In positive feedback, the initial stimulus produces a response that exaggerates or enhances its effects. Suppose that the thermostat in Figure 1-3a is connected to a heater rather than to an air conditioner. Now when room temperature rises, the thermostat turns on the heater, causing a further rise in room temperature. Room temperature will continue to rise until someone switches off the thermostat, turns off the heater, or intervenes in some other way. This kind of escalating cycle is often called a positive feedback loop.

In the body, positive feedback loops are often incorporated into control mechanisms in which a potentially dangerous or stressful process must be completed quickly. For example, the immediate danger from a severe cut is the loss of blood, which can lower blood pressure and reduce the efficiency of the heart. The body's response has been diagrammed in Figure 1-5. Damage to the blood vessel wall releases chemicals that begin the process of blood clotting. As clotting gets under way, each step releases chemicals that accelerate the process. This escalating process is a positive feedback loop that ends with the formation of a blood clot, which patches the vessel wall and stops the bleeding. Blood clotting will be examined more closely in Chapter 19. Labor and delivery, another example of positive feedback in action, will be discussed in Chapter 29.

Why is homeostatic regulation important to humans?

What happens to the body when homeostasis breaks down?

Why is positive feedback helpful in blood clotting but unsuitable for the regulation of body temperature?

fap5_clinicalsm Homeostasis and Disease

When homeostatic regulation fails, whether as a result of infection, injury, or genetic abnormality, organ systems begin to malfunction. The result is a state known as illness, or disease. Chapters 2–29 devote considerable attention to the mechanisms responsible for a variety of human diseases. An understanding of normal homeostatic mechanisms can usually enable you to draw conclusions about what might be responsible for the signs and symptoms characteristic of many diseases. A sign is an objective physical indication of a disease. Examples include swellings, fevers, or sounds of abnormal breathing. Symptoms are subjective—things that a person experiences and describes but that aren't otherwise detectable. Examples include pain, nausea, and anxiety. Major organizational and functional patterns relevant to clinical practice are discussed in the Applications Manual. Homeostasis and Disease

FIGURE 1-3 The Control of Room Temperature. (a) A thermostat turns on an air conditioner (or heater) to keep room temperature near the desired set point. Whether room temperature rises or falls, the thermostat (a control center) triggers an effector response that restores normal temperature. When room temperature rises, the thermostat turns on the air conditioner, and the temperature returns to normal.
FIGURE 1-5 Positive Feedback: Blood Clotting. Positive feedback loops are important in accelerating processes that must proceed to completion rapidly. In this example, positive feedback accelerates the clotting process until a blood clot forms and stops the bleeding.
©2003 Pearson Education, Inc., publishing as Benjamin Cummings