Exam Tips: Gas Exchange

Adaptations always need reasons: When asked to explain alveolar adaptations, state the feature AND why it helps. "Thin walls" alone scores 0 at explanation level — add "so there is a short diffusion distance, allowing gases to diffuse quickly."

Breathing mechanism must include pressure: Describe what happens to diaphragm and intercostal muscles, what happens to chest volume, what this does to pressure, and then why air moves. All four steps are needed for full marks.

Gas transport in context: When asked how oxygen is transported, specify that it is mainly carried by haemoglobin in red blood cells, forming oxyhaemoglobin. Do not just say "in the blood."

Compare inspired and expired air: Inspired ~21% O₂, ~0.04% CO₂. Expired ~16% O₂, ~4% CO₂. Nitrogen stays roughly constant (~79%). These numbers appear in data interpretation questions.

Disease questions — link structure to function: For emphysema, asthma, or lung cancer, always connect the structural damage to the gas exchange effect. "Smoking causes cilia to stop working, so mucus builds up, blocking airflow and reducing ventilation, so less fresh air reaches alveoli, reducing the concentration gradient."

Link to other topics: Gas exchange links directly to Topic 3 (diffusion), Topic 11 (circulation), Topic 12 (blood), and Unit 3 (respiration). Exam questions often span two topics — be ready to explain how gas exchange supplies oxygen for cellular respiration.