Ganong 24e chapter 39 - Acidifying the Urine (1)

Question 1

Which parts of the nephron excrete H+ ions into the tubular fluid?

Answer 1

Proximal and Distal tubules.

Question 2

What is the role of carbonic anhydrase in the renal tubules?

Answer 2

Carbonic anhydrase catalyzes the formation of H₂CO₃ from CO₂ + H₂O. This occurs in the proximal tubule cells. HCO₃^-+ H⁺ is then formed, with H⁺ then being transported into the tubular lumen in exchange for Na⁺, whilst bicarbonate diffuses back into the interstitial fluid.

Question 3

How is H⁺ “fixed” in the tubular fluid?

Answer 3

There are several buffering mechanisms for this. H⁺ binds with HCO₃^- which is quickly converted to H₂O + CO₂. The carbon dioxide then diffuses back into the cell. In the distal tubules and collecting ducts, H⁺ binds with HPO₄^2-. In the proximal and distal tubules, hydrogen ions bind to NH₃ to form ammonium ions. NH₃ readily diffuses across cell membranes, but ammonium ions do not.

Question 4

What is the lowest pH that the urine can be?

Answer 4

4.5

Question 5

How does the pH change along the nephron?

Answer 5

There is a moderate drop in pH in the proximal tubule, but much of the secreted H⁺ throughout the nephron does not correspond to a drop in pH, because it is buffered in the tubular fluid.

Question 6

Describe what happens to HCO₃^- in the renal tubules when there is respiratory acidosis.

Answer 6

Respiratory acidosis is due to an increase in P_CO2, which causes acidosis due to H₂CO₃. The kidneys excrete more H⁺ in exchange for HCO₃^-, as a compensatory mechanism, and so the plasma bicarbonate level increases as respiratory acidosis progresses from acute to chronic.