Blood is the medium of transport of $\mathrm{O}_2$ and $\mathrm{CO}_2$. Nearly $20-25 \%$ of $\mathrm{CO}_2$ is transported by RBCs as carbaminohaemoglobin whereas $70 \%$ of it is carried as bicarbonate through plasma. About $7 \%$ of $\mathrm{CO}_2$ is carried in dissolved state through plasma. The largest fraction of $\mathrm{CO}_2$ is converted to bicarbonate ions $\left(\mathrm{HCO}_3^{-}\right)$and transported in plasma. When $\mathrm{CO}_2$ diffuses into the RBCs, it combines with $\mathrm{H}_2 \mathrm{O}$, forming carbonic acid $\left(\mathrm{H}_2 \mathrm{CO}_3\right) . \mathrm{H}_2 \mathrm{CO}_3$ is unstable and quickly dissociates into hydrogen ions and bicarbonate ions.