How is water reabsorbed in the proximal tubule?

• A. Facilitated diffusion
• B. Osmosis
• C. Active transport
• D. Pinocytosis

Answer: (B)

Water reabsorption in the proximal tubule happens by osmosis. As solutes, especially sodium, are actively reabsorbed from the tubular fluid into the surrounding tissue, they create an osmotic gradient. Water moves passively across the tubule epithelium through aquaporin channels, following that gradient from the lumen into the epithelial cells and then into the interstitium and capillaries. This process is iso-osmotic, meaning water follows solute reabsorption in roughly equal proportion, so the filtrate remains about the same osmolarity as it moves along the proximal tubule. Paracellular solvent drag also contributes a bit due to the segment's high permeability. In short, water reabsorption is driven by osmosis, not active transport, diffusion through carriers, or endocytosis.