It is very efficient, since more than 25,000 mmoles/day of sodium is filtered into the nephron, but only ~100 mmoles/day, or less than 0.4% remains in the final urine.
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 cotransporterSGLT and the Na-H antiporter. 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.