How are NaCl and water reabsorbed in the proximal convoluted tubule?
Sodium ions (Na+) are pumped from tubule cell to interstitial fluid, increasing Na+ concentration to a level that drives diffusion of Na+ into blood. As Na+ is pumped out of the cell, more Na+ passively diffuses in from filtrate to maintain equilibrium of concentration. Enough Na+ moves out of the tubule and into blood that an electrical gradient is established (blood is positive relative to filtrate). Electrical attraction between oppositely charged particles drives diffusion of negative ions in filtrate, such as chloride (Cl–), into blood. As ion concentration in blood increases, osmosis of water from the tubule occurs. Thus active transport of sodium creates a situation that promotes passive transport of negative ions and water.