<P> Renal reabsorption of sodium (Na) is a part of renal physiology . It uses Na - H antiport, Na - glucose symport, sodium ion channels (minor). It is stimulated by angiotensin II and aldosterone, and inhibited by atrial natriuretic peptide . </P> <P> It is very efficient, since more than 25,000 m moles / day of sodium is filtered into the nephron, but only ~ 100 m moles / day, or less than 0.4% remains in the final urine . </P> <P> Most of the reabsorption (65%) occurs in the proximal tubule . In the latter part it is favoured by an electrochemical driving force, but initially it needs the cotransporter SGLT and the Na - H antiporter . Sodium passes along an electrochemical gradient (passive transport) from the lumen into the tubular cell, together with water and chloride which also diffuse passively . Water is reabsorbed to the same degree, resulting in the concentration in the end of the proximal tubule being the same as in the beginning . In other words, the reabsorption in the proximal tubule is isosmotic . </P> <P> Sodium is reabsorbed in the thick ascending limb of loop of Henle, by Na - K - 2Cl symporter and Na - H antiporter . It goes against its chemical driving force, but the high electrical driving force renders the overall electrochemical driving force positive anyway, availing some sodium to diffuse passively either the transcellular or paracellular way...</P>

How much sodium is actively reabsorbed by the proximal segment of the nephron