The majority of water is reabsorbed by osmosis in the:

• A. loop of Henle
• B. proximal convoluted tubule
• C. distal convoluted tubule
• D. collecting duct

Answer: (B)

Verified step by step guidance

1. Understand the function of each part of the nephron: The nephron is the functional unit of the kidney, and its components include the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct. Each part plays a specific role in filtration, reabsorption, and secretion.
2. Focus on the proximal convoluted tubule: This is the first segment of the nephron after the glomerulus. It is responsible for reabsorbing the majority of water, ions, and nutrients from the filtrate back into the bloodstream.
3. Explain the mechanism of water reabsorption: Water is reabsorbed by osmosis, which is driven by the concentration gradient created by the active transport of sodium ions (Na⁺) and other solutes out of the filtrate into the surrounding interstitial fluid.
4. Compare the proximal convoluted tubule to other nephron segments: While the loop of Henle, distal convoluted tubule, and collecting duct also contribute to water reabsorption, the proximal convoluted tubule reabsorbs the largest volume of water due to its extensive surface area and high permeability.
5. Conclude why the proximal convoluted tubule is the correct answer: Based on its structure and function, the proximal convoluted tubule is the primary site for water reabsorption by osmosis in the nephron.