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.

Regardless of the factor or event being regulated—the variable—all homeostatic control mechanisms have at least three interdependent components (Figure 1.4). The first component, the receptor, is some type of sensor that monitors the environment and responds to changes, called stimuli, by sending information (input) to the second component, the control center. Input flows from the receptor to the control center along the so-called afferent pathway. The control center, which determines the set point (the level or range at which a variable is to be maintained), analyzes the input it receives and then determines the appropriate response or course of action.

The third component, the effector, provides the means for the control center’s response (output) to the stimulus. Information flows from the control center to the effector along the efferent pathway. The results of the response then feed back to influence the stimulus, either depressing it (negative feedback) so that the whole control mechanism is shut off or enhancing it (positive feedback) so that the reaction continues at an even faster rate. (To help you remember the difference between “afferent” and “efferent,” you might note that information traveling along the afferent pathway approaches the control center and efferent information exits from the control center.)

Mity te, to wiara w to, ze istnieje antykoncepcja uniwersalna, stuprocentowa oraz że stosunek przerywany to pewny środek antykoncepcyjny. Wszystkie trzy mity są oczywiście nieprawdziwe. W odniesieniu do dwóch pierwszych – każdy organizm posiada konkretne i specyficzne predyspozycje i nie sposób przewidzieć, jak zareaguje na podawany środek. Skuteczność metod określa skala Pearla, opracowana na podstawie obserwacji metody w środowisku. Żadna nie otrzymała wskaźnika najniższego. Jeśli zaś chodzi o stosunek przerywany to warto wiedzieć, ze plemniki znajdują się również w śluzie wydzielanym przed wytryskiem i refleks nie zawsze zdaje tu egzamin.