Frequently asked questions about acid-base chemistry (Proceedings)
Apr 01, 2010
CVC IN WASHINGTON, D.C. PROCEEDINGS
A proper understanding of the terms acidosis, alkalosis, acidemia, and alkalemia is necessary to differentiate simple from mixed acid base disorders. Acidosis and alkalosis refer to the pathophysiologic processes that cause net accumulation of acid or alkali in the body, whereas acidemia and alkalemia refer specifically to the pH of extracellular fluid. In acidemia, the extracellular fluid pH is less than normal and the [H+ ] is higher than normal. In alkalemia, the extracellular fluid pH is higher than normal and the [H+ ] is lower than normal. Due to the effectiveness of compensatory mechanisms, animals can have acidosis or alkalosis but not acidemia or alkalemia. For example, a dog with chronic respiratory alkalosis may have a blood pH that is within the normal range. Such a patient has alkalosis, but does not have alkalemia.
The primary acid base disorders are divided into metabolic and respiratory disturbances: metabolic acidosis, metabolic alkalosis, respiratory acidosis, and respiratory alkalosis. The Henderson-Hasselbach equation in its clinically relevant form emphasizes the relationship between the metabolic and respiratory systems in determining extracellular fluid pH:
Traditionally, the kidneys have been considered responsible for regulation of the metabolic component (blood bicarbonate concentration, [HCO3 - ]) and the lungs for regulation of the respiratory component (partial pressure of CO2, [pCO2]). In this form, the Henderson-Hasselbach equation makes it clear that the pH of extracellular fluid is determined by the ratio of the bicarbonate concentration and pCO2.