S2 Science: Respiration, Circulation And Excretion (Paper 3 (challenge))

S2 Science: Respiration, Circulation and Excretion (Paper 3 (challenge))

2026-11-14

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Tutor Prep Guide · Respiration and gas exchange, features of alveoli, blood components and the three vessel types, the four heart chambers and double circulation, kidney filtration and reabsorption, dialysis and sweat

S2 Science: Respiration, Circulation and Excretion (Paper 3 (challenge))

This paper is calculation based: minute volume, beats per hour, converting a breath count into a rate, cardiac output, percentage of oxygen used, plasma volume, the energy ratio of aerobic to anaerobic respiration, kidney filtrate and reabsorption rate, heart rate increase, and finally the unit trap in a dialysis volume, all checked in Python.

Direct answers　Minute volume is rate times tidal volume, cardiac output is stroke volume times heart rate, percentages can only be subtracted on the same base, an increase is divided by the original value, mL differs from L by a factor of 1 000, and rate questions need matching time units.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	20
Difficulty mix	Foundation 3 · Intermediate 3 · Challenge 3 · Error redo 1
Chapter	Chapter: S2 Science: Respiration, Circulation and Excretion
Level	S2

02　Questions (10 items)

FoundationA person breathes 15 times a minute, taking in 0.5 L each time. What is the minute volume?
FoundationThe heart beats 72 times a minute. How many beats in one hour?
FoundationYou count 30 breaths in 30 seconds. What is the rate per minute?
IntermediateStroke volume is 70 mL and heart rate 72 per minute. What is the cardiac output in L per minute?
IntermediateInhaled air is 21% oxygen and exhaled air 16%. How many mL of oxygen are used per 100 mL of air?
IntermediateThe body holds 5 L of blood and plasma is 55% of it. What volume of plasma is that?
ChallengeAerobic respiration yields 38 ATP per glucose molecule while anaerobic yields only 2. How many times more is that?
ChallengeThe kidneys make 180 L of filtrate a day but pass only 1.5 L of urine. How much is reabsorbed and what percentage is that?
ChallengeResting heart rate is 70 per minute and rises to 150 after exercise. What is the percentage increase?
Error redoIn dialysis blood flows at 500 mL per minute for 4 hours. A student answers 500 times 4 equals 2 000 mL, or 2 L. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	7.5 L	Minute volume equals breathing rate times tidal volume, so 15 times 0.5, keeping the units consistent.
2	4 320 beats	An hour has 60 minutes, so multiply 72 by 60 after making the time units the same.
3	60 breaths	Thirty seconds is half a minute, so multiply by 2; that is far above the normal adult rate of 12 to 20.
4	5.04 L per minute	Cardiac output is 70 times 72, giving 5 040 mL, which is divided by 1 000 to become litres.
5	5 mL	The two percentages can be subtracted directly because both refer to the same volume of air.
6	2.75 L	5 times 0.55 is 2.75; plasma is mostly water with nutrients and waste dissolved in it.
7	19 times	38 divided by 2 is 19, which is why anaerobic respiration cannot last long during hard exercise.
8	178.5 L reabsorbed, which is 99.2%	180 minus 1.5 is 178.5, and dividing by 180 and multiplying by 100 gives the percentage, showing most water and nutrients are taken back.
9	About 114.3%	The increase is 150 minus 70, divided by the original 70 and multiplied by 100.
10	The 4 hours were not changed to minutes; it is 500 times 240, giving 120 000 mL or 120 L	The rate is per minute, so the time must be in minutes, otherwise the answer is out by a factor of 60.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

For respiration and circulation questions ask three things: whether the question is about structure, process or numbers, which way the air or blood is moving, and whether the units have been converted correctly.Tutor Registration →

Questions and answers are written originally by the TutorZone team and are not taken from any school, publisher or examination authority paper. Every value is checked by computer.