In the presence of vasopressin, which part of nephrons become more permeable to water
**Core Concept**
The nephron's ability to reabsorb water is regulated by vasopressin, also known as antidiuretic hormone (ADH). Vasopressin acts on the collecting ducts of the nephron to increase water permeability, thereby facilitating water reabsorption and concentrating the urine.
**Why the Correct Answer is Right**
When vasopressin binds to its receptors in the collecting ducts, it activates a signaling cascade that increases the expression and insertion of aquaporin-2 water channels into the apical membrane of collecting duct cells. This results in increased water permeability of the collecting ducts, allowing water to be reabsorbed from the filtrate into the interstitium. The increased water reabsorption helps to concentrate the urine and reduce water loss in the urine.
**Why Each Wrong Option is Incorrect**
* **Option A:** The loop of Henle is not the primary site of vasopressin action in the nephron. While the loop of Henle is involved in water reabsorption, vasopressin's primary effect is on the collecting ducts.
* **Option B:** The proximal convoluted tubule is involved in bulk reabsorption of water and solutes, but it is not the primary site of vasopressin action. Vasopressin's effect on water permeability is more pronounced in the collecting ducts.
* **Option D:** The glomerulus is not directly affected by vasopressin. Vasopressin's action is downstream of the glomerulus, in the collecting ducts.
**Clinical Pearl / High-Yield Fact**
Vasopressin is essential for regulating water balance in the body, particularly in the presence of dehydration or water loss. Defects in vasopressin regulation or its receptors can lead to conditions such as diabetes insipidus, characterized by excessive thirst and polyuria.
**Correct Answer: C. Collecting ducts**