Describe in detail the transport of carbon dioxide as bicarbonate. What happens in when carbon dioxide enters red blood cells in the capillaries of body tissues and what happens when carbon dioxide exits the red blood cells in the capillaries of the lungs?

Carbon dioxide is transported in three forms. 7% is in the dissolved form in the plasma, 70% is in the form of bicarbonate ions and 20-25% as carbaminohemoglobin, HbCO2.

Explanation:

Transport as bicarbonate ions: 70% of the CO2 is converted into bicarbonate ion and transported in plasma.

Near the tissues, as the partial pressure of CO2 is high so it diffuses into the blood from the tissues. With in the RBC it reacts with water to form carbonic acid i,e. H2CO3. As carbonic acid is unstable, it immediately dissociates into H+ and HCO3- ions. Carbonic anhydrase in the enzyme present with in the RBC which takes part in these reactions. The carbonate ion formed in the RBC quickly diffuses into the plasma and is transported to the lungs in the form of sodium carbonate.

Near the alveoli, the process is reversed that is, bicarbonate ion reenter the RBC and combines with H+ ion to form carbonic acid which then quickly splits into CO2 and H2O in the presence of carbonic anhydrase enzyme. Finally CO2 is diffused from blood to lungs where partial pressure of CO2 is low.