Carbon Dioxide Transport

Overview
  • Gaseous carbon dioxide is produced in large quantities by metabolically-active tissues and diffuses into the blood stream where it is transported to the lungs for elimination. Carbon Dioxide is transported within blood in multiple forms which include transport as a simple dissolved gas, bound by protein, and a chemically-modified form (See: Gases in Liquids). We discuss the relative contributions and mechanisms of these forms of carbon dioxide transport separately below.
Dissolved Gas
  • Roughly 5-10% of carbon dioxide is transported from tissues to the lungs in a simple dissolved form. In this process, metabolically-generated carbon dioxide gas simply diffuses into local capillaries, travels to the lungs via venous blood, and is exhaled .
Chemically-modified
  • Overview
    • Transport of carbon dioxide in a chemically-modified form accounts for nearly 70% of total carbon dioxide transport and is achieved by carbon dioxide conversion to bicarbonate. The processes which are responsible for this transport require multiple steps as detailed below.
  • Mechanism
    • Gaseous CO2 derived from metabolically-active cells diffuses in a dissolved state into the cytosol of erythrocytes present in local capillaries. Erythrocytes possess large amounts of carbonic anhydrase which catalyze reaction of the diffused CO2 with a molecule of H2O to form carbonic acid (H2CO3). It should be pointed out that generation of carbonic acid from carbon dioxide and water is a normally very slow kinetic process and requires catalysis by erythrocyte carbonic anhydrase to achieve rates viable for carbon dioxide transport. Once generated, carbonic acid spontaneously dissociates into a free hydrogen ion (H+) and bicarbonate (HCO3-).
    • The resultant free hydrogen ions appear to be absorbed by the erythrocytic hemoglobin molecule which may promote enhanced oxygen unloading in the periphery due to the Bohr Effect. The resultant bicarbonate ion is transported across the erythrocyte membrane into the plasma in exchange for a chloride ion using an electroneutral Bicarbonate-Cl Antiporter. Bicarbonate then travels in venous plasma to the lungs where the above reactions occur in reverse, leading to generation of gaseous CO2 which is exhaled. Reversal of these reactions in the lungs is enhanced by the presence of high oxygen partial pressures as discussed further in the Haldane Effect.
Protein-bound
  • Transport of carbon dioxide in a protein-bound form accounts for nearly 20% of total carbon dioxide transport and is achieved by reversible binding of carbon dioxide to hemoglobin. The process begins by diffusion of metabolically-generated gaseous CO2 into erythrocytes located in local tissue capillaries. Dissolved CO2 can spontaneously and reversibly react with the free amino-termini in hemoglobin polypeptides, resulting in the generation of "Carbaminohemoglobin" molecules. The carbaminohemoglobin compounds are transported via venous blood within eythrocytes to the pulmonary vasculature where the reaction reverses, releasing a molecule of CO2 that is subsequently exhaled. Release of carbon dioxide bound by hemoglobin is aided by the higher partial pressures of oxygen in the lungs as described in the Haldane Effect.


Transport of Carbon Dioxide in Venous Blood
Carbon dioxide diffuses from peripheral cells into venous blood and is transported to the lungs in three distinct chemical forms. Roughly 10% is transported as a simple dissolved gas. The remainder enters local erythrocytes and encounters one of two fates. A proportion binds directly to hemoglobin, generating carbaminohemoglobin, and is transported within red blood cells to the lung, accounting for 30% of CO2 transport. A much larger proportion is converted to bicarbonate and hydrogen by carbonic anhydrase. The bicarbonate is exported into the venous blood and travels to the lungs in this chemically-modified form of CO2, accounting for 60% of total CO2 transport. The remaining hydrogen ion binds to hemoglobin and enhances oxygen unloading via the Bohr Effect.


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