In the capillaries of the lungs, the purple-red deoxygenated haemoglobin of the venous blood becomes the bright-red oxyhaemoglobin of the arterial blood.
In the tissues of the body oxyhaemoglobin gives up its oxygen and picks up carbon dioxide to be returned to the lungs for release into the atmosphere.
95% of the CO2 generated in the tissues is carried in the red blood cells, about one-half of this is directly bound to haemoglobin
Red Blood cell Image from
