Which of the following diagrams is correct with regard to the proximal (P) and distal (D) tubule of the Nephron (showing which su…

Question

Which of the following diagrams is correct with regard to the proximal (P) and distal (D) tubule of the Nephron (showing which substances are reabsorbed into and secreted out of the tubule)?

1. A. PCT: reabsorbs HCO3^-/NaCl, secretes H^+/H2O; DCT: reabsorbs NaCl/H2O/HCO3^-, secretes K^+/H^+
2. B. PCT: reabsorbs HCO3^-/NaCl/H2O, secretes H^+/NH3; DCT: reabsorbs NaCl/H2O/HCO3^-, secretes K^+/H^+ (Correct answer)
3. C. PCT: reabsorbs HCO3^-/NaCl/H2O, secretes H^+/NH3; DCT: reabsorbs H2O/HCO3^-, secretes NaCl/H^+
4. D. PCT: reabsorbs NaCl/HCO3^-, secretes H^+/H2O; DCT: reabsorbs NaCl/NH3, secretes K^+/H2O

Correct Answer

Option B — PCT reabsorbs HCO3^-/NaCl/H2O and secretes H^+/NH3; DCT reabsorbs NaCl/H2O/HCO3^- and secretes K^+/H^+ (Option 2)

Detailed Solution & Explanation

The correct answer is Option 2.

Key Points

• The PCT reabsorbs Na$^+$, Cl$^-$, HCO$_3^-$ and water, and selectively secretes H$^+$, ammonia (NH$_3$) and K$^+$ into the filtrate to maintain ionic and acid-base balance.
• The DCT reabsorbs HCO$_3^-$ (and Na$^+$/water) and selectively secretes H$^+$ and K$^+$.
• Option 2 depicts these correctly.