AS Level Biology chapter 9: Gas exchange MCQs with answers
By Team Megalecture · Updated 4 Nov 2026
12 exam-style questions on 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. What is the function of the cartilage rings in the trachea?
A
to secrete mucus that traps dust and bacteria before they reach the alveoli
B
to allow rapid diffusion of oxygen through the wall of the trachea into the blood
C
to support the trachea and stop it collapsing when air pressure inside falls
D
to contract and narrow the trachea, so that less air enters when dust is inhaled
Show answer
Answer: C. Cartilage keeps the trachea open during inspiration, while the gap at the back allows food to pass down the oesophagus.
2. In which order does air pass through the gas exchange system when breathing in?
A
trachea → bronchioles → bronchi → alveoli
B
bronchi → trachea → bronchioles → alveoli
C
trachea → bronchi → bronchioles → alveoli
D
trachea → alveoli → bronchi → bronchioles
Show answer
Answer: C. The trachea divides into two bronchi, which branch into many bronchioles ending in the alveoli.
3. What are the functions of goblet cells and cilia in the trachea?
A
goblet cells secrete mucus to trap particles; cilia beat to sweep the mucus towards the throat
B
goblet cells secrete enzymes that digest bacteria; cilia beat to move the digested bacteria into the blood
C
goblet cells produce surfactant to reduce surface tension; cilia beat to pump air out of the alveoli
D
goblet cells absorb oxygen from the air; cilia beat to push the air down towards the alveoli
Show answer
Answer: A. Mucus traps dust and pathogens, and the beating cilia sweep it away from the lungs to be swallowed.
4. What is the function of the smooth muscle in the walls of the bronchioles?
A
it keeps the airways permanently open so that air can always flow freely
B
it contracts to narrow the airways and relaxes to widen them
C
it exchanges gases with the blood in the capillaries next to it
D
it moves mucus upwards by rhythmic contraction towards the throat
Show answer
Answer: B. Contraction of smooth muscle narrows the bronchioles, controlling airflow; cartilage keeps the larger airways open.
5. How is a steep oxygen concentration gradient maintained between alveolar air and the blood?
A
by ventilation bringing in fresh air and blood flow carrying oxygenated blood away from the alveoli
B
by a layer of mucus that dissolves oxygen and holds it at the alveolar surface
C
by active transport of oxygen from the alveoli into the red blood cells
D
by smooth muscle in the alveolar walls pumping air in and out
Show answer
Answer: A. Breathing keeps alveolar oxygen high and circulation keeps capillary oxygen low, so diffusion continues quickly.
6. What is the role of elastic fibres in the walls of the alveoli?
A
they contract during expiration to squeeze air out of the alveoli, as muscle does
B
they trap dust and bacteria in a layer of mucus on the alveolar surface
C
they stretch as the alveoli fill and recoil in expiration, helping to push air out
D
they produce surfactant, which stops the alveoli sticking together
Show answer
Answer: C. Elastic fibres stretch during inspiration and recoil passively during expiration. They do not contract, unlike muscle.
7. Which part of the gas exchange system has cartilage in its wall?
A
bronchioles
B
alveoli
C
capillaries
D
bronchi
Show answer
Answer: D. Cartilage is found in the trachea and bronchi. The bronchioles and alveoli have none.
8. During an asthma attack the bronchioles narrow and airflow falls. Which tissue in the walls of the bronchioles contracts to cause this?
A
squamous epithelium
B
elastic fibres
C
cartilage
D
smooth muscle
Show answer
Answer: D. Bronchioles have smooth muscle in their walls: when it contracts the airway narrows, and when it relaxes the airway widens. Bronchioles have no cartilage, and elastic fibres recoil passively rather than contract.
9. In a lung disease the wall separating alveolar air from the blood thickens from 0.5 μm to 2.0 μm. All other factors stay the same. By what factor does the rate of diffusion of oxygen across the wall change?
A
it falls to one quarter of its original rate
B
it does not change, because the concentration gradient is unchanged
C
it falls to one half of its original rate
D
it rises to four times its original rate
Show answer
Answer: A. By Fick's law, the rate of diffusion is inversely proportional to the diffusion distance. The distance has increased four times (2.0 ÷ 0.5), so the rate falls to one quarter. This is why thin alveolar walls are an adaptation for gas exchange.
10. A transverse section of an airway shows C-shaped rings of cartilage, ciliated epithelium with goblet cells, and mucous glands. Which airway is this?
A
bronchiole
B
alveolus
C
trachea
D
capillary
Show answer
Answer: C. C-shaped rings identify the trachea. Bronchi have irregular plates of cartilage and bronchioles have none.
12, each with the answer and a short explanation, across 1 topics: The gas exchange system. Every chapter test draws ten of them, so you can take it many times.
Is the AS Level 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.
