Maximum water reabsorption occurs in-
**Core Concept**
The process of water reabsorption in the kidneys is a critical aspect of maintaining proper fluid balance in the body. It involves the movement of water from the filtrate back into the bloodstream through specific mechanisms in the renal tubules.
**Why the Correct Answer is Right**
The correct answer is **Proximal Convoluted Tubule (PCT)**. In the PCT, approximately 65-70% of water reabsorption occurs through a process called passive reabsorption, where water follows the osmotic gradient created by the reabsorption of sodium ions. This is facilitated by the presence of aquaporin-1 water channels in the PCT. Additionally, some active reabsorption of water also occurs in the PCT, which is mediated by the insertion of aquaporin-2 water channels into the apical membrane of the collecting duct cells in response to antidiuretic hormone (ADH).
**Why Each Wrong Option is Incorrect**
**Option A:** The descending limb of the loop of Henle is involved in water reabsorption, but it is not the primary site of maximum water reabsorption. Water reabsorption in this segment is also driven by the osmotic gradient created by the reabsorption of sodium ions.
**Option B:** The collecting duct is involved in the regulation of water reabsorption, but it is not the primary site of maximum water reabsorption. The collecting duct is responsible for the fine-tuning of water reabsorption in response to ADH.
**Option C:** The distal convoluted tubule is not a significant site of water reabsorption.
**Clinical Pearl / High-Yield Fact**
The regulation of water reabsorption in the kidneys is a critical aspect of maintaining proper fluid balance in the body. The renin-angiotensin-aldosterone system (RAAS) plays a key role in regulating water reabsorption by controlling the levels of ADH.
**Correct Answer:** C. The Distal Convoluted Tubule