Carbon dioxide is transported in the blood primarily in the form of bicarbonate (HCO3-),which is released when carbonic acid dissociates. Bicarbonate can buffer H+, and thus helps to maintain a normal arterial pH. Hypoventilation raises, and hyperventilation lowers, the carbonic acid concentration of the blood.
Carbon dioxide is carried by the blood in three forms: (1) as dissolved CO2—carbon dioxide is about twenty-one times more soluble than oxygen in water, and about one-tenth of the total blood CO2 is dissolved in plasma; (2) as carbaminohemo-globin—about one-fifth of the total blood CO2 is carried attached to an amino acid in hemoglobin (carbaminohemoglobin should not be confused with carboxyhemoglobin, which is a combination of hemoglobin and carbon monoxide); and (3) as bicarbonate ion, which accounts for most of the CO2 carried by the blood.
Carbon dioxide is able to combine with water to form carbonic acid. This reaction occurs spontaneously in the plasma at a slow rate, but it occurs much more rapidly within the red blood cells because of the catalytic action of the enzyme carbonic an-hydrase. Since this enzyme is confined to the red blood cells, most of the carbonic acid is produced there rather than in the plasma. The formation of carbonic acid from CO2 and water is favored by the high PCO2 found in tissue capillaries (this is an example of the law of mass action, described in chapter 4).
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.