To interpret water-balance data, describe the pattern with numbers, then explain it with ADH and the kidney. Describing first keeps your explanation tied to the table.
This lesson follows tracing filtration, reabsorption and urine formation. Here the same ideas are applied to numbers in a table.
What is the three-step routine?
- Describe: state what changed, using values from the table.
- Compare: give the difference between the two conditions.
- Explain: link the difference to ADH and the kidney.
Each step can earn a mark, so a full answer needs all three.
Worked example with invented data
These figures are invented for practice. They describe one student on two days.
| Day 1 (cool, normal drinking) | Day 2 (hot, long sports practice) | |
|---|---|---|
| Water drunk (litres) | 1.5 | 1.5 |
| Water lost as sweat (litres) | 0.5 | 1.0 |
| Urine volume (litres) | 1.0 | 0.5 |
| Urine colour | Pale | Dark |
Question: Explain the difference in urine between the two days.
- Describe: urine volume fell from 1.0 to 0.5 litres, and the colour changed from pale to dark.
- Compare: sweat loss rose from 0.5 to 1.0 litres while water drunk stayed at 1.5 litres.
- Explain: more water was lost as sweat, so the blood became more concentrated. The hypothalamus detected this, more ADH was released, and more water was reabsorbed in the kidneys. So the urine had a smaller volume and a darker colour.
The mistake that loses marks
The common slip is to describe only: “Urine volume was lower on Day 2.” The marker also needs the reason.
| Part | Describing only | Full answer |
|---|---|---|
| Numbers | “Lower” | “Fell from 1.0 to 0.5 litres” |
| Cause | Missing | “More sweat, blood more concentrated” |
| Mechanism | Missing | “More ADH, more water reabsorbed” |
The fix is to write “because” after your numbers, then name the cause and the hormone.
A routine for any table
- Underline the factor that changed between conditions.
- Quote the values and the difference.
- Connect the change to water loss or intake.
- Finish with ADH and the amount of water reabsorbed.
The biological data interpretation trainer provides more tables to practise this on.
Check yourself
Invented data: Day 3 (cold, little sweating), water drunk 2.4 litres, sweat 0.3 litres, urine 2.2 litres, colour very pale. Explain the urine on Day 3 compared with Day 2.
Answer
Urine volume rose from 0.5 to 2.2 litres, and the colour changed from dark to very pale. More water was drunk and less was lost as sweat, so the blood was more dilute. Less ADH was released, so less water was reabsorbed and more dilute urine formed.
What to study next
Set two responses side by side in comparing regulatory responses without personal health advice. To log the marks you drop on tables, use the mistake log and paper-error review.
If you want a teacher to go through data tables with you, see online one-to-one Biology tuition.