In the presence of vasopressin the greatest fraction of filtered water is reabsorbed in which pa of the nephron?
**Core Concept**
The antidiuretic hormone (ADH), also known as vasopressin, plays a crucial role in regulating water reabsorption in the kidneys. In the presence of ADH, the collecting ducts of the nephron become highly permeable to water, allowing for increased water reabsorption. This process is essential for maintaining proper water balance and preventing dehydration.
**Why the Correct Answer is Right**
In the presence of vasopressin, the collecting ducts undergo a change in permeability due to the insertion of aquaporin-2 water channels into the apical membrane. This increase in permeability allows for the reabsorption of a significant fraction of filtered water back into the bloodstream. The collecting ducts are responsible for the final regulation of water reabsorption, and vasopressin's effect on this region is critical for maintaining proper water balance.
**Why Each Wrong Option is Incorrect**
**Option A:** The proximal convoluted tubule (PCT) is responsible for the reabsorption of approximately 70% of filtered water, but vasopressin does not primarily affect this region.
**Option B:** The loop of Henle is involved in the concentration of urine, but it is not the primary site of water reabsorption in the presence of vasopressin.
**Option C:** The distal convoluted tubule (DCT) is responsible for the reabsorption of sodium and potassium ions, but it is not the primary site of water reabsorption in the presence of vasopressin.
**Clinical Pearl / High-Yield Fact**
The effect of vasopressin on the collecting ducts is a critical mechanism for maintaining proper water balance in the body. In conditions such as diabetes insipidus, where vasopressin levels are low, the kidneys are unable to properly reabsorb water, leading to an excessive production of dilute urine.
**Correct Answer: C. Collecting ducts. Collecting ducts are the primary site of water reabsorption in the presence of vasopressin.**