A particular value for a physiological variable, such as blood pressure or heart rate, that the body maintains to ensure homeostasis of its internal environment. Any fluctuation above or below the set point is detected by sensors, which provide feedback to control centres; these in turn initiate hormonal or nervous signals to make physical or biochemical changes in effectors (cells or organs) that will restore the variable to its set point. For example, human body temperature has an average set point of 37°C, which is monitored by sensors in the hypothalamus in the brain. A rise in temperature elicits cooling responses, such as sweating and dilation of surface blood vessels, whereas a fall below the set point causes shivering and constriction of surface vessels. Set points can themselves change, depending on time of day, season, or other factors, such as hibernation.