The state of balance in the internal environment of the body it includes control of the water balance of the blood, body temperature, blood sugar level and blood urea level. Each of these internal factors is maintained by a separate mechanisms the central nervous system and the endocrine system. Before these systems can bring about any change they have to be supplied with information regarding the body state. Living cells and all the chemical reactions are controlled by enzymes; the enzymes are very sensitive to the conditions in which they work (PH). A slight fall in temperature or a rise in acidity may slow down or stop an enzyme from working and thus prevent an important reaction from taking place in the cell.

Cell membrane controls the substances which enter and leave the cell, but it is the tissue fluid which supplies or removes these substances, and it is therefore important to keep the composition of the tissue fluid as steady as possible. If the tissue fluid were too concentrated, it would withdraw water from the cells by osmosis and the body would be dehydrated. If the tissue fluid were too dilute, the cells would take up too much water from it by osmosis and the tissues would become waterlogged and swollen.