Biological Systems MCAT Practice Exam 2025 – Complete Study Resource

The part of the nephron primarily responsible for reabsorbing water is the collecting duct. This section interacts with the fine-tuning of water reabsorption in response to the body's needs. When the body requires water conservation, antidiuretic hormone (ADH) is released, which increases the permeability of the collecting duct to water. This mechanism allows for the reabsorption of a significant amount of water back into the bloodstream, concentrating the urine and reducing water loss.

While the proximal convoluted tubule does reabsorb a significant amount of water, it is primarily known for reabsorbing solutes like glucose, amino acids, and ions along with some water. The Loop of Henle does also play a role in the concentration of urine and the creation of a hyperosmotic medullary interstitium, which aids in water reabsorption, but the major site for final water reabsorption is in the collecting duct. The distal convoluted tubule has limited reabsorption of water compared to the collecting duct and is influenced by hormonal regulation without being the primary site for reabsorption.