Loop of Henle

• The loop of Henle is responsible for absorbing water.
• The loop has 2 regions:

  1. The thin descending limb with walls is permeable to water.
  2. The thick ascending limb which is impermeable to water and permeable to salts.
• The loop acts as a counter current multiplier.

  Counter current- filtrate flows in opposite directions in the descending limb and collection duct compared to the ascending limb.

  Multiplier- the concentration of the filtrate is multiplied as you move down the loop.
• As the filtrate moves up the ascending limb, sodium and chloride ions are actively transported out into the interstitial fluid.

  Results: There is a gradient of sodium ions in the interstitial fluid moving down the medulla.

  Lower down in medulla = more ions = lower water potential.
• Descending limb is permeable to water. So water moves out of the filtrate, into the interstitial fluid by osmosis downa water potential gradient. The water enters blood capillaries by osmosis and is carried away to the renal vein.
• Descending Limb. The higher sodium ion concentration (lower water potential) of the medulla interstitial region maintains a water potential gradient for water to exit the filtrate by osmosis all along the descending limb.