Life is a constant adaptation to changing conditions. Homeostasis preserves life and health in spite of large external variations. This is accomplished by maintaining a relatively constant internal environment. This internal environment is the fluid that bathes cells.
Conditions controlled by homeostasis include body temperature, blood pressure, and the pH and composition of the blood. The setpoint is the target or desired value for the controlled condition. Homeostatic control mechanisms have a sensor that monitors the controlled condition, a control center that compares the output of the receptor with a set-point, and an effector that brings the controlled condition back toward the set-point when it is disrupted.
Negative feedback systems keep controlled conditions stable around a setpoint. If a stimulus disrupts a controlled condition, control loops restore the disturbance to normal values.
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Making a Difference in the World 