TRANSPORT OF CARBON DIOXIDE

TRANSPORT OF CARBON DIOXIDE

Carbon dioxide is transported by the blood from cells to the alveoli.

Carbon dioxide is transported in the blood in four ways:

1. As dissolved form (7%)

2. As carbonic acid (negligible)

3. As bicarbonate (63%)

4. As carbamino compounds (30%).

AS DISSOLVED FORM

Carbon dioxide diffuses into blood and dissolves in the fluid of plasma forming a simple solution. Only about 3 mL/100 mL of plasma of carbon dioxide is transported as dissolved state. It is about 7% of total carbon dioxide in the blood.

AS CARBONIC ACID

Part of dissolved carbon dioxide in plasma combines with the water to form carbonic acid. Transport of

carbon dioxide in this form is negligible.

AS BICARBONATE

About 63% of carbon dioxide is transported as bicarbonate. From plasma, carbon dioxide enters the

RBCs. In the RBCs, carbon dioxide combines with water to form carbonic acid. The reaction inside RBCs

is very rapid because of the presence of carbonic anhydrase. This enzyme accelerates the reaction.

Carbonic anhydrase is present only inside the RBCs and not in plasma. That is why carbonic acid formation is at least 200 to 300 times more in RBCs than in plasma.

Carbonic acid is very unstable. Almost all carbonic acid (99.9%) formed in red blood corpuscles, dissociates into bicarbonate and hydrogen ions. Concentration of bicarbonate ions in the cell increases more and more.Due to high concentration, bicarbonate ions diffuse through the cell membrane into plasma.

Chloride Shift or Hamburger Phenomenon

Chloride shift or Hamburger phenomenon is the exchange of a chloride ion for a bicarbonate ion across

RBC membrane. It was discovered by Hartog Jakob Hamburger in 1892.

Chloride shift occurs when carbon dioxide enters the blood from tissues. In plasma, plenty of sodium chloride is present. It dissociates into sodium and chloride ions. When the negatively charged bicarbonate

ions move out of RBC into the plasma, the negatively charged chloride ions move into the RBC in order to

maintain the electrolyte equilibrium (ionic balance). Anion exchanger 1 (band 3 protein), which acts

like antiport pump in RBC membrane is responsible for the exchange of bicarbonate ions and chloride

ions. Bicarbonate ions combine with sodium ions in the plasma and form sodium bicarbonate. In this form, it is transported in the blood. Hydrogen ions dissociated from carbonic acid are buffered by hemoglobin inside the cell.

Reverse Chloride Shift

Reverse chloride shift is the process by which chloride ions are moved back into plasma from RBC shift. It

occurs in lungs. It helps in elimination of carbon dioxide from the blood. Bicarbonate is converted back

into carbon dioxide, which has to be expelled out. It takes place by the following mechanism:

When blood reaches the alveoli, sodium bicarbonate in plasma dissociates into sodium and bicarbonate

ions. Bicarbonate ion moves into the RBC. It makes chloride ion to move out of the RBC into the plasma, where it combines with sodium and forms sodium chloride. Bicarbonate ion inside the RBC combines with

hydrogen ion forms carbonic acid, which dissociates into water and carbon dioxide. Carbon dioxide is then expelled out.

AS CARBAMINO COMPOUNDS

About 30% of carbon dioxide is transported as carbamino compounds. Carbon dioxide is transported in

blood in combination with hemoglobin and plasma proteins. Carbon dioxide combines with hemoglobin to

form carbamino hemoglobin or carbhemoglobin. And it combines with plasma proteins to form carbamino

proteins. Carbamino hemoglobin and carbamino proteins are together called carbamino compounds.

Carbon dioxide combines with proteins or hemoglobin with a loose bond so that, carbon dioxide is

easily released into alveoli, where the partial pressur of carbon dioxide is low. Thus, the combination of

carbon dioxide with proteins and hemoglobin is a reversible one. Amount of carbon dioxide transported

in combination with plasma proteins is very less compared to the amount transported in combination with

hemoglobin. It is because the quantity of proteins in plasma is only half of the quantity of hemoglobin.

CARBON DIOXIDE DISSOCIATION CURVE

Carbon dioxide is transported in blood as physical solution and in combination with water, plasma

proteins and hemoglobin. The amount of carbon dioxide combining with blood depends upon the partial

pressure of carbon dioxide. Carbon dioxide dissociation curve is the curve that demonstrates the relationship between the partial pressure of carbon dioxide and the quantity of carbon

dioxide that combines with blood.

Normal Carbon Dioxide Dissociation Curve

Normal carbon dioxide dissociation curve shows that the carbon dioxide content in the blood is 48 mL%

when the partial pressure of carbon dioxide is 40 mm Hg and it is 52 mL% when the partial pressure of

carbon dioxide is 48 mm Hg. Carbon dioxide content becomes 70 mL% when the partial pressure is about 100 mm Hg

 

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