It holds oxygen at P O2 values at which hemoglobin is releasing it Delivering oxygen to the tissues is only half of the respiratory function of the blood. Blood also must take waste products such as carbon dioxide away from the tissues. Carbon dioxide diffuses through cell membranes, moving from its site of production into the blood. Red blood cells have an enzyme called carbonic anhydrase which speeds up the reaction. This facilitates the diffusion of carbon dioxide from tissues, into the plasma, and into the red blood cells.
Plasma carries most of the carbon dioxide as bicarbonate ions.
Some carbon dioxide is also carried by deoxygenated hemoglobin. The combination of carbon dioxide and hemoglobin is called carboxyhemoglobin . When carbon dioxide and bicarbonate reach the gas exchange surfaces, carbon dioxide diffuses into the respiratory medium.