Our life is dependent on chemical reactions that occur in our body cells. The substrates for the chemical reactions are circulated to the cells via the internal environment (plasma and interstitial fluid). Thus, it is important that the chemical factors in the internal environment are properly regulated. In addition, the cellular chemical reactions are also affected by the physical factors of the internal environment (e.g. Principles of Homeostasis temperature, pressure). These factors should also be regulated. The process of regulation of the chemical and physical factors in the internal environment so that they are appropriate for normal cellular chemical reactions is called homeostasis. It is arguably one of the most important concepts in human physiology.