| Under normal circumstances, the kidney reabsorbs all of the bicarbonate filtered into the proximal tubule. The main reason that the body is so keen to hang on to bicarbonate is that it is bicarbonate buffers pH. The pH of any solution containing a proton donor/acceptor (buffer), such as carbonic acid/bicarbonate depends on the relative concentrations of the base (bicarbonate) and the acid (carbonic acid). If you know the concentration of each and the equilibrium constant for the reaction (K, pK is the log of 1/K), you can calculate pH. |  |
| The concentration of carbonic acid in the body fluids is proportional to the partial pressure of CO2, in fact it is equal to 0.03 x pCO2 (measured in mm Hg) so our equation becomes the Henderson-Hasselbalch equation |  |
| Add in the usual value for bicarbonate (24 mM), pCO2 (40) and the numerical value for pK (6.1) and (surprise, surprise) you get pH 7.4 |  |
There are other buffering systems in the body, all trying to reach their equilibrium pH. None of the others make it because the bicarbonate system dominates and therefore sets the acid/base ratio of all the other buffers.