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It is stressed in the passage that haemoglobin ----.
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.