Vasopressin decreases the volume of urine primarily by causing:
**Core Concept**
Vasopressin, also known as antidiuretic hormone (ADH), is a hormone secreted by the posterior pituitary gland that plays a crucial role in regulating water balance in the body. It primarily acts on the kidneys to decrease urine production and increase water reabsorption.
**Why the Correct Answer is Right**
Vasopressin exerts its effects by binding to V2 receptors in the collecting ducts of the kidneys. This binding causes an increase in the permeability of the collecting ducts to water, allowing more water to be reabsorbed back into the bloodstream. As a result, the volume of urine produced by the kidneys decreases. The V2 receptors are coupled to G-proteins, which activate adenylate cyclase, leading to an increase in cyclic AMP (cAMP) levels. The increased cAMP levels activate protein kinase A (PKA), which in turn phosphorylates and activates aquaporin-2 water channels. These channels allow water to flow into the cell, increasing water reabsorption.
**Why Each Wrong Option is Incorrect**
**Option A:** Vasopressin does not primarily decrease urine production by increasing the glomerular filtration rate (GFR). While vasopressin may have some effects on GFR, its primary mechanism of action is through increasing water reabsorption in the collecting ducts.
**Option B:** Vasopressin does not primarily decrease urine production by increasing the secretion of potassium ions into the urine. Vasopressin's effects on potassium secretion are not its primary mechanism of action.
**Option C:** Vasopressin does not primarily decrease urine production by causing a decrease in blood volume. While vasopressin may have some effects on blood volume, its primary mechanism of action is through increasing water reabsorption in the collecting ducts.
**Clinical Pearl / High-Yield Fact**
One important clinical correlation to remember is that central diabetes insipidus, a condition caused by a deficiency of vasopressin, is characterized by an inability to concentrate urine and produce concentrated urine. This leads to excessive thirst and polyuria.
**Correct Answer:** C.