Carbon Dioxide Transport!

Today, I would like to share with you about the mechanism of transporting carbon dioxide from respiring tissues into the lung. It is quiet complex but very efficient.

1.     About 7% of carbon dioxide from respiring cells diffuses into blood plasma and interstitial fluid. Another 23% binds to amino groups of haemoglobin to form carbaminohaemoglobin (HbCO2). Another 70% is transported as bicarbonate ion (HCO3-).

2.     Firstly, CO2 diffuses into blood plasma, interstitial fluid and red blood cell. 7% of CO2 carry out by blood plasma, 23% binds to amino group of haemoglobin to form carbaminohaemoglobin, HbCO2 while the rest 70% reacts with water in red blood cell to form carbonic acid, H2CO3.

3.     Then, carbonic acid (H2CO3) dissociates into hydrogen ion, H+ and bicarbonate ion, HCO3- catalyzed by carbonic anhydrase. The hydrogen ion, H+ binds to haemoglobin forming haemoglobinic acid, HHb. Hence, haemoglobin can act as buffer to lower the pH of blood.

4.     While the bicarbonate ion, HCO3- moves out of the red blood cell. The movement of bicarbonate ion, HCO3- out of the cell causing the exchange of one chloride ion for a bicarbonate ion known as chloride shift. This move in and out maintains the electrochemical-neutrality of the cell.

5.     In lungs, the 7% of dissolved CO2 in plasma diffuse into alveolus. While the 23% of CO2 that binds to amino group of haemoglobin forming carbaminohaemoglobin splits into CO2 and haemoglobin and CO2 diffuses out into the alveolus. The haemoglobinic acid dissociates into hydrogen ion, H+ and haemoglobin. The bicarbonate ion, HCO3- reenters the cell and binds with the hydrogen ion, H+ to form carbonic acid H2C03. Then, the carbonic acid H2CO3 dissociates to form water and carbon dioxide, CO2 catalyzed by carbonic anhydrase enzyme. Carbon dioxide, CO2 diffuses out into the alveolus. Process repeats.