Free water clearance is decreased by?
**Core Concept**
Free water clearance (C_H2O) is a measure of the volume of plasma that is cleared of solute-free water per unit time. It is an important parameter in assessing the regulation of water balance in the body, particularly in the kidneys. The ability to calculate C_H2O is crucial in evaluating the body's response to various stimuli, such as changes in blood osmolality or the presence of diseases like diabetes insipidus.
**Why the Correct Answer is Right**
The correct answer is related to the concept of free water clearance being inversely proportional to the serum osmolality. When serum osmolality increases, the kidneys reabsorb more water to concentrate the urine, thereby decreasing free water clearance. This is achieved through the insertion of more aquaporin-2 water channels in the collecting duct, which increases water reabsorption. This mechanism helps to maintain water balance in the body by reducing the amount of free water lost in the urine.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not directly relate to the factors that affect free water clearance. Free water clearance is not directly affected by the concentration of glucose in the blood.
**Option B:** This option is incorrect because it implies that free water clearance is increased by an increase in serum osmolality, which is actually the opposite of what occurs.
**Clinical Pearl / High-Yield Fact**
A key point to remember is that free water clearance is a measure of the kidneys' ability to regulate water balance, and it is affected by changes in serum osmolality. This concept is crucial in understanding various renal and endocrine disorders, such as diabetes insipidus and SIADH.
**Correct Answer: C. Increased serum osmolality.**