Oxygen Transport in Blood
All four respiratory pigments are adapted to load and unload oxygen effectively in I the habitats where they have evolved, whether animals live on land where the air contains 210 ml of oxygen per litre or in fresh water containing 8.0 ml / litre or in the sea containing 6.4 ml / litre. The loading unloading reaction can be written as
The direction of this reaction depends on Po2 of the environment and bond strength or affinity between haemoglobin and oxygen. In the lungs where Po, is high almost all deoxyhaemoglobin molecules bind to oxygen. Low Po2 in the systemic capillaries promotes unloading. Similarly strong affinity favours loading and weak bonding favours unloading. Haemoglobin has a bond strength which permits 97% of haemoglobin to combine with oxygen when leaving the lung. At the same time the bond is sufficiently weak to permit unloading in tissues. Under normal resting conditions 22% of the oxygen is unloaded. This satisfies the oxygen need of the body simultaneously maintaining a reserve that is utilised during emergency conditions.