Transport of gases
- Blood is the medium of transport for O2 and CO2.
- About 97 per cent of the oxygen is transported by RBCs and the remaining 3 per cent of O2 is carried through the plasma.
- Nearly 20-25 per cent of CO2 is transported by RBCs whereas 70 per cent of it is carried as bicarbonate and about 7 per cent of CO2 is carried in a dissolved state through plasma.
Transport of oxygen
- Haemoglobin is a red coloured iron containing pigment present in the RBCs.
- O2 can bind with haemoglobin in a reversible manner to form oxyhaemoglobin.
- Binding of oxygen with haemoglobin is primarily related to partial pressure of O2 and partial pressure of CO2, hydrogen ion concentration and temperature are the other factors which can interfere with this binding.
- A sigmoid curve is obtained when percentage saturation of haemoglobin with O2 is plotted against the pO2 and the curve is called the oxygen dissociation curve.
- pCO2, H+ concentration have effect on binding of O2 with haemoglobin.
- In the alveoli, where there is high pO2, low pCO2, lesser H+ concentration and lower temperature, the factors are all favourable for the formation of oxyhaemoglobin, and where low pO2, high pCO2, high H+ concentration and higher temperature exist, the conditions are favourable for dissociation of oxygen from the oxyhaemoglobin.
- Every 100 ml of oxygenated blood can deliver around 5 ml of O2 and each haemoglobin molecule can carry a maximum of four molecules of O2.
Fig. formation of haemoglobin
Transport of carbon dioxide
- CO2 is carried by haemoglobin as carbamino-haemoglobin, which is related to the partial pressure of CO2.
- When pCO2 is high and pO2 is low as in the tissues, more binding of carbon dioxide occurs whereas, when the pCO2 is low and pO2 is high as in the alveoli, dissociation of CO2 from carbamino-haemoglobin takes place.
- RBCs contain a very high concentration of the enzyme, carbonic anhydrase, which facilitates the following reaction in both directions.
- At the tissue site where partial pressure of CO2 is high due to catabolism, CO2 diffuses into blood (RBCs and plasma) and forms HCO3 – and H+.
- CO2 trapped as bicarbonate at the tissue level and transported to the alveoli and is released out as CO2.
- Every 100 ml of deoxygenated blood delivers approximately 4 ml of CO2 to the alveoli.