Transport of Carbon Dioxide

Transport of Carbon Dioxide Forms of Carbon Dioxide Carbon dioxide is carried in the blood in three forms: dissolved, attached to hemoglobin, and converted to bicarbonate ions. Dissolved CO2 accounts for 7–10 percent of the carbon dioxide carried in the blood. This is also the only form of carbon dioxide that diffuses from the tissues into the blood and from the blood into the alveoli for expulsion from the body. Here, we will examine in depth the transfer of carbon dioxide using hemoglobin or by the formation of bicarbonate. Carbaminohemoglobin and Bicarbonate Carbon dioxide can bind to any protein and form a carbamate compound. The protein found in the highest concentration in red blood cells is hemoglobin, and 20–23 percent of the CO2 carried in the blood is bound to hemoglobin in the form of carbaminohemoglobin. In the capillaries of the systemic tissues, CO2molecules attach to the terminal amino acids of the alpha and beta chains of the hemoglobin molecule. Deoxygenated hemoglobin (hemoglobin with no or less than the maximal oxygen bound, abbreviated HHb), such as that found in metabolically active tissues, binds CO2 easily. In the capillaries of the lungs, the elevated levels of oxygen found in alveoli force the carbon dioxide off the hemoglobin molecule and oxidize the protein, freeing up hydrogen ions. Although some carbon dioxide is transported as carbaminohemoglobin, the majority, about 70 percent, is dissolved in the blood as bicarbonate ions that arise from the reversible reactions discussed below. Carbon dioxide in the presence of water can be reversibly converted to carbonic acid. Carbonic acid is not very stable and readily dissociates into a hydrogen ion and a bicarbonate ion. In fact, this is why carbonated beverages are acidic. Carbon dioxide is added to the drink mixture under pressure and dissolves in the beverage. When the CO2 has bubbled out of the beverage, it tastes flat because the acid is gone. The same thing happens in red blood cells, except that red blood cells contain an enzyme called carbonic anhydrase (CA), which is capable of facilitating one million reactions per second per enzyme molecule. Because of the enzyme, most of the CO2 dissolved in the blood is quickly converted to carbonic acid, which breaks down to form, hydrogen ions, and bicarbonate ions. The chemical reaction for this process is the following: CO2- (in the presence of CA) + H2O ⇆ H2CO3 ⇆ H++ HCO3 Where H2O is water, CA is carbonic anhydrase, H2CO3 is carbonic acid, H+ is a hydrogen ion, and HCO3- is a bicarbonate ion. The second part of the reaction, which produces the hydrogen and bicarbonate ions, does not have an enzyme, but depends on the dissociation of the weak acid. This series of reactions provides buffering for the blood. Carbon dioxide production occurs in many tissues, especially muscle. The carbon dioxide produced diffuses from the tissue of origin into a systemic capillary and dissolves in the plasma. A small amount of the carbon dioxide is transported this way. Most of the CO2 that diffused into the plasma diffuses into a red blood cell and reacts with intracellular water molecules to produce hydrogen and bicarbonate ions. Remembering that the reactions are reversible, it makes sense that levels of accumulated products will drive the direction of the reaction sequences. The dissociation of carbonic acid is driven by the relative concentration of carbonic acid compared to the relative levels of bicarbonate(carbonic acid’s conjugate base). A build-up of bicarbonate in the RBCs would slow or halt the dissociation of carbonic acid. This build-up doesn’t usually happen because, RBCs have a membrane channel that allows bicarbonate to leave the RBC and enter the blood plasma. To maintain electric neutrality inside the RBC and in the plasma, every time a negative bicarbonate ion leaves the red blood cell it is exchanged for a negative chloride ion from the plasma. This exchange is called the chloride shift. The bicarbonate ion in the plasma becomes part of the blood’s buffering system, maintaining blood pH within a narrow range. Deviation from this range compromises organ function and can cause death. The hydrogen ion liberated from the conversion of CO2 to bicarbonate binds to a a deoxygenated hemoglobin molecule causing it to become reduced. Deoxygenated hemoglobin easily picks up a molecule of CO2, creating carbaminohemoglobin. Hemoglobin is an important buffering agent for the hydrogen ions produced from the conversion of carbon dioxide to bicarbonate ions. If this buffering did not occur, the intracellular fluid of the red blood cell would become progressively more acidic, resulting in deterioration of cell functions. Some CO2 from the tissues can be found as as bicarbonate ions and dissolved CO2 in the plasma. The remainder of the carbon dioxide is attached to hemoglobin or it is still in the carbonic acid form and will stay in the red blood cells. When the blood enters the pulmonary capillaries, gaseous carbon dioxide in the plasma diffuses into the alveoli. Some of the bicarbonate diffuses from the plasma into the red blood cells, and a chloride ion passes back into the plasma, reversing the chloride shift that occurred in the capillaries in the systemic tissues. The high partial pressure of oxygen in the alveoli causes the carbaminohemoglobin to dissociate into deoxyhemoglobin, a hydrogen ion, and a molecule of carbon dioxide. The released CO2 is available for diffusion. The free hydrogen ion combines with a bicarbonate ion and reforms carbonic acid. The carbonic acid is converted back to carbon dioxide and water under the influence of carbonic anhydrase. .

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