The loop of Henle

• What is the role of the loop of Henle?

  To lower the water potential of the tissue fluid in the medulla so that water can move from the filtrate in the collecting duct into the medulla by osmosis. The water is then reabsorbed back into the blood. This allows us to produce concentrated urine
• What happens in the ascending limb of the loop of Henle?

  • as filtrate moves up the ascending limb, Na+ and Cl- are pumped out into the tissue fluid of the medulla by active transport
  • this requires lots of energy from ATP
  • walls of the ascending limb are impermeable to water, so water remains in the filtrate
  • there is now a high concentration of Na+ and Cl- in the medulla, so it has a very low water potential
• What happens in the descending limb of the loop of Henle?

  • at the top of the descending limb, the filtrate has a high water potential (equal to that of the blood)
  • walls of the descending limb are highly permeable to water and most solutes
  • so as filtrate passes down, water moves from a high water potential in the descending limb to a low water potential in the medulla by osmosis
  • filtrate becomes more concentrated due to the loss of water and the gain of urea by active transport
  • at the bottom of the descending limb, the filtrate is very concentrated
• The filtrate becomes increasingly concentrated as it moves down the descending limb and increasingly dilute as it moves up the ascending limb. This is the countercurrent multiplier, and it allows concentrated urine to be produced