Numerous technological devices rely on carefully designed control systems that are able to precisely maintain the value of a specific variable. Control systems governed by negative feedback achieve regulation by subtracting the variable output from the control input. Modern science has discovered that the human body was employing negative feedback long before the age of sophisticated technology.
In the context of anatomy and physiology, a negative feedback system operates through three interrelated components: a sensor, an integrator and an effector. The sensor detects a change in a certain physiological variable and communicates this change to the integrator, which processes the information and decides what sort of action, if any, should be taken. The integrator then sends instructions to the effector, which is an organ or gland that implements the changes specified by the integrator. The resulting desirable change in the controlled variable is repeatedly "fed back" to the integrator via the sensor, and the integrator then gradually decreases the intensity of its recommended action until the variable reaches a stable, appropriate value.
The body prefers to maintain a constant internal temperature of about 37 degrees Celsius (98.6 degrees Fahrenheit). Biological temperature sensors in the body monitor the temperature of the blood and report deviations to the hypothalamus, which is the integrator in this system. The primary effectors are blood vessels near the skin, muscles and sweat glands. If the blood temperature is too low, the hypothalamus tells blood vessels to constrict and muscles to shiver. If the temperature is too high, blood vessels dilate and sweat glands excrete sweat.
All bodily activity is powered by the energy that individual cells extract from glucose molecules. Cellular performance is optimal when the concentration of glucose in the bloodstream remains within a fairly narrow range. Eating causes blood glucose levels to rise, and this change is detected by sensor cells in the pancreas. In response, the pancreas releases insulin, which initiates several biochemical processes that use up this extra glucose. If glucose levels are too low, the pancreas releases glucagon, which causes the liver to release glucose, previously stored as glycogen, into the bloodstream. Negative feedback ensures that these actions are gradual and self-correcting. In other words, small deviations lead to small, compensating adjustments that tend to maintain a stable environment.
A variety of serious muscular and nervous-system disorders can occur when the blood does not contain a proper concentration of calcium. If calcium levels are too low, the parathyroid gland releases parathyroid hormone, which initiates various corrective actions. Two of these represent divergent methods of pursuing the same goal. Additional calcium is released into the bloodstream by cells that break down calcium-rich bone tissue, and less calcium is excreted in urine. When calcium levels are too high, the thyroid gland releases calcitonin, which has the opposite effect. More calcium is excreted in urine, and less calcium is released from bone tissue.