Blood is the medium of transport for \(\mathrm{O}_2\) and \(\mathrm{CO}_2\). About 97 per cent of \(\mathrm{O}_2\) is transported by RBCs in the blood. The remaining 3 per cent of \(\mathrm{O}_2\) is carried in a dissolved state through the plasma. Nearly 20-25 per cent of \(\mathrm{CO}_2\) is transported by RBCs whereas 70 per cent of it is carried as bicarbonate. About 7 per cent of \(\mathrm{CO}_2\) is carried in a dissolved state through plasma.

Transport of Oxygen

Haemoglobin is a red coloured iron containing pigment present in the RBCs. \(\mathrm{O}_2\) can bind with haemoglobin in a reversible manner to form oxyhaemoglobin. Each haemoglobin molecule can carry a maximum of four molecules of \(\mathrm{O}_2\). Binding of oxygen with haemoglobin is primarily related to partial pressure of \(\mathrm{O}_2\). Partial pressure of \(\mathrm{CO}_2\), 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 \(\mathrm{O}_2\) is plotted against the \(\mathrm{pO}_2\). This curve is called the Oxygen dissociation curve (Figure 14.5) and is highly useful in studying the effect of factors like \(\mathrm{pCO}_2, \mathrm{H}^{+}\)concentration, etc., on binding of \(\mathrm{O}_2\) with haemoglobin. In the alveoli, where there is high \(\mathrm{pO}_2\), low \(\mathrm{pCO}_2\), lesser \(\mathrm{H}^{+}\)concentration and lower temperature, the factors are all favourable for the formation of oxyhaemoglobin, whereas in the tissues, where low \(\mathrm{pO}_2\), high \(\mathrm{pCO}_2\), high \(\mathrm{H}^{+}\) concentration and higher temperature exist, the conditions are favourable for dissociation of oxygen from the oxyhaemoglobin. This clearly indicates that \(\mathrm{O}_2\) 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 \(\mathrm{O}_2\) to the tissues under normal physiological conditions.

Transport of Carbon dioxide

\(\mathrm{CO}_2\) is carried by haemoglobin as carbamino-haemoglobin (about 20-25 per cent). This binding is related to the partial pressure of \(\mathrm{CO}_2\). \(\mathrm{pO}_2\) is a major factor which could affect this binding. When \(\mathrm{pCO}_2\) is high and \(\mathrm{pO}_2\) is low as in the tissues, more binding of carbon dioxide occurs whereas, when the \(\mathrm{pCO}_2\) is low and \(\mathrm{pO}_2\) is high as in the alveoli, dissociation of \(\mathrm{CO}_2\) from carbamino-haemoglobin takes place, i.e., \(\mathrm{CO}_2\) 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 minute quantities of the same is present in the plasma too. This enzyme facilitates the following reaction in both directions.

\(
\mathrm{CO}_2+\mathrm{H}_2 \mathrm{O} \stackrel{\begin{array}{c}
\text { Carbonic } \\
\text { anhydrase }
\end{array}}{\rightleftarrows} \mathrm{H}_2 \mathrm{CO}_3 \stackrel{\begin{array}{c}
\text { Carbonic } \\
\text { anhydrase }
\end{array}}{\rightleftarrows} \mathrm{HCO}_3^{-}+\mathrm{H}^{+}
\) (Figure 14.4)

At the tissue site where partial pressure of \(\mathrm{CO}_2\) is high due to catabolism, \(\mathrm{CO}_2\) diffuses into blood (RBCs and plasma) and forms \(\mathrm{HCO}_3^{-}\) and \(\mathrm{H}^{+}\). At the alveolar site where \(\mathrm{pCO}_2\) is low, the reaction proceeds in the opposite direction leading to the formation of \(\mathrm{CO}_2\) and \(\mathrm{H}_2 \mathrm{O}\). Thus, \(\mathrm{CO}_2\) trapped as bicarbonate at the tissue level and transported to the alveoli is released out as \(\mathrm{CO}_2\) (Figure 14.4). Every 100 ml of deoxygenated blood delivers approximately 4 ml of \(\mathrm{CO}_2\) to the alveoli.

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