Pathophysiological aspect of metabolic acid-base disorders

Maintaing the arterial pH values (in normal range of 7,35-7,45) is one of the main principles of homeostasis. Regulatory responses, including chemical buffering (extracellular, intracellular, sceletal), the regulation of pCO2 by the respiratory system, and the regulation of [HCO3-] by the kidneys, act in concert to maintain normal arterial pH value. The main extracellular chemical buffer is bicarbonate-carbonic acid buffer system. The kidneys contribute to the regulation of hydrogen (and bicarbonate) in body fluids in two ways. Proximal tubules are important in bicarbonate reabsorption and distal tubules excrete hydrogen ion (as ammonium ion or titratable acid). There are four simple acid-base disorders: metabolic acidosis and metabolic alkalosis; respiratory acidosis and respiratory alkalosis. Metabolic acidosis can occur because of an increase in endogenous acid production (such as lactate and ketoacids), loss of bicarbonate (as in diarrhea), or accumulation of endogenous acids (as in renal failure). Metabolic acidosis can also be with high and normal (hyperchloremic metabolic acidosis) anion gap. Renal tubular acidosis (RTA) is a form of hyperchloremic metabolic acidosis which occurs when the renal damage primarily affects tubular function. The main problem in distal RTA is reduced H+ excretion in distal tubule. Type 2 RTA is also called proximal RTA because the main problem is greatly impaired reabsorption of bicarbonate in proximal tubule. Impaired cation exchange in distal tubule is the main problem in RTA type 4. Metabolic alkalosis occurs as a result of net gain of [HCO3-] or loss of nonvolatile acid from extracellular fluids. Metabolic alkalosis can be associated with reduced or increased extracellular volume.

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