MEDizzy - A 92-year-old man presents with dehydration following four

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Renal and Urinary Physiology

A 92-year-old man presents with dehydration following four days of persistent diarrhea. Under these circumstances, hypotonic fluid would be expected in which of the following?

ExplanationWhen a person is dehydrated, ADH secretion increases. In the presence of ADH, the cortical and medullary collecting tubules become permeable to water, and the filtrate within these portions of the nephron reaches osmotic equilibrium with the interstitial fluid surrounding them. The ascending limb of the loop of Henle is not affected by ADH and so remains impermeable to water. As sodium and other electrolytes are reabsorbed from the ascending limb, its filtrate becomes hypotonic. The glomerular filtrate and proximal tubular fluid remain isotonic to plasma, which in the case of dehydration is higher than normal.