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Blood flow to systemic venule

Figure

Carbon dioxide produced by tissue cells is transported in the blood in a dissolved state, combined with hemoglobin or in the form of bicarbonate ions (HCO3-).

compete for binding sites—a hemoglobin molecule can transport both gases at the same time.

Carbon dioxide combining with hemoglobin forms a loosely bound compound called carbaminohemoglobin (kar-bam" i-no-he"mo-glo"bin). This molecule readily decomposes in regions where the PCO2 is low, releasing its carbon dioxide. Although this method of transporting carbon dioxide is theoretically quite effective, carbaminohemoglobin forms relatively slowly. Only about 15%-25% of the total carbon dioxide is carried this way.

The most important carbon dioxide transport mechanism involves the formation of bicarbonate ions (HCO3-). Recall that carbon dioxide reacts with water to form carbonic acid (H2CO3). This reaction occurs slowly in the blood plasma, but much of the carbon dioxide diffuses into the red blood cells. These cells contain an enzyme, carbonic anhydrase (kar-bon'ik an-hi'dras), which speeds the reaction between carbon dioxide and water.

The resulting carbonic acid dissociates almost immediately, releasing hydrogen ions (H+) and bicarbonate ions (HCO3-):

These new hydrogen ions might be expected to lower blood pH, but this reaction occurs in the systemic capillaries, where deoxyhemoglobin is generated. Deoxyhe-moglobin is an excellent buffer because hydrogen ions combine readily with it. The bicarbonate ions diffuse out of the red blood cells and enter the blood plasma. At

Capillary wall Red blood cell

Plasma

Capillary wall Red blood cell

Plasma

Figure

As bicarbonate ions (HCO3-) diffuse out of the red blood cell, chloride ions (Cl-) from the plasma diffuse into the cell, thus maintaining the electrical balance between ions. This exchange of ions is called the chloride shift.

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