Carbon dioxide is produced by respiring tissues and must be carried in the blood to the lungs, where it diffuses into the alveoli and is exhaled. It is transported in three ways.1. Dissolved in the plasma (about 5%). A small amount of carbon dioxide dissolves directly in the blood plasma.2. As carbaminohaemoglobin (about 10%). Some carbon dioxide combines with the amine groups of the haemoglobin polypeptide chains inside red blood cells, forming carbaminohaemoglobin. It does not bind to the haem groups, so it does not compete directly with oxygen.3. As hydrogencarbonate ions (about 85%). Most carbon dioxide diffuses into red blood cells, where the enzyme carbonic anhydrase catalyses its reaction with water to form carbonic acid. This dissociates into hydrogen ions and hydrogencarbonate ions:CO2 + H2O → H2CO3 → H+ + HCO3− The hydrogencarbonate ions diffuse out of the red blood cell and are carried in solution in the plasma. To balance the loss of negative charge, chloride ions move into the red blood cell from the plasma. This is the chloride shift. The hydrogen ions stay in the cell and are taken up by haemoglobin.
In the lungs, where the partial pressure of carbon dioxide is low, all of these processes reverse. Hydrogencarbonate ions re-enter the red blood cells, carbon dioxide is re-formed and released from haemoglobin, and it diffuses out of the blood into the alveoli down its concentration gradient.
In the lungs, where the partial pressure of carbon dioxide is low, all of these processes reverse. Hydrogencarbonate ions re-enter the red blood cells, carbon dioxide is re-formed and released from haemoglobin, and it diffuses out of the blood into the alveoli down its concentration gradient.