A 24-year-old man is brought to clinic after a weekend hiking trip in a desert climate. He reports poor access to water and now has dizziness when standing. Physical exam shows dry mucous membranes and orthostatic hypotension. Labs: serum Na+ 150 mEq/L, urine osmolality 950 mOsm/kg. The physician explains that the patient’s ability to concentrate urine depends on specialized nephrons that extend deep into the medulla and are accompanied by straight vessels that run alongside the loop.

Which of the following renal structures is most directly responsible for generating and maintaining the medullary osmotic gradient needed for this high urine osmolality in this patient?