As a result of catalysis by carbonic anhydrase within the red blood cells, large amounts of carbonic acid are produced as blood passes through the systemic capillaries. The buildup of carbonic acid concentrations within the red blood cells favors the dissociation of these molecules into hydrogen ions (protons, which contribute to the acidity of a solution) and HCO3- (bicarbonate), as shown by this equation:
The hydrogen ions (H+) released by the dissociation of carbonic acid are largely buffered by their combination with de-oxyhemoglobin within the red blood cells. Although the unbuffered hydrogen ions are free to diffuse out of the red blood cells, more bicarbonate diffuses outward into the plasma than does H+. As a result of the "trapping" of hydrogen ions within the red blood cells by their attachment to hemoglobin and the outward diffusion of bicarbonate, the inside of the red blood cell gains a net positive charge. This attracts chloride ions (Cl-), which move into the red blood cells as HCO3- moves out. This exchange of anions as blood travels through the tissue capillaries is called the chloride shift (fig. 16.38).
The unloading of oxygen is increased by the bonding of H+ (released from carbonic acid) to oxyhemoglobin. This is the Bohr effect, and results in increased conversion of oxyhemoglo-bin to deoxyhemoglobin. Now, deoxyhemoglobin bonds H+ more strongly than does oxyhemoglobin, so the act of unloading its oxygen improves the ability of hemoglobin to buffer the H+ released by carbonic acid. Removal of H+ from solution by combining with hemoglobin (through the law of mass action), in turn, favors the continued production of carbonic acid and thereby improves the ability of the blood to transport carbon dioxide. Thus, carbon dioxide increases oxygen unloading, and oxygen unloading increases carbon dioxide transport.
When blood reaches the pulmonary capillaries (fig. 16.39), deoxyhemoglobin is converted to oxyhemoglobin. Since oxyhe-moglobin has a weaker affinity for H+ than does deoxyhemoglo-bin, hydrogen ions are released within the red blood cells. This attracts HCO3- from the plasma, which combines with H+ to form carbonic acid:
Under conditions of lower PCO2, as occurs in the pulmonary capillaries, carbonic anhydrase catalyzes the conversion of carbonic acid to carbon dioxide and water:
carbonic anhydrase h2co3->- CO2 + h2o low P
Red blood cells
Red blood cells
■ Figure 16.38 Carbon dioxide transport and the chloride shift.
Carbon dioxide is transported in three forms: as dissolved CO2 gas, attached to hemoglobin as carbaminohemoglobin, and as carbonic acid and bicarbonate. Percentages indicate the proportion of CO2 in each of the forms. Notice that when bicarbonate (HCO3-) diffuses out of the red blood cells, Cl-diffuses in to retain electrical neutrality. This exchange is the chloride shift.
In summary, the carbon dioxide produced by the cells is converted within the systemic capillaries, mostly through the action of carbonic anhydrase in the red blood cells, to carbonic acid. With the buildup of carbonic acid concentrations in the red blood cells, the carbonic acid dissociates into bicarbonate and H+, which results in the chloride shift. A reverse chloride shift operates in the pulmonary capillaries to convert carbonic acid to H2O and CO2 gas, which is eliminated in the expired breath (fig. 16.39). The PCo2, carbonic acid, H+, and bicarbonate concentrations in the systemic arteries are thus maintained relatively constant by normal ventilation. This is required to maintain the acid-base balance of the blood (fig. 16.40), as discussed in chapter 13 and in the next section.
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