Plasma and red blood cells transport carbon dioxide. This is because they are readily soluble in water.

(1) Through plasma:

About 7% of CO2 is carried in a dissolved state through plasma. Carbon dioxide combines with water and forms carbonic acid. 

                                                      ${\mathrm{CO}}_2 + {\mathrm{H}}_2\mathrm{O} \to {\mathrm{H}}_2{\mathrm{CO}}_3$
$\left(\mathrm{Carbonic} \mathrm{acid}\right)$

Since the process of forming carbonic acid is slow, only a small amount of carbon dioxide is carried this way.

(2) Through RBCs:

About 20 – 25% of ${\mathrm{CO}}_2$ is transported by the red blood cells as carbaminohaemoglobin. Carbon dioxide binds to the amino groups on the polypeptide chains of hemoglobin and forms a compound known as carbaminohaemoglobin.

(3) Through sodium bicarbonate:

About 70% of carbon dioxide is transported as sodium bicarbonate. As CO2 diffuses into the blood plasma, a large part of it combines with water to form carbonic acid in the presence of the enzyme carbonic anhydrase. Carbonic anhydrase is a zinc enzyme that speeds up the formation of carbonic acid. This carbonic acid dissociates into bicarbonate (${{\mathrm{HCO}}_3}^{-}$) and hydrogen ions (${\mathrm{H}}^{+}$).

                                                ${\mathrm{CO}}_2 + {\mathrm{H}}_2\mathrm{O} \stackrel{\mathrm{Carbonic} \mathrm{anhydrase}}{\to } {\mathrm{H}}_2{\mathrm{CO}}_3$
${\mathrm{H}}_2{\mathrm{CO}}_3 \underset{\mathrm{anhydrase}}{\overset{\mathrm{Carbonic}}{\to }} {{\mathrm{HCO}}_3}^{-} + {\mathrm{H}}^{+}$