TABLE 71 Pathophysiology of Metabolic Alkalosis

Net H+ loss from the ECF

A loss of protons from the body occurs primarily through either the kidneys or the GI tract

When H+ losses exceed the daily H+ load produced by metabolism and diet, a net negative H+ balance results; because the H+ loss results in generation of a HCO3-, increases in serum [HCO3-] result

GI proton loss generally occurs in stomach; in this setting, H+ secretion by the luminal gastric parietal cell H+ ATPase leaves a HCO3- to be reclaimed at the basolateral surface

In kidney, there is also coupling between net acid excretion and bicarbonate generation

Net bicarbonate or bicarbonate precursor addition to the ECF

HCO3- administration or addition of substances that generate HCO3- (e.g., lactate, citrate) at a rate greater than that of metabolic H+ production leads to an increase in ECF [HCO3-]

TABLE 7-1 (Continued)

With normal kidney function ECF [HCO3-] will not increase significantly; this occurs because as serum [HCO3-] exceeds the PT for HCO3- reabsorption, the kidney excretes the excess HCO3-; as a result serum bicarbonate will not rise unless there is a change in renal bicarbonate handling (maintenance factor)

Loss of fluid from the body that contains Cl_ in greater concentration and bicarbonate in lower concentration than serum-contraction alkalosis

If this type of fluid is lost ECF volume must contract and if contraction is substantial enough, a measurable increase in serum [HCO3-] develops

Bicarbonate is now distributed in a smaller volume resulting in an absolute increase in ECF [HCO3-]

Abbreviations: ECF, extracellular fluid; GI, gastrointestinal;

PT, plasma threshold

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