The loop of Henle as a counter-current multiplier

• Sodium ions are actively transported out of the ascending limb of the loop of Henle using ATP provided by the many mitochondria in the cells of its walls.
• Due to sodium ions moving out of the ascending limb, this creates a low water potential in the region of the medulla between the two limbs (the interstitial region). Since the thick ascending limb walls are almost impermeable to water, very little is lost by osmosis.
• The walls of the descending limb are very permeable to water, so it passes out of the filtrate via osmosis into the interstitial space. The water enters the blood capillaries in this region via osmosis and is carried away.
• As we move down the descending limb, the filtrate progressively loses water, lowering its water potential. At the bottom of the hairpin loop, the water potential is at its lowest.
• At the base of the ascending limb, sodium ions diffuse out of the filtrate and as it moves up the ascending limb these ions are also actively pumped out. Therefore, the filtrate also develops a progressively higher water potential.
• In the interstitial space between the ascending limb and the collecting duct, there is a gradient of water potential with the highest water potential in the cortex and an increasingly lower water potential the further into the medulla one goes.
• The collecting duct is permeable to water and so, as the filtrate moves down it, water passes out of it via osmosis. This water passes into the blood vessels that occupy this space, and is carried away.
• As water passes out of the filtrate its water potential is lowered. However, the water potential is also lowered in the interstitial space and so water continues to move out by osmosis down the whole length of the collecting duct. The counter-current multiplier ensures that there is always a water potential gradient drawing water out of the tubule.