Calculate a supplied reabsorption balance
Find returned and remaining amounts in an invented closed model.
Section 13.1 · Excretion in humans
Extended only (Supplement); Biology 0610, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked
Before you begin
- Recall cell metabolism, blood transport, amino acids and the distinction between excretion and egestion.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How is remaining amount calculated in this closed model?
What you will learn
- Subtract return from filtered amount
- Check the quantifiers against the values
Calculate a supplied reabsorption balance
Invented closed balance: filtered minus returned gives remaining amount. The percentages are model values, and concentration is not supplied. Original teaching model.
| Substance | Filtered | Returned | Remaining |
|---|---|---|---|
| Water | 40 | 36 | 4 |
| Glucose | 10 | 10 | 0 |
| Ions | 12 | 8 | 4 |
| Feature | Meaning |
|---|---|
| Water | 40 filtered − 36 returned = 4 remaining |
| Glucose | 10 − 10 = 0 remaining |
| Ions | 12 − 8 = 4 remaining |
| Water fraction returned | 36/40 = 90% in this model |
Subtract return from filtered amount
The invented teaching model begins with forty water units, ten glucose units and twelve ion units in the filtered contents. The stated return amounts are thirty-six water, ten glucose and eight ion units. Subtract return from the corresponding filtered amount to find what remains under this closed, simplified accounting.
The remaining amounts are four water units, zero glucose units and four ion units. Thirty-six out of forty water units is ninety percent, supporting most water in this particular model; ten out of ten glucose is all glucose, and eight out of twelve ions is some ions. These numbers illustrate the normal-outline quantifiers, not fixed biological percentages.
Check the quantifiers against the values
The model uses amounts, not concentrations. Four remaining ion units do not establish an ion concentration without a corresponding volume and defined units. Returning water can change concentration even when a dissolved amount falls, so do not substitute an amount comparison for a concentration calculation without the required information.
A valid balance keeps the same substance and same accounting boundary on both sides. Do not subtract glucose return from water input or turn negative amounts into plausible urine values. Urea processing and other routes are not numerically specified in this model; the exercise does not imply that every real kidney route has been measured or that all filtered urea always remains unchanged.
Pause and explain
Which set of remaining amounts is correct?
Worked example
Find water remaining after thirty-six of forty model units return.
Show the worked solution
- Use input minus return.
- 40 − 36 = 4.
Answer Four invented water units remain under the stated closed model.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Calculate all three remaining amounts using the supplied model.
Give me a hint
Subtract each material’s own return.
Compare my reasoning
- Water: 40 − 36 = 4; glucose: 10 − 10 = 0.
- Ions: 12 − 8 = 4.
Water 4 units; glucose 0 units; ions 4 units, under the stated closed model.
Look for these in your work
- Keep substances matched
- Calculate all three
Calculate the fraction of water returned and explain why it fits most.
Give me a hint
Compare returned water with filtered water.
Compare my reasoning
- 36 ÷ 40 = 0.9 = 90%.
- Nine tenths is the larger portion.
90% of model water returns, which is most in this invented example; it is not a universal kidney percentage.
Look for these in your work
- Calculate the fraction
- Retain the model limit
The remaining ions are four model units. Is their concentration established?
Give me a hint
A concentration also needs a volume and defined units.
Compare my reasoning
- The amount is given.
- The final volume and concentration units are not.
No; four units is an amount. A corresponding volume and defined units are needed to calculate concentration.
Look for these in your work
- Distinguish amount and concentration
- Name the missing information
Common mistakes
- Treating model percentages as universal.
- Equating amount with concentration.
What can you explain now?
Which set of remaining amounts is correct?
Compare with the explanation
Water 4; glucose 0; ions 4
40−36, 10−10 and 12−8.
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Make it stick
Calculate the three remaining amounts and explain why an amount alone is not concentration.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.