Final answer:
The mechanism of water reabsorption in the renal tubules is through osmosis, driven by an osmotic gradient established by the active transport of sodium ions, and is also influenced by hormones regulating aquaporin channel insertion.
Step-by-step explanation:
The correct option is A:
The mechanism of water reabsorption by the renal tubules is primarily via osmosis. This process is facilitated in part by the active transport of sodium ions which creates an osmotic gradient that drives water movement. Additionally, water reabsorption is influenced by various hormones that regulate the insertion of aquaporin channels into the cell membranes of the renal tubules.
Key segments of the nephron employ different mechanisms for solute and water reabsorption. In the proximal convoluted tubule (PCT), sodium ions are reabsorbed by active transport mechanisms, which then drives the reabsorption of water via osmotic forces due to the presence of aquaporin channels. The loop of Henle, particularly its descending limb, also contributes to the reabsorption of water by taking advantage of the osmotic gradient created by the ascending limb and the surrounding interstitial fluid. In the distal parts of the nephron, like the collecting ducts, the hormone ADH can significantly influence water reabsorption by causing the insertion of additional aquaporin channels, thus modifying the permeability of these tubular segments of the nephron.