Oxygen is carried by the erythrocytes of the blood. Oxygen combines with the haemoglobin present in these cells forming a compound called oxyhaemoglobin. Haemoglobin is a complex corn pound which has a great affinity for oxygen. Haemoglobin enables the blood to carry about so times more oxygen than the plasma alone can miry.
While passing through the alveolar capillaries haemoglobin becomes saturated with oxygen.
100ml of blood contains about zooms of oxygen:
HbHaemoglobin+O2Oxygen⇌HbO2Oxyhaemoglobin
Oxyhaemoglobin is a very unstable compound when blood reaches in the tissue where oxygen tension is low. Oxyhaemoglobin breaks into haemoglobin and oxygen diffuse into the cells. The amount oxyhaemoglobin formed depends upon the partial pressure of oxygen in the blood. It has bright scarlet colour, while reduced haemoglobin has a dull purplish colour. This accounts for the colour difference between arterial and venous blood.