 Dear students, in this topic we shall discuss the transport of carbon dioxide in blood. You know that cells release carbon dioxide generated by the cellular metabolism into the intracellular spaces. Cells say carbon dioxide intracellular spaces may dissolve in the tissue fluid which surrounds the cells. This dissolved carbon dioxide then diffuses into the blood capillaries, where it enters the transport system. Carbon dioxide is transported in the blood from tissues to the capillaries of the lungs. The final route is to come to the lungs capillaries from where it has to diffuse out to the environment. This transport of carbon dioxide occurs in three different forms as carboxyhemoglobin by binding two plasma proteins and as bicarbonate ions. So there are three methods to transport carbon dioxide into the blood. We will discuss these three methods in detail one by one. The first we shall discuss the transport of carbon dioxide as carboxyhemoglobin. Hemoglobin's protein, that is, globin protein, its amino group binds carbon dioxide to it and as a result carboxyhemoglobin is formed. About 23% of carbon dioxide is carried in this bound form with the hemoglobin. 7% carbon dioxide, blood may be binded with plasma proteins and transported while the major portion that is 70% of carbon dioxide is carried in the plasma as bicarbonate ions. How do bicarbonate ions form? We will discuss this in detail. When carbon dioxide is included in the blood in the capillaries of the plasma, it combines with water to form carbonic acid. Carbon dioxide is added with water to form carbonic acid. This carbonic acid is an unstable compound. So it splits quickly and it ionizes to produce hydrogen ions and bicarbonate ions. So till the blood comes out of the tissue, the carbon dioxide is mostly bicarbonate ions. So the carbon dioxide throughout its circulation is in the form of bicarbonate ions. In the lungs, all these reactions occur in the reverse order. That is, bicarbonate ions combine with hydrogen ions to form the carbonic acid. The carbonic acid splits into water and carbon dioxide and the carbon dioxide thus generated in the lungs diffuses out from the capillaries to the alveolar space from where it is delivered to the environment through exhaled air.