Question

What are the major transport mechanisms for $CO_{ 2 }$? Explain.

Answer 1

Hint: $CO_{ 2 }$ 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 $CO_{ 2 }$ are:
-In dissolved form through Plasma- About 7 percent of $CO_{ 2 }$ is transported through this mechanism. The $CO_{ 2 }$ 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 $CO_{ 2 }$ is converted into ${ { H{ C }{ O }_{ 3 }^{ - } } }$ and transported into the plasma. Then the $CO_{ 2 }$, 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 ${ { H{ C }{ O }_{ 3 }^{ - } } }$ ions. Carbonic anhydrase is the enzyme that helps to undergo this process.
$C{ O }_{ 2 }+{ H }_{ 2 }{ O }\rightleftharpoons { H }_{ 2 }{ C }{ O }_{ 3 }\rightleftharpoons { H }^{ + }+{ H }{ C }{ O }_{ 3 }^{ - }$
The ${ { H{ C }{ O }_{ 3 }^{ - } } }$ 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 $CO_{ 2 }$ and ${ { H }_{ 2 }{ O } }$ are formed from hydrogen and ${ { H{ C }{ O }_{ 3 }^{ - } } }$ ions.
${ H }{ C }{ O }_{ 3 }^{ - }+{ H }^{ + }\rightleftharpoons { H }_{ 2 }{ C }{ O }_{ 3 }\rightleftharpoons { H }_{ 2 }{ O }+C{ O }_{ 2 }$
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 $CO_{ 2 }$. Finally the $CO_{ 2 }$ diffuses from blood to the lungs, where partial pressure of $CO_{ 2 }$ is low.
-By RBCs as Carbaminohemoglobin (${ { Hb } }C{ O }_{ 2 }$)- 20-25 percent of $CO_{ 2 }$ is transported as Carbaminohaemoglobin. When $CO_{ 2 }$ enters the RBC, it forms a reversible compound ${ { Hb } }C{ O }_{ 2 }$.
$CO_{ 2 }$ binds with the amino group of globin proteins present which are a part of Haemoglobin.
$Hb+{ C }{ O }_{ 2 }\rightleftharpoons { Hb{ C }{ O }_{ 2 } }$

Note: $CO_{ 2 }$ has a higher solubility than oxygen, therefore, only 3 percent of oxygen is transported in dissolved form as compared to 7 percent of $CO_{ 2 }$.
-Carbonic Anhydrase, an enzyme which helps in dissociating Carbonic Acid into hydrogen and ${ { H{ C }{ O }_{ 3 }^{ - } } }$ ions, is present in very high concentration in the RBC and in small quantities in the plasma.