Reabsorption of water

Which part of the nephron is responsible for reabsorbing the remaining water into blood?

The loop of Henle.

Summarise how the loop of Henle is responsible for reabsorbing the remaining water back into the blood.

The role of the loop of Henle is to lower the water potential of the tissue fluid in the medulla; this causes water to move out of filtrate and into the medulla by osmosis, and then this is reabsorbed back into the blood.

The mechanism of water reabsorption in the loop of Henle is known as what?

Counter current multiplier.

Why is the mechanism called a ‘counter current multiplier’?

The filtrate flows in the 2 limbs in opposite directions (counter-current), and the concentration of solutes is increased (multiplied).

Describe the role of the ascending limb in water reabsorption as the filtrate moves up the ascending limb of the loop of Henle.

• The walls of the ascending limb are impermeable to water.

• As filtrate flows up the ascending limb of the loop of Henle, Na+ and Cl^- ions are actively transported out of the filtrate in the tubule into the tissue fluid of the medulla.

• This creates a high concentration of Na+ and Cl^- ions in the tissue fluid of the medulla, lowering the water potential.

Describe the role of the descending limb in water reabsorption as the filtrate moves up the descending limb of the loop of Henle.

• The walls of the descending limb are permeable to water.

• As filtrate flows down the descending limb, water moves out by osmosis, into the tissue fluid of the medulla, which has a low water potential (due to the ascending limb actively transporting sodium and chloride ions into it).

• From there, it moves into the vasa recta i.e. the capillaries surrounding the loop of Henle.

• As water is lost from the descending limb, the filtrate becomes more and more concentrated; at the bottom of the loop, the filtrate is most concentrated with the lowest water potential.

Describe how water also moves out of the collecting duct.

• The collecting duct runs into the medulla, passing through the region of low water potential.

• Water moves out of the collecting duct by osmosis, down a water potential gradient, and is reabsorbed back into the blood.

Explain how desert animals having a longer loop of Henle allows them to be adapted to their environment.

A longer loop of Henle means that more ions can be actively transported into the medulla, lowering the water potential in the medulla even more.

As a result, more water can be reabsorbed by osmosis (i.e. less water is lost in urine).