simplified

• descending limb before ascending limb
• • ascending limb contains enzymes that actively transport sodium ions out of the limb and into the medulla - wp of medulla decreases
  • wp gradient between medulla and loop of henle
• • only descending limb is permeable to water so water moves from descending limb to medulla
• countercurrent mechanism

  • up ascending limb wp increases as it loses sodium ions
  • down descending limb wp decreases