A 14-year-old adolescent girl presenting with polyuria is subsequently diagnosed with Type I diabetes mellitus. The polyuria results from an osmotic diuresis that involves primarily which part of the renal tubule?

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Quizplus · Accepted Answer

In Type I diabetes mellitus, high blood glucose levels lead to glucose spilling into the urine, causing osmotic diuresis. This primarily affects the proximal tubule, where glucose reabsorption occurs. When the reabsorption capacity is exceeded, glucose remains in the tubule, drawing water with it and causing polyuria. The proximal tubule reabsorbs the majority (about two-thirds) of filtered salt and water. Both the luminal salt concentration and the luminal osmolality remain constant (and equal to plasma values) along the entire length of the proximal tubule. Water and salt are reabsorbed proportionally because the water is dependent on and coupled with the active reabsorption of Na⁺. The water permeability of the proximal tubule is high, and therefore a significant transepithelial osmotic gradient is not possible. Sodium is actively transported, mainly by basolateral sodium pumps, into the lateral intercellular spaces; water follows. Increased glucose filtration in diabetes will osmotically prevent water reabsorption at this site.

A 14-Year-Old Adolescent Girl Presenting with Polyuria Is Subsequently Diagnosed