I got to researching this based on some Magnesium Acetate comments. Got way over my head, and ended up reading several discussions by Nephrologist, Dr. Frederic Coe. One of many is linked to below.
http://kidneystones....into-the-urine/
Bicarbonate
A Better Buffer than Most
A buffer keeps pH relatively constant by taking up protons when they enter a solution and giving them up when alkali enters. It is a kind of shock absorber.
At the beginning, evolution favored bicarbonate. It is a buffer of considerable virtue in that it can take up protons or release them, like common buffers do, but has a special trait.
Bicarbonate is forever in equilibrium with carbon dioxide gas (CO2). When bicarbonate takes on a proton to become carbonic acid, much of that acid becomes carbon dioxide gas. When protons are taken out of blood, CO2 gas forms new carbonic acid which donates a new proton to the solution, and essentially bicarbonate appears in solution ‘out of thin air’. That it flows from solution into thin air and back makes bicarbonate a more stable buffer than those which live only in solution so it was an excellent choice.
What Kidneys do with Bicarbonate
It is this very molecule, bicarbonate, which the kidneys traffic in when they respond to alkali or acid loads, and it is, of course, CO2 the lungs regulate in blood under the control of the brain.
The figure is from the ‘A’ panel of a lovely drawing in a lively and engaging review. Being small, bicarbonate is filtered, and being the main buffer of the blood almost all of what is filtered must be reclaimed. So the proximal tubule cells, which do most of that reclamation, busy themselves forever with that task.
The way they do it is the simplest way. They add protons (H+) to bicarbonate in the tubule fluid, which becomes, as I have said, carbonic acid that transforms into carbon dioxide (CO2), which gas passes through the cell walls into the interior. Note, ‘CA’ is carbonic anhydrase an enzyme which speeds up the process of the transformation. In the cell, the CO2 becomes carbonic acid. Because protons are being pumped into the tubule fluid, protons are stripped off the carbonic acid so it becomes bicarbonate. The bicarbonate enters the blood with Na via the NBCe1A transporter.
There are two proton pumps. One uses ATP for energy to move the protons. The other (NHE3) uses the low Na in the cell as a gradient; sodium moves in through a channel like a revolving door, which makes one proton go out for every Na that moves in. At the blood side of the cell, the ancient ‘Great’ ATPase pumps Na out and potassium in, as it does in most cells that live on Earth. NHE3, the exchanger, is the molecule we met a few paragraphs above. It is increased by Endothelin 1 via the ET1b receptor.
At the top of the left side of the picture is citrate, our little slice of this massive structure. A few scraps of proton add to citrate so it has 2, not 3 nnegative sites, and can be reabsorbed. Its gene is regulated by endothelin 1 so when NHE3 is increased so is NaDC1."