Kidney Regulation of Body Fluid pH
Hydrogen ions (HÌ) secreted into the lumen of the nephron combine with bicarbonate ions (HCOÓÍ), phosphate ions (HPOÔÍÂ), and NHÓ in the filtrate.  Bicarbonate ions and phosphate ions enter the nephron with the filtrate.  The combination of hydrogen ions with these molecules prevents the pH of the urine from decreasing to very low levels.