O Level / IGCSE Biology chapter 9: Human gas exchange MCQs with answers
By Team Megalecture · Updated 4 Nov 2026
12 exam-style questions on Human gas exchange, each with the answer and a short explanation. Ten of them are below to try. The chapter test draws ten fresh questions each time and is marked the moment you finish, with your score by topic.
Try each one, then open the answer. These ten stay the same; the test draws new ones every time.
1. How do goblet cells and ciliated cells protect the gas exchange system?
A
goblet cells make enzymes that digest bacteria; cilia beat to push air into the lungs
B
goblet cells make mucus to trap dust and pathogens; cilia move the mucus to the throat
C
ciliated cells make mucus that traps dust; goblet cells sweep the mucus to the throat
D
goblet cells absorb oxygen from the air; cilia beat to pump air down to the alveoli
Show answer
Answer: B. Goblet cells produce mucus that traps dust and pathogens, and the ciliated cells beat to move the mucus towards the throat, where it is swallowed or coughed out.
2. Air breathed in contains 21% oxygen and air breathed out contains 16% oxygen. About what percentage of the oxygen breathed in is absorbed into the body?
A
16%
B
5%
C
24%
D
76%
Show answer
Answer: C. The oxygen taken in is 21 − 16 = 5 percentage points of the air. As a percentage of the 21% breathed in this is 5 ÷ 21 × 100 = 24%.
3. Air breathed in contains about 0.04% carbon dioxide. Air breathed out contains about 4%. How many times more carbon dioxide is there in the air breathed out?
A
10 times
B
100 times
C
4 times
D
1000 times
Show answer
Answer: B. 4 ÷ 0.04 = 100, so there is about 100 times as much carbon dioxide in expired air, because respiring cells produce it. Subtracting the percentages, or dividing the wrong way round, gives the wrong answers.
4. Limewater turns milky faster when expired air is bubbled through it than when inspired air is. Why?
A
expired air contains more oxygen than inspired air does
B
expired air contains more nitrogen, which reacts with limewater
C
expired air contains more carbon dioxide, which turns limewater cloudy
D
expired air is warmer, which makes limewater react faster
Show answer
Answer: C. Limewater turns milky when carbon dioxide is bubbled through it. Expired air contains about 100 times more carbon dioxide than inspired air, so the change is faster.
5. In which order does air pass through these structures on the way to the alveoli?
A
trachea, larynx, bronchiole, bronchus
B
larynx, trachea, bronchus, bronchiole
C
larynx, bronchus, trachea, bronchiole
D
bronchiole, bronchus, trachea, larynx
Show answer
Answer: B. Air passes the larynx, down the trachea, into a bronchus, then smaller bronchioles, and finally the alveoli.
6. Compared with inspired air, expired air contains
A
less oxygen, more carbon dioxide and more water vapour
B
less oxygen, less carbon dioxide and the same water vapour
C
less oxygen, more carbon dioxide and less water vapour
D
more oxygen, less carbon dioxide and more water vapour
Show answer
Answer: A. Respiration in body cells uses oxygen and produces carbon dioxide and water vapour, so expired air has less oxygen and more carbon dioxide and water vapour than inspired air.
7. Which feature of a gas exchange surface increases the rate of gas exchange?
A
a small blood supply so that the gases do not travel too far
B
walls that are one cell thick, giving a short diffusion distance for the gases
C
a dry surface, so that the oxygen does not dissolve in it
D
thick walls supported by cartilage rings to keep the airway open
Show answer
Answer: B. Gas exchange surfaces are very thin, so gases have a short distance to diffuse. They are also moist, with a good blood supply and a large surface area.
8. During exercise, the rate and depth of breathing increase. What is the explanation?
A
muscles need more nitrogen from the air to make proteins
B
the lungs grow larger during exercise, so they can hold more air
C
more carbon dioxide is made, and the brain increases breathing to remove it
D
breathing faster cools the body, which is detected by the skin and lungs
Show answer
Answer: C. Respiring muscles produce more carbon dioxide, which is detected by the brain. It increases the rate and depth of breathing to remove the carbon dioxide and supply more oxygen.
9. Which muscles contract during forced expiration?
A
the external intercostal muscles
B
the internal intercostal muscles
C
the diaphragm
D
the ciliary muscles
Show answer
Answer: B. Internal intercostals pull the ribs down and in, making the thorax smaller and pushing air out forcefully.
10. At rest, a person takes 15 breaths per minute of 0.5 dm3 each. During exercise they take 30 breaths per minute of 2.0 dm3 each. How many times greater is the volume breathed per minute during exercise?
A
2
B
4
C
8
D
15
Show answer
Answer: C. At rest 15 × 0.5 = 7.5 dm3/min; exercise 30 × 2.0 = 60 dm3/min. 60 ÷ 7.5 = 8. Looking at rate or depth alone gives 2 or 4.
12, each with the answer and a short explanation, across 1 topics: Human gas exchange. Every chapter test draws ten of them, so you can take it many times.
Is the O Level / IGCSE Biology chapter 9 test free?
Yes. The test, the answers and the explanations are free and need no sign-up. Sign in only if you want your scores kept across devices.
How is the chapter test marked?
The moment you press Finish: your score, the right answer and an explanation for every question, and the ones you got wrong saved in My mistakes to retry.
