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LEARNING OBJECTIVES

Learning Objectives

  • The student will be able to identify primary acid/base disorders given patient laboratory results and the degree of compensation.

  • The student will be able to distinguish between the roles of the lungs and kidneys in maintenance of acid/base homeostasis.

  • The student will be able to identify common causes of metabolic acidosis and alkalosis, and of respiratory acidosis and alkalosis.

  • The student will be able to use the calculation of the anion gap to identify causes of metabolic acidosis.

Although the human diet is essentially neutral with respect to its pH, various aerobic and anaerobic metabolic processes create large amounts of acid daily from the catabolism of carbohydrates, fats, and proteins. The body maintains a constant [H+] such that blood and extracellular pH is near 7.4 and intracellular pH is about 7.1. The buffer most responsible for this is the HCO3/H2CO3 system, regulated by the coordinated behavior of the lungs and kidneys. Aerobic metabolism yields gaseous CO2 that combines with water to form volatile carbonic acid, H2CO3. The lungs are primarily responsible for elimination of CO2. Anaerobic metabolism of glucose and fat, as well as some protein catabolism yield fixed or nonvolatile acids such as lactic acid, sulfuric acid, and phosphoric acid, most being excreted by the kidneys. In one 24-hour period, a 70-kg person produces 70-100 mmol of such nonvolatile acids versus 20 moles CO2, all without appreciable change in plasma pH. An effective V̇A controls CO2 excretion, while the kidneys excrete nonvolatile acids and reabsorb or regenerate HCO3. The lungs and kidneys maintain a balance of CO2 and HCO3, and thus pHa near 7.4 (Fig. 17.1).

FIGURE 17.1

Acidosis and alkalosis refer to primary processes that increase or decrease blood [H+], respectively. Acidemia refers to a blood pH <7.36 and alkalemia to a blood pH >7.44. Compensation occurs as either the metabolic (renal) or respiratory response that attempts to restore blood pH when altered by some primary process.

BASIC CATEGORIES OF ACIDOSIS AND ALKALOSIS

Acidosis and alkalosis are terms that describe primary processes modulating [H+] in patient populations. Each may be metabolic or respiratory in origin, based on whether the primary deviation is in arterial [HCO3] or Paco2, respectively. Respiratory disorders are caused by inappropriate excretion of CO2 that raises or lowers [H2CO3] in body fluids. Metabolic disorders reflect primary changes in plasma [HCO3] due to excessive intake, production, or loss of HCO3 or inappropriate handling of H+ and anions from dissociated nonvolatile acids. Primary acid-base disorders and the compensations that must follow to restore pH to normal are ...

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