5.2.5 Counter current multiplier system

Question 1

What are the four stages in order of the counter current multiplier system?

Answer 1

• The Ascending limb
• Descending limb
• Bottom of ascending limb
• Collecting duct.

Question 2

Describe what happens at the ascending limb.

Answer 2

• Sodium and chlorine ions actively pumped out into the medulla.
• Impermeable to water therefore water stays inside.
• Medulla water potential decreases due to high concentration of ions.

Question 3

Describe what happens at the descending limb.

Answer 3

• Lower water potential in medulla means water moves out of descending limb via osmosis into the medulla.
• Filtrate becomes more concentrated as it descends.
• Water now reabsorbed into capillary network associated with descending limb.

Question 4

Describe what happens at the bottom of the ascending limb.

Answer 4

• Sodium and chlorine ions diffuse out into medulla.
• Water potential decreased further.
• Ascending limb impermeable to water so water stays inside.

Question 5

• Describe what happens at the collecting duct.

Answer 5

• All previous stages have lowered water potential in the medulla.
• Water moves out of collecting duct via osmosis.
• Water is reabsorbed into the blood through associated capillary networks.