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O Level Biology: Human Gas Exchange MCQs — Biology

15 multiple-choice questions on O Level Biology: Human Gas Exchange from Biology, each with the correct answer and, where available, a teacher's explanation. Read through them to revise the chapter, then practise the same questions one by one to test yourself.

1. What percentage of atmospheric air is carbon dioxide?

  1. 0.04%
  2. 21%
  3. 4%
  4. 78%

Answer: A) 0.04%

Explanation: Air is about 0.04% CO₂. O Level Biology · Topic 9 Human Gas Exchange · 9.1 Human gas exchange

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2. Cilia in the trachea

  1. sweep mucus up to the throat
  2. make mucus
  3. exchange gases
  4. produce antibodies

Answer: A) sweep mucus up to the throat

Explanation: Beating cilia move mucus away from the lungs. O Level Biology · Topic 9 Human Gas Exchange · 9.1 Human gas exchange

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3. Which is the correct order of air passing into the lungs?

  1. larynx → bronchioles → trachea → alveoli
  2. bronchi → trachea → alveoli → bronchioles
  3. alveoli → bronchioles → bronchi → trachea
  4. trachea → bronchi → bronchioles → alveoli

Answer: D) trachea → bronchi → bronchioles → alveoli

Explanation: Air goes trachea, bronchi, bronchioles, alveoli. O Level Biology · Topic 9 Human Gas Exchange · 9.1 Human gas exchange

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4. Where does gas exchange take place?

  1. Trachea
  2. Bronchi
  3. Larynx
  4. Alveoli

Answer: D) Alveoli

Explanation: Alveoli are the gas exchange surface. O Level Biology · Topic 9 Human Gas Exchange · 9.1 Human gas exchange

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5. During exercise, breathing becomes faster and deeper mainly to

  1. remove extra carbon dioxide and supply oxygen
  2. cool the body
  3. make more mucus
  4. slow the heart

Answer: A) remove extra carbon dioxide and supply oxygen

Explanation: More respiration produces more CO₂. O Level Biology · Topic 9 Human Gas Exchange · 9.1 Human gas exchange

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6. Limewater turns milky faster when you breathe out through it because expired air contains

  1. more nitrogen
  2. less water
  3. more carbon dioxide
  4. more oxygen

Answer: C) more carbon dioxide

Explanation: CO₂ turns limewater milky. O Level Biology · Topic 9 Human Gas Exchange · 9.1 Human gas exchange

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7. Air enters the lungs during inspiration because

  1. the ribs move down
  2. the volume of the thorax increases and pressure decreases
  3. the diaphragm relaxes
  4. the pressure in the lungs increases

Answer: B) the volume of the thorax increases and pressure decreases

Explanation: Lower pressure inside draws air in. O Level Biology · Topic 9 Human Gas Exchange · 9.1 Human gas exchange

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8. Why do alveoli have a rich capillary network?

  1. To make mucus
  2. To maintain a steep concentration gradient
  3. To keep the airways open
  4. To trap dust

Answer: B) To maintain a steep concentration gradient

Explanation: Blood flow carries gases away, keeping the gradient. O Level Biology · Topic 9 Human Gas Exchange · 9.1 Human gas exchange

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9. Which feature of a gas exchange surface gives a short diffusion distance?

  1. Large surface area
  2. Thin walls
  3. Ventilation
  4. Good blood supply

Answer: B) Thin walls

Explanation: A thin wall gives a short distance. O Level Biology · Topic 9 Human Gas Exchange · 9.1 Human gas exchange

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10. The function of goblet cells is to

  1. make mucus that traps pathogens and dust
  2. beat to move mucus
  3. keep the trachea open
  4. exchange gases

Answer: A) make mucus that traps pathogens and dust

Explanation: Goblet cells secrete mucus. O Level Biology · Topic 9 Human Gas Exchange · 9.1 Human gas exchange

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11. When breathing in, the external intercostal muscles

  1. contract, moving the ribs down
  2. contract, moving the ribs up and out
  3. relax, moving the ribs down
  4. relax, moving the ribs out

Answer: B) contract, moving the ribs up and out

Explanation: External intercostals raise the ribcage. O Level Biology · Topic 9 Human Gas Exchange · 9.1 Human gas exchange

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12. During inspiration, the diaphragm

  1. does not move
  2. contracts and flattens
  3. relaxes and domes upwards
  4. moves up into the chest

Answer: B) contracts and flattens

Explanation: Contraction flattens the diaphragm, increasing volume. O Level Biology · Topic 9 Human Gas Exchange · 9.1 Human gas exchange

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13. Compared with inspired air, expired air contains

  1. more oxygen and less carbon dioxide
  2. less nitrogen
  3. less water vapour
  4. more carbon dioxide and less oxygen

Answer: D) more carbon dioxide and less oxygen

Explanation: Respiration uses O₂ and produces CO₂. O Level Biology · Topic 9 Human Gas Exchange · 9.1 Human gas exchange

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14. What keeps the trachea open?

  1. The diaphragm
  2. Cilia
  3. Mucus
  4. Rings of cartilage

Answer: D) Rings of cartilage

Explanation: C-shaped cartilage stops collapse. O Level Biology · Topic 9 Human Gas Exchange · 9.1 Human gas exchange

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15. What percentage of atmospheric air is oxygen?

  1. 78%
  2. 16%
  3. 0.04%
  4. 21%

Answer: D) 21%

Explanation: Air is about 21% oxygen. O Level Biology · Topic 9 Human Gas Exchange · 9.1 Human gas exchange

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