In a normally functioning kidney, which part of the tubule has the lowest permeability to water during antidiuresis?
## Core Concept
The question tests the understanding of renal physiology, specifically the regulation of water permeability along different segments of the renal tubule during anti-diuresis. Anti-diuresis is a state where the body aims to conserve water, concentrating the urine. This process involves the action of antidiuretic hormone (ADH), also known as vasopressin, which increases water permeability in certain parts of the renal tubule.
## Why the Correct Answer is Right
The correct answer, **D. Collecting Duct**, is right because during anti-diuresis, ADH acts on the collecting ducts to increase their permeability to water. However, even under the influence of ADH, the **thick ascending limb of the loop of Henle (D)** inherently has very low permeability to water. This segment is impermeable to water due to its structure and the absence of aquaporin-1 water channels, which are present in other segments like the proximal convoluted tubule and descending limb of the loop of Henle. The collecting duct's permeability to water is increased during anti-diuresis, but among the given options and in a general sense, the thick ascending limb of the loop of Henle has the lowest permeability to water under all conditions.
## Why Each Wrong Option is Incorrect
- **Option A:** Proximal Convoluted Tubule (PCT) has high permeability to water due to the presence of aquaporin-1 water channels, allowing for significant water reabsorption.
- **Option B:** Descending Limb of the loop of Henle is highly permeable to water, especially in the thin segment, facilitated by aquaporin-1.
- **Option C:** The permeability characteristics can vary, but generally, these segments do not have the lowest permeability compared to the thick ascending limb.
## Clinical Pearl / High-Yield Fact
A key point to remember is that ADH primarily acts on the collecting ducts to increase water reabsorption, leading to more concentrated urine during anti-diuresis. The thick ascending limb of the loop of Henle's low permeability to water is crucial for the countercurrent multiplication process that helps in concentrating the urine.
## Correct Answer: D. Collecting Duct is incorrect based on explanation; Correct Answer should reflect thick ascending limb
**Correct Answer: D. Thick Ascending Limb.**