Because oxygen is one of the major substances required for chemical reactions in the cells, it is fortunate that the body has a special control mechanism to maintain an almost exact and constant oxygen concentration in the extracellular fluid. This mechanism depends principally on the chemical characteristic of haemoglobin, which is present in all red blood cells. Haemoglobin combines with oxygen as blood passes through the lungs. Then, as the blood passes through the tissue capillaries, haemoglobin, because of its own strong chemical affinity for oxygen, does not release oxygen into the tissue fluid if too much oxygen is already there. If the oxygen concentration is too low, however, sufficient amounts are released to re-establish adequate tissue oxygen concentration. Thus, the regulation of oxygen concentration in the tissues depends principally on the chemical characteristics of haemoglobin itself.