Kidney Regulation of Body Fluid pH
Once hydrogen ions combine with bicarbonate ions, they form an equilibrium with the carbonic acid (HÒCOÓ) in the filtrate.  Some of the carbonic acid molecules dissociate to form water and carbon dioxide.  The carbon dioxide passes into the cells of the nephron.  As the pH of the extracellular fluid decreases, more HÒPOÔÍ and NHÔÌ ions appear in the urine.