The vasa recta and countercurrent multiplication.
D000755:Anemia, Sickle Cell / Q000378:metabolism; D000818:Animals; D001783:Blood Flow Velocity; D001794:Blood Pressure / Q000502:physiology; D006801:Humans; D007668:Kidney / Q000098:blood supply*; D007684:Kidney Tubules / Q000033:anatomy & histology* / Q000502:physiology*; D014508:Urea / Q000378:metabolism*; D014556:Urine; D014662:Vasoconstrictor Agents / Q000494:pharmacology; D014665:Vasodilator Agents / Q000494:pharmacology