4.12: Positive and Negative Feedback Loops
Animal organs and organ systems constantly adjust to internal and external changes through a process called homeostasis ("steady state"). Examples of these changes include regulation of the level of glucose or calcium in the blood or internal responses to external temperatures. Homeostasis requires maintaining an internal dynamic equilibrium:
- Dynamic because it constantly adjusts to the changes that the body's systems encounter.
- Equilibrium because body functions are kept within specific ranges. Even an animal that is apparently inactive is maintaining this homeostatic equilibrium.
The goal of homeostasis is the maintenance of equilibrium around a point or value called a set point. While there are normal fluctuations from the setpoint, the body's systems will usually attempt to go back to this point. A change in the internal or external environment is called a stimulus and is detected by a receptor; the response of the system is to adjust the deviation parameter toward the set point.
Control of Homeostasis
When a change occurs outside an animal's environment, an adjustment must be made. The receptor senses the change in the environment, then sends a signal to the control center (in most cases, the brain) which in turn generates a response that is signaled to an effector. The effector is a muscle (that contracts or relaxes), or a gland that secretes. These signals and responses are controlled by unconscious autonomic feedback loops. Negative feedback loops maintain homeostasis. Positive feedback loops push the organism further out of homeostasis but may be necessary for life functions. Thus, homeostasis is controlled by the nervous and endocrine systems of mammals.
It is possible to adjust a system's set point. When this happens, the feedback loop works to maintain the new setting. An example of this is blood pressure: over time, the normal or set point for blood pressure can increase as a result of continued increases in blood pressure. The body no longer recognizes the elevation as abnormal, and no attempt is made to return to the lower set point. The result is a regular higher blood pressure that can harm the body. Medication can lower blood pressure and reduce the set point in the system to a more healthy level. This is known as a process of alteration of the set point in a feedback loop.
Changes can be made in a group of body organ systems to maintain a set point in another system. This is called acclimatization. This occurs, for instance, when an animal migrates to a higher altitude than that to which it was originally accustomed. In order to adjust to the lower oxygen levels at a higher altitude, the body increases the number of red blood cells circulating in the blood to ensure adequate oxygen delivery to the tissues. As another example of acclimatization, some animals have seasonal changes in their coats: a heavier coat in the winter ensures adequate heat retention, and a lighter coat in the summer assists in keeping body temperature from rising to harmful levels.