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AS · Practice questions · Alveoli, gas exchange and the airway tissues

Short path, steep gradient, air keeps moving.

Six original Cambridge-style questions on gas exchange in the alveoli and the tissues that keep air flowing, from partial pressures to asthma.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Explain fast diffusion

Area, distance, gradient.

01
Recall
[3 marks]

State the functions of cartilage, smooth muscle and elastic fibres in the gas exchange system.

  • Cartilage: supports the trachea and bronchi, keeping them open so they do not collapse during inhalation. ✓
  • Smooth muscle: contracts or relaxes to change the diameter of the airways, controlling airflow. ✓
  • Elastic fibres: stretch during inhalation and recoil during exhalation, helping push air out of the alveoli. ✓
02
Explain
[4 marks]

Describe how oxygen moves from the air in an alveolus into a red blood cell in a capillary.

  • Oxygen dissolves in the moisture lining the alveolus. ✓
  • It diffuses down its partial pressure (concentration) gradient, from higher in the alveolar air to lower in the blood. ✓
  • It crosses the squamous epithelium of the alveolus and the endothelium of the capillary, a distance of less than 1 μm. ✓
  • It diffuses through the plasma into the red blood cell and combines with haemoglobin. ✓
03
Explain
[4 marks]

Explain how ventilation and blood flow maintain a steep concentration gradient for oxygen across the alveolar wall.

  • Ventilation replaces alveolar air with fresh air, keeping the oxygen concentration in the alveoli high. ✓
  • It also removes carbon dioxide, keeping its concentration in the alveoli low. ✓
  • Blood flow carries oxygenated blood away from the alveoli. ✓
  • And brings deoxygenated blood with a low oxygen concentration, so oxygen keeps diffusing in. ✓
04
Calculation
[2 marks]

The alveolar wall is 0.2 μm thick and the capillary endothelium is 0.3 μm thick. Calculate the total diffusion distance between alveolar air and blood plasma in nanometres, and comment on its size.

  • 0.2 + 0.3 = 0.5 μm = 500 nm. ✓
  • Less than 1 μm, a very short diffusion distance, so diffusion is fast. ✓
05
Analysis
[3 marks]

Suggest why a person with asthma finds it harder to breathe out than to breathe in during an attack.

  • The smooth muscle in the bronchiole walls contracts, so the airways are narrowed. ✓
  • Breathing out relies on elastic recoil and the bronchioles are compressed further as the chest gets smaller, so they narrow more. ✓
  • Breathing in widens the airways as the lungs expand, so air enters more easily than it leaves. ✓
06
Data
[3 marks]

In the lab's alveolus model, alveolar pO2 starts at 13.3 kPa and blood pO2 at 5.3 kPa. When ventilation is switched off, both lines move towards each other. Explain this result.

  • Without ventilation, the oxygen that diffuses into the blood is not replaced in the alveolus, so alveolar pO2 falls. ✓
  • Blood still takes up oxygen, so its pO2 rises. ✓
  • The gradient shrinks, so the rate of diffusion falls towards zero. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.