The primary function of the proximal convoluted tubule is to reabsorb most of the materials needed from the filtrate as it passes through the kidneys, explains HowStuffWorks. The proximal convoluted tubule reabsorbs approximately 65 percent of sodium and most of the water from the filtrate.
Molecules are reabsorbed from the filtrate using specialized transporters located on the nephron. The majority of the sodium transporters are present in the proximal convoluted tubule. Each type of transporter targets a specific type of molecule. Transporters function actively or passively. Active transporters usually require an energy source called adenosine triphosphate. Passive transporters do not require energy to function.
Water is reabsorbed in the proximal convoluted tubule by a process called osmosis. Generally, wherever sodium is reabsorbed, water follows. Sodium is gradually reabsorbed by sodium-potassium pumps placed over the basal membrane of the proximal convoluted tubule. As solutes are removed from the proximal convoluted tubule, the filtrate becomes more hypotonic. When the filtrate leaves the proximal convoluted tubule, it is isotonic. The filtrate is isotonic because it is osmotically balanced due to the continuous reabsorption of water and solute. The rate of flow of the filtrate and the concentration of small molecules within the filtrate are major factors that can affect the process of reabsorption.