20.9–20.13: How does homeostasis work?

Why do we yawn? (Hint: it’s about homeostasis.)

Figure 20.15: A negative feedback loop restores the internal environment to a set point.

For animals to maintain homeostasis with regard to a particular physiological variable, they must have a set point for that variable, a target value or range to which the organism can return. Most vertebrates have set points for a variety of physiological variables, including body temperature, water-solute balances, blood sugar levels, blood pH, and tissue O₂ and CO₂ concentrations.

In the face of changes in its external environment, how does an organism maintain its homeostasis, its constant internal environment? The most common method involves the regulatory mechanism of negative feedback, in which sensors detect a change in the internal environment and trigger structures called effectors to oppose or reduce the change. This cycle of detection, response, and change is called a negative feedback loop (FIGURE 20-15).

An example of a negative feedback loop that we encounter in everyday life is the regulation of room temperature in a house, which involves a sensor (the thermostat) and two effectors (a furnace and an air conditioner). The thermostat contains a sensor that detects the temperature in the house. If the temperature drops too low, the thermostat triggers an effector—the furnace—to turn on, bringing the room temperature back up to the desired level. Once the temperature reaches a specified level, the thermostat tells the furnace to shut off. Similarly, if the temperature gets too high, the thermostat can trigger the air conditioner to turn on, which brings the temperature back down to the desired level, after which the air conditioner shuts off. In both cases, a negative feedback system senses a change in the temperature, triggers events that reverse the change, and restores the house’s internal environment to its desired level.

In the human body, negative feedback systems are the most common method used to maintain the internal environment within a narrow range. These systems include those that maintain body temperature, regulate levels of sugar in the bloodstream, and control the levels of salt and other solutes in body fluids. We explore these systems in detail later in this chapter.

Figure 20.16: Regulate or conform? Two strategies to cope with a changing environment.

It is important to note that while some groups of organisms, called regulators, maintain homeostasis for a certain variable (as we’ve just described for temperature), other organisms, called conformers, may have no set point for that variable, and the variable may fluctuate with external changes. Animals differ in the traits for which they are regulators and those for which they are conformers. Most fishes, for example, are conformers with regard to the temperature of the surrounding water but are regulators when it comes to the concentration of salt in their blood and tissues (FIGURE 20-16).

In addition to negative feedback systems, organisms also have a small number of positive feedback systems, in which deviations from conditions normally found in the internal environment cause an increase or acceleration of the change, in the same direction. Such systems push the body away from homeostasis and often initiate cascading processes that increase the rate at which the system deviates from the normal range. For this reason, positive feedback systems, if unchecked, can become highly unstable.

An example of a positive feedback system is the blood-clotting process (FIGURE 20-17). Injury to a blood vessel causes platelets—cellular fragments involved in clotting—to release their Super Glue–like contents, which begin to seal any rip or tear in the blood vessel (see Section 21-7). Within the bloodstream, the contents released from a platelet have another effect as well: they cause other platelets to release their contents. This is a positive feedback system, because the release of some blood-clotting molecules into the bloodstream causes the release of more such molecules, which causes the release of still more. The result is that a dangerous situation, injury to a blood vessel, is quickly remedied.

Figure 20.17: Patching a tear. The blood clot that forms after an injury is a result of a positive feedback system.

Generally, positive feedback systems are part of larger, negative feedback systems, which, after a short period of positive feedback, bring the variable (and the organism) back to within its original, normal range of internal environmental conditions. After blood clotting has stopped the loss of blood from a damaged vessel, other chemical signals interrupt the positive feedback loop, stop the further release of blood-clotting molecules, and even begin to break down blood clots that no longer serve a function.