The primary function of erythrocytes (red blood cells) is to exchange oxygen and carbon dioxide through tiny blood vessels called capillaries. In the lungs, oxygen diffuses into the blood, hemoglobin molecules release carbon dioxide picked up from body tissues. This allows oxygen to attach to the hemoglobin molecules and it can be carried to the rest of the body. Hemoglobin is a protein that makes blood red and carries oxygen throughout the circulation. The adult form of hemoglobin contains 2 alpha chains and 2 beta chains. When CO2 is removed from tissues, a portion of it is dissolved in the plasma and converted to bicarbonate. A majority of the CO2 is taken up by the RBCs and follows one of three transport pathways. 1. The CO2 is dissolved into the RBC cytoplasm. 2. CO2 is converted, by carbonic anhydrase, into bicarbonate which is exchanged at the cell membrane for a chloride ion (involved in the Chloride shift). This bicarbonate removal increases CO2 uptake into the cell. 3. CO2 is carried by carbaminohemoglobin which can be transported to the lung for removal. After offloading of oxygen into tissues, hemoglobin has an increased affinity for carbon dioxide and hydrogen ions (Haldane effect).

Red Blood Cell Gas Exchange References