Answer: D (hemoglobin)
) An iron-containing protein in the blood of many animals that, in vertebrates, carries oxygen from the lungs to the tissues of the body and carries carbon dioxide from the tissues to the lungs. Hemoglobin is contained in the red blood cells of vertebrates and gives these cells their characteristic color. Hemoglobin is also found in many invertebrates, where it circulates freely in the blood. See Note at red blood cell.
Did You Know?
Red Blood Cells
Blood contains many cell types, but the distinctive red color comes from the aptly named red blood cells
(RBCs). RBCs have their rich red color because of a vitally important iron-containing protein called hemoglobin. The protein picks up oxygen molecules as the blood exchanges gases in the lungs. The RBCs then carry 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. Just as importantly, after the RBC drops off its load of oxygen, its hemoglobin picks up carbon dioxide, the waste product of those brain and muscle cells, and brings it back to the lungs to be breathed out. All animals have some oxygen distribution system, but only vertebrate animals use RBCs. In some invertebrate animals, 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.