- The body maintains close control of the blood pH to ensure optimal function of enzymatic reactions within the cells.
- The relationship between dissolved carbon dioxide and carbonic acid and the consequent ability of the respiratory system to affect the acidity of the blood are critical to maintenance of an appropriate blood pH.
- Although the respiratory system is able to compensate for increased production of carbonic acid, elimination of fixed acids produced by the metabolism of proteins requires a functioning renal system.
- The primary intracellular buffers, which accept or release hydrogen ions to minimize changes in pH in the setting of an acidosis or alkalosis, are proteins, phosphates, and hemoglobin in RBCs. The primary source of buffer in the extracellular fluid is bicarbonate.
- The terms acidosis and alkalosis refer to processes that lead to a reduction or increase in serum pH, respectively.
- The terms acidemia and alkalemia refer to the status of the blood pH—below normal in the case of acidemia and above normal in the case of alkalemia.
- The four primary acid-base disorders are respiratory acidosis, respiratory alkalosis, metabolic acidosis, and metabolic alkalosis.
- Respiratory acidosis is characterized by an increased PaCO2, a decreased pH, and a mild increase in the serum bicarbonate concentration.
- Respiratory alkalosis is characterized by a decreased PaCO2, an increased pH, and a mild decrease in the serum bicarbonate concentration.
- Metabolic acidosis is characterized by a reduced bicarbonate concentration and a low pH. Conditions that lead to a metabolic acidosis may be divided into two categories: those associated with a normal anion gap and those that result in an increased anion gap.
- Metabolic alkalosis is characterized by an increased bicarbonate concentration and an elevated pH.
- Disorders of the respiratory and renal systems, when they lead to primary acid-base disturbances, generate compensatory processes in the other system to minimize the effect of the primary problem on the blood pH. Whereas respiratory compensation can occur within seconds to minutes, renal compensation may take 2 to 5 days before it is complete.
- The analysis of an acid-base disturbance requires a systematic approach that includes an assessment of the blood pH, the PaCO2, serum bicarbonate level, and the anion gap.
- Because of the delay in the response of the renal system to primary acid-base abnormalities of the respiratory system, one can categorize respiratory system acid-base disorders as acute or chronic. In an acute respiratory system disturbance, the pH changes by 0.08 units for every change of 10 mm Hg in the PaCO2; for a chronic disorder, the pH changes by 0.03 units for every change of 10 mm Hg in the PaCO2.
- If there is evidence of an abnormal pH but the PaCO2 has moved in a direction that is opposite of what you expect for a primary respiratory disorder, then you are dealing with a primary metabolic disturbance with respiratory compensation.
- A complete understanding of acid-base disorders requires integration of respiratory, renal, and GI system physiology.