Introduction: Urine formation starts when glomerular filtration delivers protein-free of plasma into a kidney tubule. Most of the water and solutes return to the blood by the tubular reabsorption machinery. Substances that are not reabsorbed, and which are added to the filtrate by the tubular secretion, end up in the urine. The kidney contributes to the acid-base balance by adjusting the tubular reabsorption of bicarbonate and tubular secretion of H+.
A decline in pH causes an increase in the bicarbonate reabsorption. The bicarbonate moves into peritubular capillaries, where it buffers the excess acid.
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