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.