In the ascending limb of the nephron loop, which of the following occurs?

• A. Water is passively reabsorbed
• B. Urea is actively reabsorbed
• C. Glucose is secreted into the filtrate
• D. Sodium and chloride ions are actively reabsorbed

Answer: (D)

Verified step by step guidance

1. Understand the structure and function of the nephron loop (loop of Henle). The nephron loop is divided into two segments: the descending limb and the ascending limb. The descending limb is permeable to water, while the ascending limb is impermeable to water but actively transports ions.
2. Recognize the role of the ascending limb in reabsorption. The ascending limb is responsible for actively reabsorbing sodium (Na⁺) and chloride (Cl⁻) ions into the surrounding interstitial fluid, which helps create the concentration gradient necessary for water reabsorption in other parts of the nephron.
3. Clarify why water is not passively reabsorbed in the ascending limb. The ascending limb is impermeable to water, meaning water cannot move across its walls, even passively. This is a key distinction from the descending limb.
4. Explain why urea and glucose are not involved in the processes of the ascending limb. Urea is primarily reabsorbed in the collecting duct, and glucose is reabsorbed in the proximal convoluted tubule, not secreted into the filtrate in the nephron loop.
5. Conclude that the correct process occurring in the ascending limb is the active reabsorption of sodium and chloride ions. This active transport is essential for maintaining the osmotic gradient in the kidney, which is critical for water balance and urine concentration.