How does urea contribute to the concentrated medulla?

1. Ammonia from protein breakdown is converted into urea by liver hepatocytes. 2. Urea is less toxic than ammonia and is also useful to the kidney. 3. Collecting ducts carry urea pumps that drive urea into the medullary interstitial spaces. 4. While water diffuses out of the descending limb, urea diffuses into its lumen from that concentrated interstitium. 5. Much of that urea is reabsorbed again once the fluid reaches the collecting duct, so it cycles rather than escaping. 6. Sodium and urea moving into the interstitium together create the hyperosmotic environment of the medulla. 7. A urea that never crossed a tubule membrane could not build any gradient at all. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 25 "The Urinary System", section 25.6 Tubular Reabsorption_