Essay on Transportation of Gases in Blood!
(I) Oxygen Transport:
Each decilitre of blood carries 19.8 ml of O2 of which 4.6 ml diffuses into tissues. 3% is transported dissolved in plasma because water has poor solubility for O2. Hence, blood plasma is a poor carrier of this gas and 97% is carried by RBCs. Four Fe2+ ions of each haemoglobin can bind with 4 molecules of O2 and it is carried as oxyhaemoglobin.
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Oxyhaemoglobin dissociates near tissues due to increase in acidity & decrease in pH. It can also be caused due to high temperature. Arterial blood carries about 20 ml of O2/100 ml of blood. Only 25% of O2 carried by the blood is reversed in tissue.
(II) Carbon dioxide Transport:
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Transport of CO2 by blood is much easier than that of oxygen due to high solubility of C02 in water (about 20 times that of O2). Each decilitre of blood carries about 3.7 ml of CO2. About 7% is transported dissolved in plasma.
About 23% loosely binds with haemoglobin as carbaminohaemoglobin in the form of bicarbonates and about 70% reacts with water forming carbonic acid in erythrocytes in the presence of enzyme carbonic anhydrase.
The carbonic acid (H2CO3) dissociates into H+ and HCO3- ions. (Concentration of carbonic acid does not increase in blood due to presence of sodium.)
So to maintain electrostatic neutrality of plasma, HCO3– ions diffuse out into plasma and CI ions enter into the RBCs. The chloride content of RBC increases when oxygenated blood becomes deoxygenated.
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This is known as “chloride shift” or “Hamburger shift”. Because of it, the Cl- content of the red cells in venous blood is therefore significantly greater than in arterial blood. The chloride shift occurs rapidly & is essentially complete in 1 second.
Fate of CO2 in blood
In plasma:
1. Dissolved
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2. Formation of carbamino compounds with plasma protein
3. Hydration, H+ buffered, HCO3– in plasma
In red blood cells:
1. Dissolved
2. Formation of carbamino-Hb
3. Hydration, H+ buffered, 70% of HCO3– enters the plasma
4. CP shifts into cells
Bohr’s Effect:
A rise in pCO2 or fall in pH decreases oxygen affinity of haemoglobin, raising the P50 value and shifts the curve to right. This is called Bohr’s effect. Conversely a fall in pCO2 and rise in pH increases oxygen affinity of haemoglobin. (P50 value is the value of pO2 at which haemoglobin is 50% saturated with oxygen to form oxyhaemoglobin).
Haldane’s Effect:
It is related to the transport of CO2 in the blood. It is based on the simple fact that oxyhaemoglobin behaves as strong acid and releases an excess of H+ ions which bind with bicarbonate (HCO, ) ions to form H2CO3 which dissociates into H2O and CO2. Secondly, due to the increased acidity, CO2 loses the power to combine with haemoglobin and form carbamino-haemoglobin. Effect of oxyhaemoglobin formation or dissociation on CO2 transport is called Haldane’s effect.
Oxygen Haemoglobin Dissociation Curve:
The relationship between the pO2 and percent saturation of haemoglobin when represented on a graph is termed as oxygen-haemoglobin dissociation curve. It is sigmoid in shape. The percentage of haemoglobin that is bound with oxygen is called percent saturation of haemoglobin.
The pO2 in arterial blood is about 95 mm Hg and percent saturation of haemoglobin at this partial pressure is 97%. 100% saturation of haemoglobin with O2 takes place at pO2 of 140 mm Hg.