Maintaining homeostasis requires that the body continuously maintain its internal conditions. Each physiological condition has a particular set point, from body temperature to blood pressure to levels of certain nutrients. A set point is the physiological value around which the normal range fluctuates. A normal range is a restricted set of values that is optimally healthful and stable. For example, the set point for normal human body temperature is approximately 37°C (98.6°F). Physiological parameters, such as body temperature and blood pressure, tend to fluctuate within a normal range a few degrees above and below that point. Control centers in the brain and other parts of the body monitor and react to deviations from homeostasis using negative feedback. Negative feedback is a mechanism that reverses a deviation from the set point. Therefore, negative feedback maintains body parameters within their normal range. Maintaining homeostasis by negative feedback goes on throughout the body at all times, and understanding negative feedback is thus fundamental to understanding human physiology.
A sensor, also referred to as a receptor, is a component of a feedback system that monitors a physiological value. This value is reported to the control center. The control center is the component in a feedback system that compares the value to the normal range. The control center activates an effector if the value deviates too much from the set point. An effector is a component in a feedback system that causes a change to reverse the situation and return the value to the normal range.
For example, to set the system in motion, a stimulus must drive a physiological parameter beyond its normal range (beyond homeostasis). This stimulus is "heard" by a specific sensor. 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 blood glucose level 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 pancreatic alpha cells, and insulin release stops. This prevents blood sugar levels from continuing to drop below the normal range.
This text is adapted from Openstax, Anatomy and Physiology 2e, Section 1.5: Homeostasis
