Each tiny red cell looks like a doughnut with a dent instead of a hole. Red cells are like tiny rafts floating through your body in a river of about five quarts of a straw-colored liquid called plasma. The red pigment, called hemoglobin, contains iron. When iron combines with oxygen it turns red. So the job of hemoglobin is to combine with oxygen.
Red cells pick up oxygen in the lungs.
The heart pumps them through sixty thousand miles of arteries, veins, and capillaries on a round-trip journey. They deliver the life-giving oxygen to every other cell in the body. They pick up carbon dioxide, a waste gas, and return to the lungs for us to breathe it out. The red cells are now ready for another load of oxygen. The shape of red cells gives them a larger surface area for exchanging oxygen and carbon dioxide gases.