All the water filtered by the kidneys must be reabsorbed into the body, in order to maintain the right levels of salts and minerals. There are three main areas of the nephron where this occurs: the proximal convoluted tubule (PCT), the descending limb of the Loop of Henle, and the collecting ducts.
The PCT is responsible for reabsorbing many substances that are not transported by other regions of the nephron, such as amino acids and glucose. The epithelial cells of the proximal PCT have transport proteins called sodium-glucose transporters in their apical membranes that capture both glucose and sodium from the tubular fluid.
ATP-powered sodium-potassium pumps remove Na+ from the cell and a separate glucose transporter transports glucose out of the cell wall. These transporters are paired with a sodium/potassium ATPase that maintains the concentration gradient between Na+ and glucose.
Each region of the nephron also has its own unique mechanism for reabsorbing substances from the nephron lumen into the bloodstream. This includes passive and active transport, as well as facilitated diffusion.
Seven members of the aquaporin (AQP) family are expressed in different parts of the nephron and each is responsible for a particular role in water reabsorption. AQP1 is localized in the apical and basolateral membranes of proximal tubules, while AQP2 is expressed in principal cells of the collecting duct.
These AQPs are essential for the maintenance of a high osmolarity of filtrate. In the juxtaglomerular segment of the kidney, these AQPs are complemented by enzymes to enhance water and solute recovery.