Which of the following diagrams is correct with regard to the proximal (P) and distal (D) tubule of the Nephron.

Answer: 2. Diagram option (2): P: HCO3-, NaCl, H2O out; H+, NH3 in. D: NaCl, H2O, HCO3- out; K+, H+ in

Key idea: The proximal tubule reabsorbs most substances and secretes some, while the distal tubule does conditional reabsorption and secretion.

In the proximal tubule, \(\mathrm{HCO_3^-}\), NaCl and water are reabsorbed, while \(\mathrm{H^+}\) and \(\mathrm{NH_3}\) are secreted. In the distal tubule, NaCl, water and \(\mathrm{HCO_3^-}\) are reabsorbed, while \(\mathrm{K^+}\) and \(\mathrm{H^+}\) are secreted. This matches diagram (2).

Other diagrams show water moving into the proximal tubule, or wrong ions in the distal tubule.