How the Kidney Clears Acid From the Body

The kidney is one of the organs that shift the pH of the blood, that is, how acidic it is. And it does this in two ways at once, not one. On one hand it rescues bicarbonate, the substance that buffers acid, and returns it to the blood. On the other it dumps what are called fixed acids into the urine.

The fine work happens in the tubule, the small tube that carries the fluid that has already passed through the filter. Its cells push hydrogen ions, acid in its simplest form, into that fluid, and at the same time pass bicarbonate into the peritubular capillaries, the vessels that wrap around the outside of the tube. It is one tube with two exits: one carries off what is left over, the other returns what is useful.

When there is metabolic acidosis, blood acidified by a cause that is not respiratory, the kidney adds another tool: it excretes ammonium in the urine.

And there is something worth being clear about, because it explains a lot: the kidney is slow. It takes days to correct, while the lung does it in minutes to hours.