The mechanism by which water is reabsorbed from proximal convoluted tubule is
**Core Concept**
The reabsorption of water from the proximal convoluted tubule (PCT) is a crucial process in maintaining proper fluid balance and concentrating the urine in the kidneys. This process is highly dependent on the reabsorption of sodium ions, which creates an osmotic gradient that facilitates water reabsorption.
**Why the Correct Answer is Right**
The reabsorption of water from the PCT is primarily driven by the active reabsorption of sodium ions (Na+) through the sodium-hydrogen exchanger (NHE3) in the luminal membrane of the PCT cells. This process is facilitated by the enzyme sodium-potassium pump (Na+/K+-ATPase) in the basolateral membrane. As sodium ions are reabsorbed, water follows passively through the aquaporin-1 (AQP1) water channels in the PCT cells, creating an osmotic gradient that favors water reabsorption.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the primary mechanism of water reabsorption in the PCT.
**Option B:** This option is incorrect because it refers to the collecting duct, where water reabsorption is regulated by antidiuretic hormone (ADH) and the insertion of aquaporin-2 (AQP2) water channels.
**Option C:** This option is incorrect because it refers to the loop of Henle, where water reabsorption is driven by the countercurrent multiplier system and the presence of aquaporin-3 (AQP3) and aquaporin-4 (AQP4) water channels.
**Clinical Pearl / High-Yield Fact**
The reabsorption of water from the PCT is highly dependent on the reabsorption of sodium ions, which is regulated by the enzyme sodium-hydrogen exchanger (NHE3). This is a critical process that helps maintain proper fluid balance and concentrating the urine in the kidneys.
**Correct Answer: C. The loop of Henle**