Water Reabsorption

Question 1

Describe the process of water reabsorption

Answer 1

1. • Na+ and Cl- are actively transported out of the ascending limb into the tissue fluid
   • the ascending limb is impermeable to water so the water stays inside the tubule
   • so the water potential of the tissue fluid decreases as there is a high concentration of ions
2. • the water potential in the descending limb and collecting duct is higher than the water potential in the tissue fluid, so water moves out the the descending limb and collecting duct by osmosis down a water potential gradient
   • this makes the filtrate more concentrated (ions can’t diffuse out)
   • the water is then reabsorbed into the blood through the capillary network
3. • Na+ and Cl- ions diffuse out into the tissue fluid near the bottom of the ascending limb
   • this further decreases the water potential in the tissue fluid
   • the ascending limb is impermeable to water so it stays in the tubule
4. The first 3 stages massively increase the ion concentration in the tissue fluid of the medulla which lowers the water potential
   
   • this causes water to move out of the collecting duct by osmosis
   • the water in the tissue fluid in the medulla is reabsorbed into the blood through the capillary network

The ascending and descending limb set up a hairpin counter current multiplier mechanism

Question 2

How does the length of an animals loop of Henle affect the amount of water they can reabsorbed from the filtrate?

Answer 2

• the longer an animals loop of Henle the more water they can reabsorbed from the filtrate
• when there’s a longer ascending limb, more ions are actively pumped out into the medulla (tissue fluid) which creates a really low water potential in the medulla
• this means more water moves out of the nephron and collecting duct into the capillaries giving very concentrated urine
• this is useful for animals that live in areas where there’s little water as they can save as much water as possible