Gas Transport (O₂ Transport & CO₂ Transport)
It is the transport of respiratory gases (O2 & CO2) from alveoli to the systemic tissues and vice versa.
O₂ Transport
- It is the transport of O2 from lungs to various tissues.
- It occurs in 2 ways:
- In physical solution (blood plasma): About 3% of O2 is carried in a dissolved state through plasma.
- As oxyhaemoglobin: About 97% of O2 is transported by haemoglobin (red coloured iron containing pigment) on RBC. O2 binds with haemoglobin (Hb) to form oxyhaemoglobin. This is called oxygenation. Hb has 4 haem units. So, each Hb molecule can carry 4 oxygen molecules. Binding of O2 depends upon pO2, pCO2, H+ ion concentration (pH) and temperature.
- In the alveoli, high pO2, low pCO2, lesser H+ ion concentration and lower temperature exist. These factors are favourable for the formation of oxyhaemoglobin.
- In tissues, low pO2, high pCO2, high H+ ions and high temperature exist. So Hb4O8 dissociates to release O2.
- Every 100 ml of oxygenated blood can deliver around 5 ml of O2 to the tissues under normal physiological conditions.
Oxygen-haemoglobin dissociation curve
- It is a sigmoid curve obtained when percentage saturation of Hb with O2 is plotted against the pO2.
- It is used to study the effect of factors like pCO2, H+ concentration etc., on binding of O2 with Hb.
CO₂ Transport
- It is the transport of CO2 from tissues to lungs.
- In tissues, pCO2 is high due to catabolism and pO2 is low. In lungs, pCO2 is low and pO2 is high. This favours CO2 transport from tissues to lungs. It occurs in 3 ways:
- As carbonic acid: In tissues, 7% of CO2 is dissolved in plasma water to form carbonic acid and carried to lungs.
- As carbamino-haemoglobin: In tissues, 20-25% of CO2 binds to Hb to form carbamino-haemoglobin. In alveoli, CO2 dissociates from carbamino-haemoglobin.
- As bicarbonates: 70% of CO2 transported by this method. RBCs contain an enzyme, carbonic anhydrase. (It is slightly present in plasma too).
- At tissue site, it facilitates the following reactions:
- In alveoli, the above reaction proceeds in opposite direction leading to the formation of CO2 and H2O.
- Every 100 ml of deoxygenated blood delivers about 4 ml of CO2 to the alveoli.
