Total plasma volume filters into the renal tubules about every 22 minutes, so all plasma would be drained away as urine in less than 30 minutes were it not for the fact that most of the tubule contents are quickly reclaimed and returned to the blood. This reclamation process, called tubular reabsorption, is a selective transepithelial process that begins as soon as the filtrate enters the proximal tubules. To reach the blood, reabsorbed substances follow either either the transcellular or paracellular route. In the transcelullar route, transported transported substances move through the luminal membrane, the cytosol, and the basolateral membrane of the tubule cell and then the endothelium of the peritubular capillaries. Movement of substances in the paracellular route between the tubule cells is limited because these cells are connected by tight junctions. In the proximal nephrons, however, these tight junctions are “leaky” and allow some important ions (Ca2+, Mg2+, K+, and some Na+) through the paracellular route.
Given healthy kidneys, virtually all organic nutrients such as glucose and amino acids are completely reabsorbed to maintain or restore normal plasma concentrations. On the other hand, the reabsorbtion of water and many ions is continuosly regulated and adjusted in response to hormonal signals. Depending of the substances transported, the reabsortion process may be passive (no ATP required) or active (at least one of its steps is driven by ATP directly or indirectly)