In the proximal convoluted tubule (PCT), which substances are reabsorbed with water?

• A. Only water is reabsorbed.
• B. Amino acids, glucose, vitamins and salts are reabsorbed with water.
• C. This segment is the primary site of filtration.
• D. No substances are secreted in this segment.

Answer: (B)

Water reabsorption in the proximal tubule happens in tandem with reabsorbing many solutes, so water follows the movement of these substances via osmotic forces and aquaporin channels. In this segment, amino acids, glucose, vitamins, and various salts are actively transported back into the blood, and water accompanies them, so they’re reabsorbed with water. Filtration, by contrast, occurs in the glomerulus, not in the proximal tubule, and the tubule does more than just reabsorb water—it reabsorbs a large load of solutes and also secretes certain waste products. The key point is that these specific solutes are reabsorbed together with water, which is why this option is correct.