The main role of hemoglobin is to transport oxygen from the lungs to tissues, and CO2 from tissues to the lungs, by unstable binding of oxygen and CO2 with bivalent iron heme. The blood stream continuously transfers erythrocytes from the place with high pressure of O2 and low pressure of CO2 (lungs) to the place with low pressure of O2 and high pressure of CO2 (tissue). The higher the pressure of CO2 in tissues, the larger amount of oxygen released by hemoglobin. When hemoglobin is completely saturated, each gram of it binds 1.34 ml of oxygen. Hemoglobin has a significant role in maintaining the concentration of hydrogen ions in the organism by binding the greatest amount of CO2 produced in tissues. If the total quantity of CO2 were in plasma as H2CO3, pH of the plasma would be 4.0, which would cause death. Therefore, 99% of CO2 enters erythrocytes and binds to hemoglobin.
Life-span of a normal erythrocyte is 100 - 120 days. Hemoglobin catabolism releases Fe, globin, and porphin ring from these cells. Iron and globin amino acids are used for a new hemoglobin synthesis, and bilirubin is formed from porphiria ring. Most of the iron from the histiomonocytic system travels to plasma where it binds to transferrin and comes to the bone marrow. Some iron remains in histiomonocytic cells as ferritin or hemosiderin.