Homeostasis is linked with the relative constancy of the chemical and physical environment in the cells and the organism itself. Human beings have about 70 trillion cells that form their own environment inside our bodies. This internal environment defends us from changes in the outside world, such as temperature, water composition, and food availability. Our cells interrelate to keep this internal environment relatively persistent despite an ever-changing outside environment. While homeostatic mechanisms uphold a relatively constant internal environment, physiological values may vary slightly in a person from time to time or from person to person. These values exist in a "normal range".
