability of the adrenal cortex to continue glucocorticoid hormone production, destroying the ability of the body to maintain blood glucose levels. If any of these occur, the result is the exhaustion phase, in which homeostasis breaks down. One or more organs may malfunction. For example, excessive potassium loss can cause heart failure.

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