Final answer:
By the time the filtrate reaches the loop of Henle, about 70% of sodium and water has been reabsorbed, due to the permeability of the descending limb to water and ion transport mechanisms in the ascending limb.
Step-by-step explanation:
By the time filtrate reaches the loop of Henle, approximately 70% of the sodium and water has been reabsorbed. The reabsorption of water and sodium in the nephron is a crucial function that allows the kidneys to regulate the volume and concentration of urine. Various parts of the nephron, including the proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT), and the collecting ducts work collaboratively in this process. In particular, the descending limb of the loop of Henle is water-permeable, which allows water to pass from the filtrate into the interstitial fluid. As the filtrate then enters the ascending limb, sodium (Na+) and chloride (Cl-) ions are reabsorbed through ion channels and active transport, creating a concentration gradient essential for urine concentration.