Metabolic Alkalosis Results From a Gain of Strong Base or Bicarbonate or a Loss of Noncarbonic Acid

Metabolic alkalosis is an abnormal process characterized by a gain of a strong base or HCO3- or a loss of an acid (other than carbonic acid). Plasma [HCO3-] and pH rise; Pco2 rises because of respiratory compensation. These changes are opposite to those seen in metabolic acidosis (see Table 25.3). A variety of situations can produce metabolic alkalosis, including the ingestion of antacids, vomiting of gastric acid juice, and enhanced renal H+ loss (e.g., as a result of hyperaldosteronism or hypokalemia). Clinical Focus Box 25.3 discusses the metabolic alkalosis produced by vomiting of gastric juice.

Chemical Buffering. Chemical buffers in the body limit the alkaline shift in blood pH by releasing H+ as they are titrated in the alkaline direction. About one third of the buffering occurs in cells.

Respiratory Compensation. The respiratory compensation for metabolic alkalosis is hypoventilation. An alkaline blood pH inhibits ventilation. Hypoventilation raises the blood Pco2 and [H2CO3], reducing the alkaline shift in pH. A l mEq/L rise in plasma [HCO3-] caused by metabolic alkalosis is accompanied by a 0.7 mm Hg rise in Pco2 (see Table 25.4). If, for example, the plasma [HCO3-] rose to 40 mEq/L, what would the plasma pH be with and without respiratory compensation? With respiratory compensation, the Pco2 should rise by 11.2 mm Hg (0.7 X 16) to 51.2 mm Hg ([CO2(d)] = 154 mmol/L). The pH is 7.51:

Without respiratory compensation, the pH would be 7.62:

Respiratory compensation for metabolic alkalosis is limited because hypoventilation leads to hypoxia and CO2 retention, and both increase breathing.

Renal Compensation. The kidneys respond to metabolic alkalosis by lowering the plasma [HCO3-]. The plasma [HCO3-] is primarily raised and more HCO3- is filtered than can be reabsorbed (see Fig. 25.7); in addition, HCO3- is secreted in the collecting ducts. Both of these changes lead to increased urinary [HCO3-] excretion. If the cause of the metabolic alkalosis is corrected, the kidneys can often restore the plasma [HCO3-] and pH to normal in a day or two.

^ABLE 25.5^ High and Normal Anion Gap Metabolic Acidosis

Condition

Explanation

High anion gap metabolicacidosis

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