Reabsorption of Water is maximum in:September 2009, September 2012, March 2013
**Core Concept**
The reabsorption of water in the kidneys is a critical process that helps maintain fluid balance in the body. It occurs mainly in the proximal convoluted tubule (PCT) and the collecting ducts under the influence of antidiuretic hormone (ADH).
**Why the Correct Answer is Right**
Water reabsorption in the kidneys is primarily facilitated by the presence of aquaporin-2 (AQP2) water channels in the collecting ducts. ADH stimulates the insertion of AQP2 channels into the apical membrane of collecting duct cells, thereby increasing water permeability and promoting water reabsorption. This process is essential for concentrating the urine and maintaining proper fluid balance in the body.
**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because the loop of Henle is primarily involved in reabsorbing ions and solutes, rather than water. While water reabsorption does occur in the descending limb of the loop of Henle, it is not the site of maximum water reabsorption.
**Option B:** This is incorrect because the distal convoluted tubule (DCT) is involved in reabsorbing potassium and calcium ions, but it is not the primary site of water reabsorption in the kidneys.
**Option C:** This is incorrect because the glomerulus is the site of filtration, where water and solutes are filtered from the blood into the nephron. It is not involved in reabsorption.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the regulation of water reabsorption in the kidneys is tightly controlled by ADH, which is released by the posterior pituitary gland in response to changes in blood osmolality. This helps maintain proper fluid balance in the body.
**Correct Answer:** D. Collecting ducts.