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While 60 percent of the US population is eligible to donate blood, only about 5 percent does. There is no substitute for human blood, which is used for numerous medical situations, including surgery for trauma, cancer treatment, organ transplants, burns, open heart surgeries, anemia, clotting disorders, and treating premature babies. The average red blood cell transfusion is 3.4 pints. Blood, which is made in the bone marrow, has four main components—red cells, platelets, plasma, and white cells. Red blood cells, or erythrocytes, transport oxygen from the lungs to the rest of the body. These disk-shaped cells contain hemoglobin, an iron-containing protein that picks up oxygen molecules as the blood exchanges gases in the lungs. The red blood cells, which can live about 120 days in the circulatory system, deliver oxygen to the far reaches of the body, where it is released for use by other cells, such as those of the brain and muscles. Red blood cells also pick up carbon dioxide and return it to the lungs to be exhaled. All animals have some form of oxygen distribution system, but only vertebrates use red blood cells. In some invertebrates, such as the earthworm, oxygen is transported using hemoglobin that is freely dissolved in the blood. Other invertebrates don't use hemoglobin at all. The horseshoe crab, for instance, uses copper instead of iron, making its blood blue instead of red.