This topic summary covers Knowledge Organiser within Gas Exchange in Humans for GCSE Biology. Lung structure, alveoli adaptations, breathing mechanism, gas transport in blood, and effects of smoking It is section 18 of 19 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 18 of 19
Practice
19 questions
Recall
23 flashcards
Knowledge Organiser
Key Terms
- Alveolus — microscopic air sac for gas exchange
- Trachea — main airway to lungs
- Bronchi — airways into each lung
- Bronchioles — smaller airways within lungs
- Diaphragm — main breathing muscle
- Intercostal muscles — rib muscles for breathing
- Concentration gradient — drives diffusion
- Ventilation — breathing (moving air in/out)
- Tidal volume — amount of air per breath
Must-Know Facts
- Alveoli: large surface area, thin walls, moist lining, good blood supply
- Inspired air: 21% O₂, 0.04% CO₂
- Expired air: ~16% O₂, ~4% CO₂
- Inspiration: diaphragm contracts (flattens down), ribs up and out, pressure drops, air in
- Expiration: diaphragm relaxes (returns up), ribs down and in, pressure rises, air out
- Breathing rate trigger: rising CO₂ (not falling O₂)
- Emphysema: alveoli merge, surface area falls, less gas exchange
Common Marks Lost
- Saying diaphragm moves down during expiration (it moves up)
- Saying we breathe out only CO₂ (exhaled air is still 16% O₂)
- Confusing breathing with respiration
- Forgetting the moist lining as an alveolar adaptation
- Not mentioning pressure changes in breathing mechanism answers
- Saying "need more oxygen" triggers faster breathing — it is rising CO₂