Urine — Science definition
What remains of the filtrate after reabsorption: mostly water with urea and surplus salts, concentrated or dilute according to what the body needs.
Where it is taught
- Kidneys, Excretion & Internal Balance — Body Organization, Transport & Movement, Life Science & Scientific Inquiry, Grade 7
Questions using urine
Problem 1
Does entering a filtrate guarantee a substance leaves in urine?
Answer and explanation
No. Some substances are returned to blood during subsequent processing.
Problem 2
Illustrative processing record for one model interval: 100 water units enter a filtrate; 96 return to the blood pathway; 4 leave through the urine pathway.
Check the model balance, calculate the returned fraction, and explain why it is not a real intake recommendation.
Answer and explanation
The model balances its 100 input units with 100 output units, and 96% return through the stated pathway. It represents selective processing rather than all filtrate being excreted. The numbers are invented for one boundary and interval; they do not describe a person’s whole water budget or prescribe fluid intake.
Add outputs: 96+4=100.
Calculate returned share: 96/100×100%.
Identify the simplified boundary and invented units.
Response criteria
- Explain the relevant biological process accurately.
- Use the supplied evidence and show calculation steps with units where needed.
- Connect a labeled model or mechanism with the claim.
- State an assumption, evidence limitation, or useful next observation.