11.1 Gas exchange in humans
- Syllabus
- 0610–2026–2027
- Topic
- 11.1
- Level
- —
Learning objectives
11.1.1Features of gas exchange surfaces• Describe the features of gas exchange surfaces in humans, limited to: large surface area, thin surface, good blood supply and good ventilation with air11.1.2Following parts of the breathing• Identify in diagrams and images the following parts of the breathing system: lungs, diaphragm, ribs, intercostal muscles, larynx, trachea, bronchi, bronchioles, alveoli and associated capillaries11.1.3Differences in composition between• Investigate the differences in composition between inspired and expired air using limewater as a test for carbon dioxide11.1.4Differences in composition between• Describe the differences in composition between inspired and expired air, limited to: oxygen, carbon dioxide and water vapour11.1.5Effects of physical activity on• Investigate and describe the effects of physical activity on the rate and depth of breathing11.1.6Internal and external intercostal• Identify in diagrams and images the internal and external intercostal muscles11.1.7Function of cartilage in the trachea• State the function of cartilage in the trachea11.1.8Ventilation mechanics• Explain the role of the ribs, the internal and external intercostal muscles and the diaphragm in producing volume and pressure changes in the thorax leading to the ventilation of the lungs11.1.9Differences in composition between• Explain the differences in composition between inspired and expired air11.1.10Link between physical activity• Explain the link between physical activity and the rate and depth of breathing in terms of: an increased carbon dioxide concentration in the blood, which is detected by the brain, leading to an increased rate and greater depth of breathing11.1.11Role of goblet cells, mucus• Explain the role of goblet cells, mucus and ciliated cells in protecting the breathing system from pathogens and particles
Explain the features of human gas exchange surfaces
Human gas exchange surfaces allow oxygen and carbon dioxide to diffuse rapidly between alveolar air and blood.
| Feature | Why it speeds exchange |
|---|---|
| large surface area | provides more area for diffusion at the same time |
| thin surface | gives a short diffusion distance |
| good blood supply | carries gases away or brings them in, maintaining steep concentration gradients |
| good ventilation with air | continually refreshes alveolar air, maintaining steep concentration gradients |
The four required features work together; describing only a moist surface does not answer this syllabus objective.
Identify the parts of the human breathing system
Air follows a branching route to the gas exchange surface, while the thorax contains the structures that ventilate the lungs.
| Group | Structures to identify |
|---|---|
| air route | larynx → trachea → bronchi → bronchioles → alveoli |
| gas exchange | alveoli with associated capillaries |
| ventilation | lungs, ribs, intercostal muscles and diaphragm |
A bronchus is one of the two main branches from the trachea; bronchioles are the narrower branches leading towards alveoli.
Investigate carbon dioxide in inspired and expired air
Limewater tests for carbon dioxide: it changes from colourless to milky/cloudy when carbon dioxide is bubbled through it.
| Stage | What to do or observe |
|---|---|
| control variables | use equal limewater volumes and pass comparable air volumes at comparable rates |
| inspired-air path | draw atmospheric air through one limewater sample |
| expired-air path | bubble exhaled air through a separate limewater sample |
| compare | expired air makes limewater turn cloudy faster because it contains more carbon dioxide |
Use a one-way arrangement so liquid cannot be sucked into the mouth, and do not share mouthpieces.
A faster cloudy change compares carbon dioxide concentration; it does not directly test oxygen or water vapour.
Describe inspired and expired air
Gas exchange changes the composition of air between inspiration and expiration.
| Component | Inspired air | Expired air |
|---|---|---|
| oxygen | about 21% | about 16%; lower |
| carbon dioxide | about 0.04% | about 4%; higher |
| water vapour | lower/variable | higher, usually saturated |
Expired air still contains substantial oxygen; respiration does not remove all oxygen from each breath.
Investigate how physical activity changes breathing
Physical activity increases both breathing rate and breathing depth.
| Stage | Fair-test decision |
|---|---|
| before activity | measure breaths per minute and depth using chest movement or a spirometer |
| activity | use the same exercise, duration and intensity for comparisons |
| after activity | repeat measurements immediately and at fixed intervals during recovery |
| outcome | compare rate and depth with resting values; both rise after activity and then return towards rest |
On a volume–time trace, more cycles per minute mean a faster rate; a larger peak-to-trough change means greater depth.
Rate is breaths per unit time; depth is the volume moved in each breath. They are different variables.
Identify internal and external intercostal muscles
Intercostal muscles occur between adjacent ribs in two layers.
| Layer | Position and fibre direction in a diagram |
|---|---|
| external intercostal | more superficial layer; fibres run downwards and forwards |
| internal intercostal | deeper layer; fibres run downwards and backwards, crossing the external layer |
The names refer to the two muscle layers between ribs, not to muscles inside versus outside the lungs.
State the function of tracheal cartilage
C-shaped rings of cartilage support the trachea and prevent it from collapsing, keeping the airway open for airflow.
The incomplete rings provide support while allowing flexibility and space for the oesophagus behind the trachea.
Cartilage keeps the airway open; mucus traps particles and cilia move mucus, so those are different functions.
Explain ventilation of the lungs
Air moves because muscle action changes thoracic volume, which changes pressure relative to atmospheric pressure.
| Phase | Muscles and ribs | Volume and pressure | Air movement |
|---|---|---|---|
| inspiration | external intercostals contract; internal relax; ribs up and out; diaphragm contracts and flattens | thoracic volume increases; pressure falls below atmospheric | air enters |
| quiet expiration | external intercostals and diaphragm relax; ribs down and in; diaphragm domes | thoracic volume decreases; pressure rises above atmospheric | air leaves |
| forced expiration | internal intercostals contract, pulling ribs down and in | volume decreases further; pressure rises | air is forced out |
Use the full chain: muscle action → rib/diaphragm movement → volume change → pressure change → air movement.
The lungs are ventilated by pressure differences; air is not pulled in directly by the diaphragm.
Explain why inspired and expired air differ
Respiring cells change the gases carried by the blood, and exchange at the alveoli changes the air that is breathed out.
| Expired-air difference | Explanation |
|---|---|
| less oxygen | oxygen diffuses from alveoli into blood and is used in aerobic respiration |
| more carbon dioxide | carbon dioxide made in respiration is carried to the lungs and diffuses into alveoli |
| more water vapour | air is warmed and humidified by moist breathing surfaces |
Ventilation moves air; respiration in cells uses oxygen and produces carbon dioxide. The two processes are linked but not identical.
Explain how physical activity controls breathing
More active muscles respire faster, increasing the carbon dioxide concentration of the blood.
| Stage | Event |
|---|---|
| 1 | physical activity increases muscle respiration |
| 2 | carbon dioxide concentration in blood increases |
| 3 | the increase is detected by the brain |
| 4 | the brain increases signals to breathing muscles |
| 5 | breathing becomes faster and deeper, removing carbon dioxide more quickly |
The required control signal is increased carbon dioxide in the blood detected by the brain, not simply ‘the lungs need more air’.
Explain how mucus and cilia protect the breathing system
Goblet cells and ciliated cells form a cleaning system that prevents pathogens and particles reaching the gas exchange surfaces.
| Component | Protective role |
|---|---|
| goblet cells | secrete mucus |
| mucus | traps pathogens and particles |
| cilia on ciliated cells | beat to move mucus towards the throat |
| removal | mucus is swallowed or expelled, taking trapped material away from the lungs |
Cilia move the mucus; they do not secrete it. Goblet cells secrete mucus but do not sweep it.