Which protein is primarily responsible for generating the medullary urea gradient essential for maximum urinary concentration, and where is it located?

• A UT-A1 and UT-A3 urea transporters in the inner medullary collecting duct, regulated by vasopressin ✓
• B NKCC2 (SLC12A1) in the thick ascending limb, actively concentrating urea in the medulla
• C UT-A2 in the thin descending limb, secreting urea from the interstitium into the lumen
• D AQP1 in vasa recta, trapping urea by osmotic equilibration

Explanation

Vasopressin (ADH) increases the permeability of the inner medullary collecting duct to urea via UT-A1 (apical) and UT-A3 (basolateral) transporters. Urea is concentrated in the tubular lumen during water reabsorption in the cortical and outer medullary collecting duct, then exits through UT-A1/A3 in the inner medullary collecting duct into the interstitium. This urea recycling (via UT-A2 in the thin descending limb returning urea to the loop) establishes and sustains the inner medullary hyperosmolar gradient, allowing maximum urine concentration up to 1200 mOsm/kg. NKCC2 generates the NaCl gradient, not urea.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.