Question 1
Paper 2Hard16 marksResearchers investigated how active sodium-ion transport in the ascending limb of the loop of Henle relates to water reabsorption in the collecting ducts. They compared three groups of mammals: normal transport, reduced transport, and transport restored after being reduced. Each group contained eight animals. Water intake, the volume of fluid entering the collecting ducts (2.0 mL h⁻¹ per animal), and collecting-duct permeability to water were the same in all groups. The treatment used to reduce transport acted selectively on active sodium-ion transport in the ascending limb. Values are group means.
| Condition | Relative rate of active Na⁺ transport in ascending limb (normal = 100) | Osmotic concentration at cortex (arbitrary units) | Osmotic concentration at outer medulla (arbitrary units) | Osmotic concentration at inner medulla (arbitrary units) | Water reabsorbed from collecting ducts (%) | Urine flow (mL h⁻¹) |
|---|---|---|---|---|---|---|
| Normal transport | 100 | 300 | 600 | 1000 | 70 | 0.60 |
| Reduced transport | 40 | 300 | 420 | 560 | 42 | 1.16 |
| Transport restored | 90 | 300 | 580 | 960 | 68 | 0.64 |
Calculate the percentage decrease in osmotic concentration at the inner medulla when transport is reduced compared with normal transport.
Compare the osmotic concentration profiles and collecting-duct outcomes in the three conditions.
Explain how active sodium-ion transport in the ascending limb can affect water reabsorption from the collecting ducts and urine flow.
In a separate test, collecting-duct segments with identical water permeability were used. The osmotic concentration of the fluid inside each segment was 300 units. The surrounding solution had the concentration shown below; its solute could not cross the duct wall. The percentage of the initial water volume inside the segment that crossed out after 10 minutes was measured.
| Osmotic concentration of surrounding solution (arbitrary units) | Water crossing out of segment (%) |
|---|---|
| 300 | 0 |
| 600 | 24 |
| 1000 | 48 |
Deduce what this test adds to the interpretation of the first experiment.
A researcher concludes: “These results establish that active sodium-ion transport in the ascending limb is the only factor controlling water reabsorption from the collecting ducts.” Evaluate this conclusion using both experiments.