The kidneys are instrumental in regulating the body’s acid-base balance by excreting hydrogen ions and reabsorbing bicarbonate. In states of acidosis, when blood pH is too low, the kidneys increase hydrogen ion secretion and bicarbonate reabsorption to neutralize excess acid. Conversely, in states of alkalosis, when blood pH is too high, the kidneys excrete bicarbonate to lower pH. The proximal tubule, distal tubule, and collecting ducts all contribute to this process through the secretion of hydrogen ions and the generation of new bicarbonate via ammonium excretion. The scale matters: about 4,300 mEq of bicarbonate is filtered each day and essentially all of it must be reclaimed (roughly 80% in the proximal tubule), while the fixed acid load to be excreted is only about 1 mEq/kg per day, carried out as ammonium and as titratable acid buffered by phosphate. Renal compensation is powerful but slow, taking 2 to 5 days to reach its full effect, which is why an acute acid-base disturbance during anesthesia is handled by ventilation and buffers, not by the kidney.
In the proximal tubule, bicarbonate is reabsorbed in tandem with sodium through the action of carbonic anhydrase, which converts carbonic acid into water and carbon dioxide. These molecules diffuse into tubular cells, where they are reconverted into bicarbonate and hydrogen ions. The hydrogen ions are secreted back into the filtrate, while the bicarbonate is reabsorbed into the bloodstream. In the collecting ducts, intercalated cells secrete hydrogen ions into the filtrate via proton pumps, while simultaneously reabsorbing bicarbonate into the bloodstream through anion exchangers.
