hard · MCAT bio-biochem

The nephron maintains a steep osmotic gradient in the renal medulla to facilitate water reabsorption.

Which of the following correctly describes the roles of the loop of Henle and Antidiuretic Hormone (ADH) in this process?

  1. A salty renal medulla is generated by the collecting duct itself, and the loop of Henle then uses that osmotic gradient to help excrete dilute urine.
  2. ADH binds specific receptors along the ascending loop of Henle to directly increase the active transport of sodium ions into the peritubular capillary blood.
  3. The descending limb of the loop of Henle actively pumps chloride and sodium ions into the surrounding interstitium, creating a comparatively dilute medullary fluid.
  4. The loop of Henle establishes the medullary osmotic gradient via countercurrent multiplication, while ADH increases water permeability in the collecting duct.

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