Porth's Essentials of Pathophysiology, 4e

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Disorders of Fluid, Electrolyte, and Acid–Base Balance

C h a p t e r 8

Transport

CO 2

3

Hemoglobin. The remaining CO 2 in the red blood cells combines with hemoglobin to form carbaminohe- moglobin (HbCO 2 ). The combina- tion of CO 2 with hemoglobin is a reversible reaction characterized by a loose bond, so that CO 2 can be easily released in the alveolar capil- laries and exhaled from the lung.

CO 2

dissolved

in plasma

CO 2

HbCO 2

Red blood cell

Hemoglobin (Hb)

20% carried as carbaminohemoglobin (HbCO 2 )

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phosphoric acid). The difference between the two types of acids arises because H 2 CO 3 is in equilibrium with dis- solved CO 2 , which is volatile and leaves the body by way of the lungs. The nonvolatile or fixed acids are not eliminated by the lungs. Instead, they are buffered by body proteins or extracellular buffers, such as HCO 3 – , and then eliminated by the kidney. Carbon Dioxide and Bicarbonate Production. Carbon dioxide, which is the end product of aerobic metabolism, is transported in the circulation as a dissolved gas (i.e., PCO 2 ), as the HCO 3 – ion, or as CO 2 bound to hemo- globin in carbaminohemoglobin (see understanding car- bon dioxide transport). Collectively, dissolved CO 2 and HCO 3 – account for approximately 77% of the CO 2 that is transported in the extracellular fluid; the remaining CO 2 travels as carbaminohemoglobin. 2 Although CO 2 is a gas and not an acid, a small percentage of the gas combines with water to form the weak H 2 CO 3 acid. The reaction that generates H 2 CO 3 from CO 2 and water is catalyzed by an enzyme called carbonic anhydrase , which is present in large quantities in red blood cells, renal tubular cells, and other tissues in the body. (see Understanding: Carbon Dioxide Transport) Because it is almost impossible to measure H 2 CO 3 , carbon dioxide measurements are commonly used when calculating pH. The H 2 CO 3 content of the blood can be calculated by multiplying the partial pressure of CO 2 (PCO 2 ) by its solubility coefficient, which is 0.03. This means that the concentration of H 2 CO 3 in the arterial blood, which normally has a PCO 2 of approximately 40 mm Hg, is 1.20 mEq/L (40 × 0.03 = 1.20), and that for venous blood, which normally has a PCO 2 of approximately 45 mm Hg, is 1.35 mEq/L.

Production of Nonvolatile Acids and Bases. The metabolism of dietary proteins and other substances results in the generation of nonvolatile acids and bases. 4 For example, the metabolism of sulfur-containing amino acids (e.g.,methonine and cysteine) results in the produc- tion of sulfuric acid ; of arginine and lysine, hydrochloric acid ; and of nucleic acids, phosphoric acid . Incomplete oxidation of glucose results in the formation of lactic acid, and incomplete oxidation of fats, the production of ketoacids . The major source of bases is the metabo- lism of amino acids such as aspartate and glutamate and the metabolism of certain organic anions (e.g., citrate, lactate, acetate). Calculation of pH The serum pH can be calculated using an equation called the Henderson-Hasselbalch equation. This equation uses the dissociation constant for the bicar- bonate buffer system (which is 6.1) plus the log of the HCO 3 – - to - PCO 2 (used as a measure of H 2 CO 3 ) ratio (normally 20:1) to determine the pH (i.e., pH = 6.1 + log of 20 = 7.4). Because the ratio is used, a change in either HCO 3 – or PCO 2 will have little or no effect on pH as long as there is an accompanying change in PCO 2 and HCO 3 – (Fig. 8-17). The pH will decrease when the ratio decreases and increase when the ratio increases. Regulation of pH The pH of body fluids is regulated by three major mecha- nisms: (1) chemical buffer systems in body fluids, which immediately combine with excess acids or bases to pre- vent large changes in pH; (2) the lungs, which control