Most homeostatic control mechanisms are negative feedback mechanisms. In these systems, the output shuts off the original stimulus or reduces its intensity. These mechanisms cause the variable to change in a direction opposite to that of the initial change, returning it to its “ideal” value; thus the name “negative” feedback mechanisms.
A good example of a nonbiological negative feedback system is a home heating system connected to a temperature-sensing thermostat (Figure 1.5). The thermostat houses both the receptor (thermometer) and the control center. If the thermostat is set at 20°C (68°F), the heating system (effector) is triggered ON when the house temperature drops below that setting. As the furnace produces heat and warms the air, the temperature rises, and when it reaches 20°C or slightly higher, the thermostat triggers the furnace OFF. This process results in a cycling of “furnace-ON” and “furnace-OFF” so that the temperature in the house stays very near the desired temperature of 20°C. Your body “thermostat,” located in a part of your brain called the hypothalamus, operates in a similar fashion.
Regulation of body temperature is only one of the many ways the nervous system maintains the constancy of the internal environment. Another type of neural control mechanism is seen in the withdrawal reflex referred to earlier, in which the hand is jerked away from a painful stimulus such as broken glass. The endocrine system is equally important in maintaining homeostasis, however, and a good example of a hormonal negative feedback mechanism is the control of blood volume by antidiuretic hormone (ADH). As blood volume drops, the hypothalamus of the brain (the control center) is stimulated to release ADH to the blood. This in turn prompts the kidneys to reabsorb more water and return it to the bloodstream. The rising blood volume then ends the stimulus for ADH release.
The body’s ability to regulate its internal environment is fundamental, and all negative feedback mechanisms have the same goal: preventing sudden severe changes within the body. Body temperature and blood volume are only two of the variables that need to be regulated. There are hundreds! Other negative feedback mechanisms regulate heart rate, blood pressure, the rate and depth of breathing, and blood levels of oxygen, carbon dioxide, and minerals. Now, let’s take a look at the other type of feedback control mechanism—positive feedback.
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