Red blood cells (= RBCs) transport respiratory gases through the blood stream so they can be exchanged at the lungs and tissues. Involved in this process is the molecule, hemoglobin. Each RBC makes and stores about 200-300 million of these complex proteins. There are so many that they take up about a third of the cell volume. As blood flows through the tissues, hemoglobin accepts CO2 from surrounding cells and releases O2. The loss of O2 causes the color of hemoglobin (and RBCs) to change from red to purple. The situation rapidly reverses in the lungs, where the hemoglobin bonds to O2 and releases CO2. An enzyme called carbonic anhydrase is also made and stored by RBCs. In the tissues, carbonic anhydrase catalyzes (= accelaerates) a reversible reaction that converts CO2 to HCO3- (= bicarbonate ions). Most of the CO2 that enters the RBCs is converted to this water-soluble ion and released into the plasma. In the lungs, bicarbonate ions enter the RBCs from the plasma and are converted back to CO2, which is exhaled.