Kidneys, Excretion & Internal Balance — Practice · Grade 7 Science

Chapter: Body Organization, Transport & Movement

What this practice covers

By the end of Kidneys, Excretion & Internal Balance you should be able to:

8 questions, written answer, multiple choice and numeric, each with a worked explanation.

Vocabulary used: excretion, filtration, reabsorption, urine, metabolism.

Problem 1

Why is excretion not identical to eliminating unabsorbed food?
Answer and explanation

Excretion removes wastes produced by metabolism; unabsorbed food is material that did not enter those cellular pathways.

Problem 2

Does entering a filtrate guarantee a substance leaves in urine?
Answer and explanation

No. Some substances are returned to blood during subsequent processing.

Problem 3

Why must a model’s interval be stated?
Answer and explanation

Input and output amounts are comparable only when they refer to the same period and boundary.

Problem 4

Does regulation require an unchanging number at every instant?
Answer and explanation

No. Regulated conditions can vary within ranges while feedback and exchanges help maintain function.

Problem 5

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.

Problem 6

A draft calls kidney processing a digestive absorption step. Correct the system relationship.
Answer and explanation

Kidneys process materials from blood and regulate excretion and balance. Digestive absorption moves food-related materials across the intestinal lining; circulation connects the systems but the steps differ.

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.

Problem 7

Which material can be excreted through lungs?
  1. Carbon dioxide
  2. Every chromosome
  3. Whole bone
  4. Every unabsorbed food piece
Answer and explanation

Carbon dioxide

Carbon dioxide produced in cellular metabolism can be transported to lungs and released.

Problem 8

A model returns 45 of 50 filtered units. What percent returns?
Answer and explanation

90 %

45/50×100=90%, for the invented model.

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