The number one reason for the circulatory system is transport of oxygen to the cells and carbon dioxide away from them.\u00a0 This is done via the blood, which is water-based.\u00a0 The immediate problem might be that gases don’t necessarily dissolve very well in water.<\/p>\n
Red blood cells contain hemoglobin (which is why they are red).\u00a0 Hemoglobin attracts and holds oxygen very effectively, enabling the red blood cells to carry the oxygen through the circulatory system to the capillaries.\u00a0 There, the oxygen is dropped off to the cells.<\/p>\n
Carbon dioxide can be carried by red blood cells (as carbaminohemoglobin), but not very effectively.\u00a0 In the blood, most carbon dioxide combines with water to form carbonic acid (H2<\/sub>CO3<\/sub>), next breaking into hydrogen ion H+<\/sup> and bicarbonate ion HCO3<\/sub>–<\/sup>. Ions travel easily in water. At the lungs, the hydrogen ion and bicarbonate ion recombine into carbonic acid, which then separates into carbon dioxide and water. The carbon dioxide is exhaled.<\/p>\n HTH:)<\/p>\n Source:<\/em><\/p>\n Mader, Sylvia S. Inquiry into Life<\/em>, 11th edition. New York: McGraw-Hill, 2006.<\/p>\n