What role does urea play in the kidney's ability to concentrate urine?
It increases water reabsorption by directly stimulating ADH release
It promotes active sodium reabsorption in the distal tubule
It contributes to the medullary osmotic gradient through recycling
It reduces the permeability of the collecting ducts to water
The Correct Answer is C
A. It increases water reabsorption by directly stimulating ADH release: ADH release is triggered by hypothalamic osmoreceptors sensing sodium concentrations and blood volume. Urea itself is not the primary physiological trigger for the neurohypophyseal release of ADH. It is a metabolic byproduct that aids the osmotic gradient.
B. It promotes active sodium reabsorption in the distal tubule: Urea transport is largely passive or facilitated and does not drive the active transport of sodium. Sodium reabsorption in the distal tubule is primarily regulated by aldosterone. Urea's role is restricted to the osmotic environment of the inner medulla.
C. It contributes to the medullary osmotic gradient through recycling: Urea is reabsorbed from the inner medullary collecting ducts and moves into the thin limbs of the loop of Henle. This recycling traps urea in the medulla, accounting for nearly 50% of its hypertonicity. This high osmolarity facilitates maximal water reabsorption.
D. It reduces the permeability of the collecting ducts to water: Urea does not alter the intrinsic water permeability of the ductal epithelium. Water permeability is strictly controlled by the presence of ADH-induced aquaporins. Urea provides the osmotic "pull" that makes that permeability effective for water recovery.
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Naxlex Comprehensive Predictor Exams
Related Questions
Correct Answer is {"dropdown-group-1":"B"}
Explanation
A. 10%: If 10% of the 180 L filtrate became urine, a person would excrete 18 L per day. This volume would lead to rapid and fatal hypovolemic shock due to massive fluid loss. Renal physiology is designed to conserve the vast majority of filtered water and solutes.
B. 1%: Normal daily urine output averages 1 to 2 L, which represents approximately 0.5% to 1% of the total glomerular filtration rate. The kidneys reabsorb over 99% of the filtered load through the renal tubules. This efficiency is critical for maintaining systemic fluid and electrolyte homeostasis.
C. 2%: A 2% excretion rate would result in nearly 3.6 L of urine daily, which exceeds the typical physiological range for a healthy adult. While possible under high fluid intake, it is not the standard baseline for filtration efficiency. The kidney's reabsorptive capacity is generally more robust.
D. 5%: This figure suggests an excretion of 9 L daily, which is characteristic of pathological states like diabetes insipidus. In a healthy state, the tubular system prevents such excessive loss through active and passive reabsorption. The standard percentage of filtrate converted to urine remains much lower.
Correct Answer is B
Explanation
B. False: The thick ascending limb is often termed the "diluting segment" because it is impermeable to water. It actively reabsorbs solutes like sodium, potassium, and chloride without accompanying water movement. This process reduces the osmolarity of the tubular fluid before it enters the distal tubule.
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