17. BREATHING AND EXCHANGE OF GASES
1. 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.
2. 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.
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