Transport of Gases in the BloodOxygen diffuses into the blood from the air, but oxygen is not extremely soluble in the plasma of the blood. Therefore, vertebrates have hemoglobin, a protein with a high affinity for oxygen so sufficient oxygen can be transported to meet metabolic demands. Hemoglobin consists of four polypeptide chains combined with an iron-containing heme group. Hemoglobin, which is red in color, is packed into red blood cells, and allows them to play their role in the transport of oxygen around the body. It is also responsible for the red color of blood.
Carbon dioxide is somewhat more soluble in the plasma than oxygen, but much of it reacts with water in the blood to form carbonic acid, H2CO3, which subsequently dissociates to form bicarbonate ions and free hydrogen ions. This is why increased levels of carbon dioxide increase the acidity (H+ concentration) of the blood. Since carbon dioxide can exist in the blood in these different forms, it also plays a major role in buffering the blood against pH changes, a major aspect of homeostasis.