Homeostasis is a general term that describes how the body is constantly adjusting in order to ensure that it continues to function in the most efficient and effective manner. A useful analogy for homeostasis is driving a car. In order to function properly, a car needs fuel, water in its radiator and a fan to prevent overheating. The body simultaneously maintains homeostasis at a chemical, cellular and systemic level. Homeostasis is maintained by a variety of control systems which detect and respond to changes in internal and external environment. The internal temperature of the human body is maintained at around 37°C. Strictly speaking, however, body temperature can be divided into two values: core temperature and shell temperature. Core temperature is the temperature of the internal organs and the blood. Shell temperature, on the other hand, is the temperature of the skin's surface and may vary considerably according to the temperature of the surrounding environment.