(N/A) Carbon dioxide $(CO_{2})$ is transported in the blood through three main mechanisms:
$(1)$ Dissolved in Plasma:
Approximately $7\%$ of $CO_{2}$ is carried in a dissolved state through blood plasma. It reacts with water to form carbonic acid $(H_{2}CO_{3})$.
$CO_{2} + H_{2}O \to H_{2}CO_{3}$
Since this reaction is slow,only a small amount is transported this way.
$(2)$ As Carbaminohaemoglobin:
About $20-25\%$ of $CO_{2}$ is transported by red blood cells $(RBCs)$ by binding to the amino groups of haemoglobin to form carbaminohaemoglobin.
$(3)$ As Bicarbonate Ions:
Approximately $70\%$ of $CO_{2}$ is transported as bicarbonate $(HCO_{3}^{-})$. $CO_{2}$ diffuses into $RBCs$ and reacts with water in the presence of the enzyme carbonic anhydrase to form carbonic acid,which then dissociates into bicarbonate and hydrogen ions $(H^{+})$.
$CO_{2} + H_{2}O \xrightarrow{\text{Carbonic anhydrase}} H_{2}CO_{3} \to HCO_{3}^{-} + H^{+}$