What is homeostasis?

Homeostasis can be defined as the maintenance of a constant internal environment despite various stresses in the external environment which may act to alter this stable state. The stable state that the body aims to maintain is called the “set point”. The body achieves homeostasis by using negative feedback. This involves sensors that detect a change in the controlled variable, a control centre and effectors that act to return the variable to the set point. These concepts may be best illustrated using an example. The body acts to maintain a temperature of around 37˚C - the set point. If the temperature of the external environment increases, for example if you stand in the sun, your body temperature will increase above 37˚C. This is detected by sensors in the body which send signals to the centre in the brain which controls body temperature, the thermoregulatory centre. This centre sends signals to various effectors in the body to decrease the body temperature again to 37˚C. For example, it may signal to sweat glands to increase sweat production and may stimulate blood vessels near the surface of the skin to dilate so more heat is lost by radiation. However if the temperature of the external environment decreases, for example if you sit in an ice bath, your body temperature will decrease below 37˚C. This is detected by sensors that signal to the same control centre, however this time the centre signals to effectors in the body to increase body temperature to the set point, for example to muscles to cause shivering. It is important to remember the sensors will also detect body temperature as it returns to the set point and will stop the effectors from working once the temperature has returned to the set point. This feedback loop is termed negative feedback. Other parameters of the body that are controlled in this way include blood glucose level, pH and blood water level. 
Why is homeostasis important? Many vital processes of the body can only occur under certain conditions. For example, many reactions of the body are controlled by enzymes that work optimally at specific temperature and pH values - enzymes may even be destroyed if these conditions change too much. If pH and temperature were not controlled, changes in the external environment may cause these conditions to change so that the reactions could not happen. Therefore, homeostasis allows organisms to survive in a range of environments.