Final answer:
The active transport of Na+ during tubular reabsorption is to create an osmotic gradient for water to be reabsorbed passively, while other solutes like Cl- move passively following the established gradient.
Step-by-step explanation:
The second step of urine formation is tubular reabsorption, which mainly occurs in the proximal convoluted tubule (PCT) of the renal tubule. The active transport of sodium (Na+) into the interstitial space is primarily to create an osmotic gradient that facilitates the passive reabsorption of water. This movement of water occurs through aquaporins, special water channels present in the PCT.
The passive reabsorption during this process involves substances like chloride (Cl-) ions and other small solutes, which follow the sodium ions due to the electrochemical gradient established by the active transport of Na+.