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Acid-Base Pathology

  • Acid-Base Disturbances describe any condition which results in changes of the extracellular fluid pH from the normal range of pH 7.35 - 7.45. Conditions which reduce ECF pH beyond 7.35 are termed acidosis, whereas those which increase ECF pH beyond 7.45 are termed alkalosis. Because the blood pH and the ECF pH are almost always equivalent, disturbances of acid-base homeostasis are diagnosed by measuring blood pH. As described below, acid-base disturbance are initiated by a primary shift in the ratio of the partial pressure of arterial carbon dioxide (PaCO2) relative to ECF bicarbonate concentration ([HCO3-]). When such a pathological shift occurs, both respiratory and renal compensatory mechanisms exist to rebalance this ratio and thus re-establish a normal extracellular fluid pH.
Primary Disturbance
  • Whatever the etiological source, all initial acid-base disturbances result in a misalignment of the normal ratio of the partial pressure of arterial carbon dioxide (PaCO2) relative to ECF bicarbonate (HCO3-) concentration. This is because the bicarbonate buffer is the principal physiological buffer of the extracellular fluid and thus any disturbances in ECF pH will be manifested by a change in the ratio of the weak acid (gaseous CO2) to weak base (HCO3-) forms of the bicarbonate buffer.
  • The quantitative relationship between ECF pH and the PaCO2-to-HCO3- ratio is discussed in the Henderson-Hasselbalch Equation page. This equation makes clear that increases in the PaCO2-to-bicarbonate ratio are associated with acidosis whereas decreases in the PaCO2-to-HCO3- ratio are associated with alkalosis. Evaluation of the PaCO2-to-HCO3- ratio is also useful in categorizing etiologies of acid-base disturbances. Acid-Base Disturbances resulting from initial changes in PCO2 are termed 'Respiratory' because they are always caused by defects in proper control of respiration. In contrast, acid-base disturbances resulting from initial changes in HCO3- are given a catch-all 'Metabolic' label, and can result from a number of different etiologies.
  • Whatever the primary shift in the PaCO2-to-bicarbonate ratio, the body can use compensatory mechanisms to realign the ratio by modulating any remaining intact processes that an affect either PaCO2 or the ECF bicarbonate concentration. For example, if the ECF pH lowers due to a primary reduction in the bicarbonate concentration, then the body can proportionally reduce the PaCO2, thus realigning the overall ratio and consequently re-establishing a nearly normal ECF pH. Alternatively, if the ECF pH lowers because of a primary increase in PaCO2, the body can then proportionally increase the ECF bicarbonate concentration, thus realigning the overall ratio and re-establishing a nearly normal ECF pH. It should be noted that although these compensatory mechanisms can largely normalize the ECF pH, they can never fully correct the blood pH to normal levels.
Acid-Base Disturbances