Transport of Gases

Blood is the medium of transport for O₂ and CO₂.

Oxygen:

· About 97 per cent of O₂ is transported by RBCs in the blood.

· The remaining 3 per cent of O₂ is carried in a dissolved state through the plasma.

Carbon dioxide:

· Nearly 20-25 per cent of CO₂ is transported by RBCs whereas 70 per cent of it is carried as bicarbonate.

· About 7 per cent of CO₂ is carried in a dissolved state through plasma.

Transport of Oxygen:

Haemoglobin:

· Hemoglobin is a red coloured iron containing pigment present in the RBCs.

· O₂ can bind with haemoglobin in a reversible manner to form oxyhaemoglobin.

· Each haemoglobin molecule can carry a maximum of four molecules of O₂.

· Binding of oxygen with haemoglobin is primarily related to partial pressure of O₂.

· Partial pressure of CO₂, hydrogen ion concentration and temperature are the other factors which can interfere with this binding.

Oxygen Dissociation Curve:

A sigmoid curve is obtained when percentage saturation of haemoglobin with O₂ is plotted against the pO₂. This curve is called the Oxygen dissociation curve and is highly useful in studying the effect of factors like pCO₂, H⁺ concentration, etc., on binding of O₂ with haemoglobin.

Oxygen dissociation curve

In Alveoli:

 In the alveoli, where there is

o High pO₂

o Low pCO₂

o Lesser H⁺ concentration

o Lower temperature

 The above factors are all favorable for the formation of oxyhaemoglobin,

In Tissues:

In the tissues, where

o Low pO₂

o High pCO₂

o High H⁺ concentration

o Higher temperature exist

The conditions are favorable for dissociation of oxygen from the oxyhaemoglobin.

This clearly indicates that O₂ gets bound to haemoglobin in the lung surface and gets dissociated at the tissues.

Every 100 ml of oxygenated blood can deliver around 5 ml of O₂ to the tissues under normal physiological conditions.

Transport of Carbon dioxide:

CO₂ is carried by haemoglobin as carbamino-haemoglobin (about 20-25 per cent). This binding is related to the partial pressure of CO₂. pO₂ is a major factor which could affect this binding.

In Tissues:

When pCO₂ is high and pO₂ is low as in the tissues, more binding of carbon dioxide occurs

In Alveoli:

· Whereas, when the pCO₂ is low and pO₂ is high as in the alveoli, dissociation of CO₂ from carbamino-haemoglobin takes place, i.e., CO₂ which is bound to haemoglobin from the tissues is delivered at the alveoli.

· RBCs contain a very high concentration of the enzyme, carbonic anhydrase and a minute quantity of the same is present in the plasma too.

· This enzyme 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 HC and H⁺.

· At the alveolar site where pCO₂ is low, the reaction proceeds in the opposite direction leading to the formation of CO₂ and H₂O.

· Thus, CO₂ trapped as bicarbonate at the tissue level and transported to the alveoli is released out as CO₂.

· Every 100 ml of deoxygenated blood delivers approximately 4 ml of CO₂ to the alveoli.