Question

What are the major transport mechanisms for $C{O}_2$? Explain.

Answer 1

Hint: $C{O}_2$ is highly soluble in the plasma and has high partial pressure in the tissues and low partial pressure in the lungs, allowing its movement in 3 different mechanisms. One mechanism occurs in the fluid component of the blood that lacks clotting factors.

Complete answer:
Major transport mechanisms for $C{O}_2$ are:
-In dissolved form through Plasma- About 7 percent of $C{O}_2$ is transported through this mechanism. The $C{O}_2$ gets dissolved in the blood plasma and is transported in the form of a solution to the lungs.
-As Bicarbonate ion- About 70 percent of $C{O}_2$ is converted into $HC{O}_3^{-}$ and transported into the plasma. Then the $C{O}_2$, diffuses in the RBC, binds with water and forms Carbonic Acid, $H_{2}C{O}_3$. Carbonic acid is unstable so it quickly dissociates into hydrogen and $HC{O}_3^{-}$ ions. Carbonic anhydrase is the enzyme that helps to undergo this process.
$C{O}_2+H_{2}O\rightleftharpoons H_{2}C{O}_3\rightleftharpoons H^{+}+HC{O}_3^{-}$
The $HC{O}_3^{-}$ ions formed in RBCs very quickly diffuse into plasma, from where they are carried to the lungs. In the lungs, at the alveolar site, the process gets reversed, and $C{O}_2$ and $H_{2}O$ are formed from hydrogen and $HC{O}_3^{-}$ ions.
$HC{O}_3^{-}+H^{+}\rightleftharpoons H_{2}C{O}_3\rightleftharpoons H_{2}O+C{O}_2$
The Bicarbonate ions enter the RBCs again and bind with hydrogen ions to form$H_{2}C{O}_3$, which quickly splits into $H_{2}O$ and $C{O}_2$. Finally the $C{O}_2$ diffuses from blood to the lungs, where partial pressure of $C{O}_2$ is low.
-By RBCs as Carbaminohemoglobin ($HbC{O}_2$)- 20-25 percent of $C{O}_2$ is transported as Carbaminohaemoglobin. When $C{O}_2$ enters the RBC, it forms a reversible compound $HbC{O}_2$.
$C{O}_2$ binds with the amino group of globin proteins present which are a part of Haemoglobin.
$Hb+C{O}_2\rightleftharpoons HbC{O}_2$

Note: $C{O}_2$ has a higher solubility than oxygen, therefore, only 3 percent of oxygen is transported in dissolved form as compared to 7 percent of $C{O}_2$.
-Carbonic Anhydrase, an enzyme which helps in dissociating Carbonic Acid into hydrogen and $HC{O}_3^{-}$ ions, is present in very high concentration in the RBC and in small quantities in the plasma.