1.   Some of the excess chloride ions in the lumen are exchanged for bicarbonate ions at the surface of the epithelium.

2.   The bicarbonate ion is invariably present in weathering solutions and is easily the most abundant anion in most surface waters.

3.   The dissolution of carbon dioxide in precipitation provides an additional source of bicarbonate ions.

4.   They do the same with calcium, and they also excrete bicarbonate ions as a byproduct of respiration.

5.   But the new research suggests bicarbonate ions also play a role.