Learning Objectives
By the end of this section, you will be able to:
- List the components of a homeostatically controlled system
- Discuss the role of homeostasis in the human body
- Contrast negative and positive feedback, giving one physiologic example of each mechanism
Maintaining a stable system requires the body to continuously monitor its internal conditions. Though certain physiological systems operate within frequently larger ranges, certain body parameters are tightly controlled homeostatically. For example, body temperature and blood pressure are controlled within a very narrow range. A set point is the physiological value around which the normal range fluctuates. For example, the set point for typical human body temperature is approximately 37°C (98.6°F). Physiological parameters, such as body temperature and blood pressure, tend to fluctuate within a range of a few degrees above and below that point. Receptors located in the body’s key places detect changes from this set point and relay information to the control centers located in the brain. The control centers monitor and send information to effector organs to control the body’s response. If these effectors reverse the original condition, the system is said to be regulated through negative feedback.
Control centers in the brain and other parts of the body monitor and react to deviations from this set point using negative feedback. Negative feedback is a mechanism that reverses a deviation from the set point, and in turn, maintains body parameters within their normal range. The maintenance of homeostasis by negative feedback goes on throughout the body at all times and an understanding of negative feedback is thus fundamental to an understanding of human physiology.
Negative Feedback
A negative feedback system has three basic components: a sensor, control center and an effector. (Figure 1.3.2a). A sensor, also referred to a receptor, monitors a physiological value, which is then reported to the control center. The control center compares the value to the normal range. If the value deviates too much from the set point, then the control center activates an effector. An effector causes a change to reverse the situation and return the value to the normal range.
In order to set the system in motion, a stimulus must drive a physiological parameter beyond its normal range (that is, beyond homeostasis). This stimulus is “heard” by a specific sensor. For example, in the control of blood glucose, specific endocrine cells in the pancreas detect excess glucose (the stimulus) in the bloodstream. These pancreatic beta cells respond to the increased level of blood glucose by releasing the hormone (insulin) into the bloodstream. The insulin signals skeletal muscle fibers, fat cells (adipocytes), and liver cells to take up the excess glucose, removing it from the bloodstream. As glucose concentration in the bloodstream drops, the decrease in concentration—the actual negative feedback—is detected by