Acidification of urine occurs at:

H+ secretion : 80%-85% H+ secretion in the PCT 10% in the Thick ascending limb And remaining secretion occurs in the late distal tubule and collecting duct. - Although secretion in the late distal tubule and collecting duct accounts for only about 5-10% of the total H+ secreted, it is impoant for acidification of urine. - In PCT, H+ secretion can be increased only about 3-4 times, although large amounts of H+ is secreted by this segment. Most of it is for the re-absorption of the filtered bicarbonate. On the other hand, H+ concentration can be increased as much as 1000-fold in the late distal tubule and collecting duct. This decreases the pH of the tubular fluid to about 4.5. This is called the limiting pH; the maximally acidic urine is thus achieved by H+ secretion by this segment.