Knowledge Organiser
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 19 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 19 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 Mistakes
- Saying the diaphragm moves down during expiration: During expiration the diaphragm relaxes and moves back up (dome shape restored), chest volume decreases, pressure rises, and air is pushed out. "Moves down" only applies to inspiration.
- Saying we breathe out only CO₂: Exhaled air is still approximately 16% O₂ — only 5% less than inhaled air. The correct comparison is: inspired 21% O₂ / 0.04% CO₂; expired ~16% O₂ / ~4% CO₂. State both gases changing.
- Confusing breathing with respiration: Breathing (ventilation) is the physical movement of air in and out of the lungs. Respiration is the chemical process inside cells that releases energy from glucose. They are completely different processes — never use the words interchangeably.
- Forgetting the moist lining as an alveolar adaptation: Four adaptations are needed for full marks — large surface area, thin walls, good blood supply, AND moist lining (gases dissolve in the moisture before diffusing). Students list the first three and miss the fourth.
- Omitting pressure changes in breathing mechanism answers: A complete answer must state: muscle movement → volume change → pressure change → air movement direction. Answers that jump from "ribs move up" to "air enters" without mentioning pressure reduction score no more than half marks.
- Saying falling oxygen triggers faster breathing: It is rising carbon dioxide (detected by receptors in the medulla) that triggers an increase in breathing rate — not falling oxygen. Write: "rising CO₂ concentration in the blood is detected, which increases breathing rate."
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Practice Questions for Gas Exchange in Humans
Which is the correct order of structures air passes through to reach the lungs?
Explain how the structure of alveoli is adapted for efficient gas exchange.
Quick Recall Flashcards
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