Which pa of the nephron is impermeable to water?
The nephron has several parts: the proximal convoluted tubule (PCT), the loop of Henle (both descending and ascending limbs), the distal convoluted tubule (DCT), and the collecting duct. Each has different permeability to water and solutes.
The core concept here is the permeability of different nephron segments to water. The question is about where water can't pass through. The loop of Henle's ascending limb is known to be impermeable to water. That's where active transport of NaCl occurs, creating a concentration gradient. The descending limb is permeable to water, allowing it to move out into the medullary interstitium. The PCT is also permeable to water. The collecting duct can be permeable depending on ADH levels.
So the correct answer should be the ascending limb of the loop of Henle. The options given are A, B, C, D, but the user didn't provide the specific labels. However, the correct answer is the ascending limb. The other options would be incorrect because the descending limb and PCT are permeable. The distal convoluted tubule and collecting duct can vary, but the ascending loop is definitely impermeable.
Clinical pearl: The ascending loop's impermeability to water is crucial for the countercurrent multiplier system, which helps in concentrating urine. This is a high-yield fact for exams like NEET and USMLE.
**Core Concept**
The nephron segments vary in permeability to water and solutes, crucial for urine concentration. The ascending limb of the loop of Henle is **impermeable to water**, allowing active reabsorption of Na⁺, K⁺, and Cl⁻ without water movement, which drives the countercurrent multiplier system.
**Why the Correct Answer is Right**
The **thick ascending limb of the loop of Henle** is impermeable to water due to the absence of aquaporin-1 channels. It actively transports Na⁺, K⁺, and Cl⁻ via the Na-K-2Cl cotransporter (NKCC2) in the luminal membrane, creating a hypertonic medullary interstitium. This solute reabsorption without water exit is essential for establishing the osmotic gradient necessary for urine concentration in the collecting duct.
**Why Each Wrong Option is Incorrect**
**Option A:** If the question lists the *descending limb of the loop of Henle*, it is **permeable to water** via aquaporin-1, allowing passive water reabsorption.
**Option B:** If the *proximal convoluted tubule (PCT)* is listed, it reabsorbs ~65% of filtered water via aquaporin-1, making it highly permeable.
**Option C:** If the *collecting duct* is listed, its permeability to water depends on antidiuretic hormone (ADH) levels, which regulate aquaporin-2 insertion. It is not inherently impermeable.
**Clinical Pearl / High-Yield Fact**
The ascending limb’s impermeability to water is the **key