Final answer:
Most sodium and water reabsorption from the tubular filtrate occurs in the proximal convoluted tubule (PCT) of the nephron. Sodium is reabsorbed by active transport, while water follows due to osmotic pressure and the presence of aquaporins. Additional water reabsorption occurs along the nephron, regulated by hormones such as ADH and aldosterone.
Step-by-step explanation:
Tubular Reabsorption and Sodium Reabsorption
In the nephron, most sodium and water are reabsorbed from the tubular filtrate in the proximal convoluted tubule (PCT). Sodium (Na+) is the most abundant ion and is reabsorbed by active transport and then transported to the peritubular capillaries. As Na+ is actively transported out of the tubule, water follows it to even out the osmotic pressure. Water is also independently reabsorbed due to the presence of aquaporins, or water channels, in the PCT, because of the low blood pressure and high osmotic pressure in the peritubular capillaries.
The control of water reabsorption is tightly regulated by hormones such as ADH and aldosterone, and indirectly by renin. Further along the nephron, additional water is reabsorbed in the loop of Henle, distal convoluted tubule (DCT), and collecting ducts, with fine adjustments in the latter depending on the body's hydration status. Each solute has a transport maximum, and any excess beyond this limit is not reabsorbed and is excreted in the urine.