Which structure in the nephron is primarily responsible for water reabsorption?

The proximal tubule is primarily responsible for water reabsorption in the nephron. This segment of the nephron initiates the process of reabsorption after the initial filtration of blood occurs in the glomerulus.

During this stage, about 65-70% of the filtered water, along with significant amounts of solutes such as sodium, glucose, and amino acids, are reabsorbed back into the bloodstream. The cells lining the proximal tubule are equipped with numerous microvilli that increase the surface area for reabsorption, allowing for efficient transport of water through both passive diffusion and various active transport mechanisms.

In contrast, while the glomerulus is involved in the filtration of blood to form urine, it does not play a direct role in reabsorption. The distal convoluted tubule also participates in reabsorption but to a lesser extent compared to the proximal tubule. Finally, the renal pelvis is part of the kidney's anatomy that collects urine and does not have a function in the reabsorption process. This differentiation emphasizes the critical role that the proximal tubule plays in maintaining water balance in the body.