In the kidney water and solutes are passed out of the bloodstream and into the proximal tubule via the slit-like structure formed by nephrin in the glomerulus. Water is reabsorbed from the filtrate during its transit through the proximal tubule, the descending loop of Henle, the distal convoluted tubule, and the collecting duct.
Aquaporin-1 (AQP1) in the proximal tubule and the descending thin limb of Henle is responsible for about 90% of reabsorption (as estimated from mouse knockouts of AQP1). AQP1 is located on both the apical and basolateral surface of epithelial cells and thus transports water through the epithelium and back into the bloodstream.
In the collecting duct epithelial cells have AQP2 on their apical surface and AQP3 and AQP4 on their basolateral surface to transport water across the epithelium. The permeability of the epithelium is regulated by vasopressin, which activates the phosphorylation and subsequent translocation of AQP2 from intracellular vesicles to the plasma membrane.