| Clinically, acidosis is a much more common disorder than alkalosis (Table 23.6) and it can be sub-classified into either respiratory or metabolic.
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| Respiratory acidosis occurs most often in lung disease and results from decreased ventilation
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| The most common cause is the chronic obstructive airways disease (COAD). Severe asthmatic attack can also result in respiratory acidosis because of bronchial constriction. Respiratory acidosis often accompanies hypoxia (respiratory failure); in such a case, an increase in pCO2 occurs together with the decrease in pO2.
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| Metabolic acidosis results from the excessive production, or inefficient metabolism or excretion, of nonvolatile acids
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| VOMITING CAN LEAD TO METABOLIC ALKALOSIS |
| A 47-year-old man came to the outpatient clinic with a history of intermittent profuse vomiting and loss of weight. He had tachycardia, reduced tissue turgor, hypotension and an abdominal succussion splash. His blood pH was 7.55 (hydrogen ion concentration 28 nmol/L) and pCO2 was 6.4 kPa (48 mmHg). His bicarbonate concentration was 35 mmol/L and then was also hyponatraemia and hypokalaemia. Despite the systemic alkalosis urine pH was only 3.5. |
| Comment. This patient presents with metabolic alkalosis caused by the loss of hydrogen ion through vomiting. Investigations showed gastric outlet obstruction of the stomach entrance due to scarring from chronic peptic ulceration. He subsequently underwent surgery for pyloric stenosis, with a good outcome. Note the increased pCO2 as a result of respiratory compensation of metabolic alkalosis. The paradoxical acid urine is probably due to depletion of chloride which then limits reabsorption of sodium in the thick ascending limb of the loop of Henle so that sodium is then exchanged for hydrogen ions and potassium in the distal nephron. |
| A classic example of metabolic acidosis is the diabetic ketoacidosis, when ketoacids, acetoacetic acid, and α-
hydroxybutyric acid accumulate in the plasma (see Chapter 20). Acidosis may also occur during extreme physical exertion, when there is accumulation of lactate generated from muscle metabolism; in normal circumstances, this lactate would be quickly metabolized after cessation of exercise.
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| However, when large amounts of lactate are generated as a consequence of hypoxia, lactic acidosis may become life-threatening, as happens, for instance, in shock.
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| Excretion of nonvolatile acids is also impaired in kidney disease (renal failure) and this also causes metabolic acidosis. Renal failure develops when the perfusion of the kidneys is inadequate (e.g. in trauma, shock, or dehydration) or if there is an intrinsic kidney disease such as glomerulonephritis (inflammatory reaction in the renal tubular tissue; see Chapter 22).
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| Excessive loss of bicarbonate can also be a cause of metabolic acidosis. This may occur when renal reabsorption mechanism is defective, but is more common when bicarbonate present in the intestinal fluid is lost as a result of severe diarrhea or surgical drainage after bowel surgery.
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