Metabolic Acidosis Is Caused by the Accumulation of Fixed Acids or the Loss of Buffer BaserWhich Decreases Blood pH

Metabolicacidosis is the most common acid-base abnormality. During metabolism, there is a continuous production of fixed acids. An increase in their production or a failure of H⁺ elimination by the kidneys is the cause of metabolic acidosis.

FIGURE 52-5 A pH-bicarbonate diagram showing the effects of metabolic acidosis and alkalosis on pH, bicarbonate (HCO₃ ) concentration, and carbon dioxide tension (Pco₂) of arterial blood. In uncompensated metabolic acidosis, there is a decrease in the bicarbonate concentration, which leads to a decrease in pH, whereas Pco₂ remains constant. Respiratory compensation results in a decrease in Pco₂, with a subsequent increase in pH and movement of data points parallel to the normal buffer line. In metabolic alkalosis, HCO₃ increases and causes an increase in pH. Respiratory compensation is by alveolar hypoventilation, which leads to an increase in Paco₂ and a return of pH toward normal. N, Normal arterial blood composition.

anaerobic metabolism that leads to lactic acidosis. Diarrhea can also cause metabolic acidosis because excessive amounts of HCO₃" (buffer) are lost in the feces. In ruminants, excessive feeding of carbohydrates can lead to increased H* production in the rumen (rumen aeidosis). The H^, ions that are absorbed cause metabolic acidosis.

The accumulation of H^f in the blood is combined with HCO₃^- and other buffers. The CO₂ produced by the combination of H^, and HCO₃' is lost through the lungs. The loss of buffer base gives rise to a base deficit, and the HCO₃^- depletion decreases the [HC0₃70.03 ∙ Pco₂I ratio; therefore the pH decreases (Figure 52-5).

The decrease in pH accompanying metabolic acidosis stimulates ventilation. This increase in alveolar ventilation has a compensatory effect on metabolic acidosis by eliminating CO₂, leading to a decreased Pco₂, which ultimately adjusts the ∣HC0₃70.03 ∙ Pco₂] ratio and pH toward normal This sequence is shown in Figure 52-5. As the Pco₂ decreases, IHCO₃') decreases along a line that parallels the normal buffer line. This decrease in [HCO₃'] is accompanied by a decrease in pH. Complete restoration of normal acid-base balance requires the restoration of the depleted base by the kidney or by therapy with intravenous fluid that contains buffers, such as bicarbonate.