USMLE (United States Medical Licensing Examination) Step 1 Practice Exam

What effect does increased tubular flow rate in the loop of Henle have on urine concentration?

1. Increases urine concentration

2. Decreases urine production

3. Increases countercurrent mechanism efficiency

4. Produces dilute urine in large amounts

The correct answer is: Produces dilute urine in large amounts

Increased tubular flow rate in the loop of Henle leads to the production of dilute urine in large amounts. This phenomenon occurs because a high flow rate diminishes the time that tubular fluid spends in contact with the osmotic gradient established by the medullary interstitium. The loop of Henle plays a crucial role in urine concentration through the countercurrent multiplier system, where the descending limb is permeable to water but not to solutes, while the ascending limb is impermeable to water and actively transports sodium and chloride ions out into the medullary interstitium. If the flow rate is increased, the filtrate moves through these structures more rapidly, which allows less time for water reabsorption in the descending limb and less time for solute reabsorption in the ascending limb. As a result, more sodium and chloride remain in the tubular fluid, which leads to the dilution of urine. This process is particularly significant during conditions of high fluid intake or diuretic use, where a rapid flow in the nephron can hinder the kidney's ability to concentrate urine effectively, leading to a larger volume of dilute urine being excreted.