Transport of CO2 IDissolved in Plasma ICarbaminohemoglobin I Bicarbonate I Chloride Shift I Haldane
Transport of CO2 in blood:
Carbon dioxide is produced as a result of complete oxidation of nutrients in the mitochondria. The amount of CO2 generated is dependent on the metabolic rate of the organism and the relative composition of the nutrients viz. carbohydrates, proteins and lipids. CO2 is carried from tissues to lung for exhalation in three ways;
Dissolved in plasma
Buffered with water as carbonic acid or in the form of HCO3-
Bound to proteins or in the form of carbaminohemoglobin.
Dissolved in plasma:
Carbon dioxide is 20 times more soluble in plasma than oxygen.
However less than 5% of the CO2 is carried in plasma due to 1) low solubility (solubility coefficient=0.0308 mmol/litre/mm Hg) of carbon dioxide in plasma & 2) lack of carbonic anhydrase enzyme in the plasma.
The solubility of CO2 in plasma is inversely proportional to temperature. i.e. as temperature increases solubility decreasesBuffered with water as carbonic acid:
Carbon dioxide combines with water in presence of carbonic anhydrase from RBC’s to produce carbonic acid as
H2O + CO2 ↔ H2CO3 ↔ HCO3- + H+
The carbonic anhydrase is present in kidneys, eyes and brain in addition to RBC’s.
The carbonic acid readily dissociates to bicarbonate ion such that the ratio of H2CO3 to HCO3- is 1:20.
Carbon dioxide diffuses easily from plasma to RBC’s and vice a versa but H+ and HCO3- cannot. There will be an increase in the osmolarity coupled with increase in cell and lysis of cell if H+ and HCO3- is not maintained. So also increased H+ ion concentration prevents formation of HCO3-. Hydrogen ions bind with and are removed from the plasma by reduced hemoglobin i.e. Hb from oxygen is released. Reduced Hb is less acidic than the oxygenated Hb. This is Haldane effect which states that at any given pCO2, the CO2 content of deoxygenated blood is always more than that of oxygenated blood.
The bicarbonate ion diffuses out of the RBC’s while chloride ion moves from plasma into the RBC’s in a phenomenon called chloride shift or Gibbs-Donnan equilibrium or Hamburger effect.
The antiporter anion exchange transporter Band-3 is responsible for chloride shift.
Bound to proteins or in the form of carbaminohemoglobin:
Carbon dioxide combines with the uncharged terminal amino group of hemoglobin or other plasma proteins to form carbamino compounds as follows;
R-NH2 + CO2 ↔ RNH-CO2 + H+
About 30% of the exhaled CO2 is carried in the form of carbaminohemoglobin.
The transport of CO2 in the form of carbaminoHb is non-significant in teleosts since there is acetylation of terminal –NH2 group to which CO2 binds.
The acetylation of terminal amino group is absent in elasmobranchs.
Lampreys lack transmembrane anion exchange transporter Band 3 in their membranes as a result, RBC’s of lamprey have high CO2 than other vertebrates.
CO2 binds with terminal amino groups of β-chains where they are in competition with organic phosphates for binding sites.
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