Figure 306

Primary active secretion of hydrogen ions through the luminal membrane of the intercalated epithelial cells of the late distal and collecting tubules. Note that one bicarbonate ion is absorbed for each hydrogen ion secreted, and a chloride ion is passively secreted along with the hydrogen ion.

instead of by counter-transport, as occurs in the early parts of the nephron.

Although the secretion of H+ in the late distal tubule and collecting tubules accounts for only about 5 per cent of the total H+ secreted, this mechanism is important in forming a maximally acidic urine. In the proximal tubules, H+ concentration can be increased only about threefold to fourfold, and the tubular fluid pH can be reduced to only about 6.7, although large amounts of H+ are secreted by this nephron segment. However, H+ concentration can be increased as much as 900-fold in the collecting tubules. This decreases the pH of the tubular fluid to about 4.5, which is the lower limit of pH that can be achieved in normal kidneys.

Combination of Excess Hydrogen Ions with Phosphate and Ammonia Buffers in the Tubuleā€”A Mechanism for Generating "New" Bicarbonate Ions

When H+ is secreted in excess of the bicarbonate filtered into the tubular fluid, only a small part of the excess H+ can be excreted in the ionic form (H+) in the urine. The reason for this is that the minimal urine pH is about 4.5, corresponding to an H+ concentration of 10-45 mEq/L, or 0.03 mEq/L. Thus, for each liter of urine formed, a maximum of only about 0.03 mil-liequivalent of free H+ can be excreted. To excrete the 80 milliequivalents of nonvolatile acid formed by metabolism each day, about 2667 liters of urine would have to be excreted if the H+ remained free in solution.

Renal Tubular interstitial Tubular cells lumen fluid Na+ + NaHPO,


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