Final answer:
Reabsorbed salt and water are returned to the ECF and blood through the peritubular capillaries or the vasa recta, both of which play essential roles in the functioning of the renal system and maintaining the countercurrent mechanism.
Step-by-step explanation:
Reabsorbed salt and water goes back into the ECF (extracellular fluid) and blood via the peritubular capillaries or the vasa recta. When considering where the majority of reabsorbed salt and water is returned to the circulation, the peritubular capillaries and the vasa recta play crucial roles in the renal system. The peritubular capillaries are involved primarily in recovery after substances are reabsorbed from the Proximal Convoluted Tubule (PCT), while the vasa recta are particularly important in maintaining the countercurrent multiplier system, which preserves the osmotic gradients necessary for water and solute recovery in the kidneys. Slow blood flow in the vasa recta is essential to allow blood cells to wisely adjust their water content without damage and to maintain the countercurrent mechanism necessary for efficient reabsorption.