9.1.59 · Gas Exchange
Structures and Functions of the Gas Exchange System — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
5 min read 10 parts Lesson 7 of 9 Unit quiz
Lesson summary
9.1.5 Structures and Functions of the Gas Exchange System — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Explain how the structures of the airways and alveoli suit their functions, using Fick's law.Three jobs — carry air to the alveoli, protect the delicate exchang…
- Mucus escalator
- Warming and humidifying
- Smooth muscle
- Elastic fibres
- Very large surface area
- Thin walls
Part 1 of 8Overview of functions
- Three jobs
- carry air to the alveoli, protect the delicate exchange surface from damage and infection, and allow efficient diffusion between air and blood.
Part 2 of 8Main function of each part
| Structure | Main function |
|---|---|
| Trachea and bronchi | Conduct air; clean and warm it; resist collapse |
| Bronchioles | Distribute air to alveoli; adjust airflow |
| Alveoli | Gas exchange |
| Capillary network | Carries blood to and from alveoli and maintains the gradient |
Part 3 of 8Trachea and bronchi
Cartilage
- Cartilage it prevents collapse.
Mucus escalator
- Mucus escalator goblet cells and mucous glands secrete mucus rich in glycoproteins that traps bacteria, spores and dust.
- Cilia beat in a coordinated way to move the mucus to the pharynx, where it is swallowed.
Warming and humidifying
- Warming and humidifying capillaries beneath the lining warm the air, and mucus and moisture humidify it.
- Warm, moist air does not damage the alveoli or dry the exchange surface.
Part 4 of 8Bronchioles
Smooth muscle
- Smooth muscle contraction narrows the lumen to restrict irritant particles or gases, and relaxation widens it so air flows faster in exercise.
Elastic fibres
- Elastic fibres they let the walls return to their original diameter.
Branching
- Branching total cross-sectional area increases, so air slows towards the alveoli, giving time for diffusion.
Part 5 of 8The alveolus as an exchange surface
Very large surface area
- Very large surface area there are roughly 300 to 500 million alveoli in the two lungs, giving roughly 70 m2.
Thin walls
- Thin walls squamous epithelium and capillary endothelium are each one cell thick, giving a diffusion distance below 1 micrometre.
Dense capillary network
- Dense capillary network blood flow removes oxygen and brings carbon dioxide, keeping gradients steep.
- Red cells are squeezed in single file, slowing them and bringing them close to the wall.
- The capillary is only 3 to 4 micrometres wide, so there is time for exchange.
Moist lining
- Moist lining oxygen dissolves in the water film before diffusing;
- surfactant stops the film collapsing the alveolus.
Elastic fibres
- Elastic fibres the alveolus stretches to take in air and recoils to expel it.
Ventilation
- Ventilation it maintains high oxygen and low carbon dioxide in the alveolus compared with the blood.
Part 6 of 8Fick's law and efficiency
Fick's law
- Fick's law rate of diffusion is proportional to ÷ diffusion distance.
- (surface area × concentration difference)
Use
- Use the alveolar adaptations raise the numerator or reduce the denominator.
Fibrosis
- Fibrosis it thickens the wall, increasing the distance.
Emphysema
- Emphysema it destroys walls, reducing the surface area.
Pulmonary oedema
- Pulmonary oedema fluid fills the alveoli, increasing the distance.
Asthma
- Asthma narrowed bronchioles lower airflow.
Part 7 of 8Other exchange surfaces
Other exchange surfaces
Four shared features
- Four shared features a large surface area, a short diffusion distance, a mechanism to maintain a gradient and a moist surface.
Fish gill
- Fish gill many thin lamellae on each filament give a large area.
- Blood and water flow in opposite directions (counter-current), so equilibrium is never reached.
Insect
- Insect air passes through spiracles into tracheae and tracheoles that reach almost every cell.
- Fluid in the tracheole ends is withdrawn during activity to shorten the diffusion path.
Leaf
- Leaf air spaces and stomata, with spongy mesophyll giving a large internal surface area.
Cost of a moist surface
- Cost of a moist surface mammals lose water vapour when breathing out.
Part 8 of 8More from the specification
More from the specification
Epithelial lining of the respiratory tract
- Ciliated pseudostratified columnar epithelium with mucus-secreting goblet cells lines the trachea and bronchi, trapping inhaled particulate matter and microorganisms.
Alveolar wall ultrastructure
- Transmission electron micrographs reveal that alveolar walls consist of squamous epithelial cells and extracellular elastic fibres, providing an extremely thin diffusion pathway.
- (<1 micrometre)
Smooth muscle and cartilage in airway regulation
- C-shaped hyaline cartilage rings prevent tracheal collapse during inhalation,
- while bronchial smooth muscle contracts or relaxes to adjust airflow resistance.
Tissue
- A group of cells of similar structure that perform the same function.
Photomicrograph
- A photograph taken through a light microscope.
Micrograph
- A photograph taken through a microscope.
Microorganism
- A living organism so small that it can only be seen clearly with a microscope.
Electron micrograph
- A photograph taken through an electron microscope.
Key terms
| Term | Meaning |
|---|---|
| Fick's law | Diffusion rate is proportional to surface area × concentration difference ÷ diffusion distance. |
| Mucus escalator | Mucus swept upward by cilia to remove trapped particles. |
| Surfactant | A substance that lowers surface tension in alveoli. |
| Counter-current flow | Flow of two fluids in opposite directions, maintaining a gradient. |
| Concentration gradient | The difference in concentration between two regions. |
| Diffusion distance | The distance a molecule must travel across an exchange surface. |
Avoid these mistakes
1
Saying the alveolar wall is thick to prevent damage — it is one cell thick for short diffusion.
2
Saying ventilation is not needed because blood flow keeps the gradient — both ventilation and circulation are needed.
3
Saying the airways get shorter diffusion distances — the alveoli, not the airways, are the exchange surface.
4
Forgetting to use Fick's law terms (area, gradient, distance) in an efficiency explanation.
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