Proximal Tubule

The proximal tubule reabsorbs ~60% of filtered NaCl and water, as well as ~90% of filtered bicarbonate and most critical nutrients, such as glucose and amino acids. The proximal tubule harnesses both cellular and paracellular transport mechanisms to promote reabsorption. The apical membrane of proximal tubular cells features an expanded surface area, supportive of reabsorptive activities. This vast surface area is provided by a dense array of microvilli, called the brush border, and together with leaky tight junctions, high-capacity fluid reabsorption occurs.