RBCs are produced in the bone marrow and released into the peripheral blood, where they may remain for approximately 120 days before senescence. Their main function is the transport of respiratory gases (oxygen and carbon dioxide) between the lungs and body tissues.
Each RBC can be thought of as an "envelope" containing hemoglobin. Each hemoglobin molecule contains iron which has a high affinity for oxygen. As a result, when an RBC passes through one of the capillaries of the lungs, it picks up oxygen. The oxygen is transported through the blood to the tissues, where it is released. Carbon dioxide from the tissues then diffuses into the RBC, where it undergoes chemical changes. About 70% of the altered carbon dioxide diffuses into the plasma, 25% binds to the hemoglobin molecule, and 5% goes into a simple solution within the red cell. In each of these three ways, carbon dioxide is transported from the body tissues back to the lungs, where it is released.