By the end of this lesson
- Describe alveolar adaptations.
- Explain oxygen and carbon-dioxide diffusion.
- Relate ventilation and perfusion to gradients.
BIG QUESTION
How can hundreds of millions of tiny air sacs exchange enough gas for the whole body?
Large area, a two-cell-thin barrier, moist lining, ventilation and blood flow combine to maintain high diffusion flux.
Many alveoli create a large surface
Millions of microscopic alveolusA tiny air sac in the lungs where gas exchange happens; alveoli have thin walls, a large surface area, a moist lining and a rich blood supply. collectively provide a very large exchange area inside a compact thorax.
Elastic fibres allow expansion and recoil, supporting repeated ventilation without making alveoli rigid sacs.
The air–blood barrier is extremely thin
Alveolar epithelium and capillary endothelium are each about one cell thick, with closely associated basement membranes.
Oxygen and carbon dioxide therefore cross a very short diffusion distance.
Moisture and surfactant support function
A thin moist lining lets gases dissolve before membrane crossing. surfactantA substance lining the alveoli that reduces surface tension, stopping the alveoli from collapsing and sticking together. lowers surface tension and helps prevent alveolar collapse.
Too much fluid, as in pulmonary oedema, lengthens diffusion distance and impairs exchange.
Ventilation and blood flow maintain gradients
Ventilation replenishes oxygen and removes carbon dioxide from alveolar air. Dense capillary flow brings low-oxygen, high-carbon-dioxide blood and carries exchanged gases away.
Haemoglobin binding helps keep dissolved oxygen concentration low in blood plasma near the exchange surface, supporting continued diffusion.
Alveolar adaptations
Link every structural feature to a diffusion variable.
| Feature | Effect on exchange | If impaired |
|---|---|---|
| Many alveoli | Large surface area. | Less area reduces total diffusion. |
| Thin air–blood barrier | Short diffusion distance. | Thickening slows exchange. |
| Moist lining | Gases dissolve before crossing. | Excess fluid increases distance. |
| Ventilation | Maintains alveolar gradients. | Stale air reduces gradients. |
| Dense capillary flow | Maintains blood-side gradients. | Poor perfusion reduces exchange. |
| Surfactant | Reduces surface tension. | Alveoli are more prone to collapse. |
Language that earns precision
- alveolus
- A microscopic air sac where lung gas exchange occurs.
- capillary endothelium
- Single-cell lining of a capillary.
- surfactant
- A substance reducing surface tension in alveolar fluid.
- perfusion
- Blood flow through tissue capillaries.
- air–blood barrier
- Thin tissue layers separating alveolar air from capillary blood.
Common misconceptions
Claim: Alveoli pump oxygen into blood.
Correction: Oxygen diffuses down a concentration gradient.
Claim: Alveolar walls are dry to let gases through.
Correction: A thin moist lining is required for gases to dissolve.
Claim: One cell thick means only one cell total separates air and blood.
Correction: Alveolar epithelium and capillary endothelium each contribute thin cellular layers.
Claim: A rich blood supply provides oxygen to alveoli.
Correction: It maintains gradients and transports gases; alveolar oxygen comes from ventilated air.
Match the answer to the command word
Link area, distance and gradient to diffusion rate.
For disease questions, state which adaptation is lost and the consequence.
Keep ventilation and blood perfusion as distinct gradient-maintaining processes.
Recall, apply, then justify
Explain four alveolar adaptations for gas exchange.
Hint
Use area, distance, moisture and gradients.
Suggested marking points
- Many alveoli provide large surface area.
- This allows extensive simultaneous diffusion.
- Alveolar/capillary walls are thin.
- This gives a short diffusion distance.
- Ventilation and blood flow maintain steep concentration gradients.
- A moist lining permits gases to dissolve before crossing membranes.
Explain why pulmonary oedema reduces oxygen uptake.
Hint
Extra fluid changes distance.
Suggested marking points
- Fluid accumulates around/in alveoli.
- The diffusion distance for oxygen increases.
- Oxygen reaches blood more slowly, reducing uptake.
State the directions of oxygen and carbon-dioxide diffusion at an alveolus.
Hint
Air versus incoming blood.
Suggested marking points
- Oxygen diffuses from alveolar air into blood.
- Carbon dioxide diffuses from blood into alveolar air.
Explain why an alveolus with normal ventilation but no blood flow contributes little useful gas exchange.
Planning hint
One side of the gradient is not renewed.
What a strong design includes
- Alveolar air may contain normal oxygen.
- Without perfusion, no incoming deoxygenated blood receives oxygen.
- Blood-side concentration gradients are not maintained.
- Carbon dioxide is not delivered for removal.
- Ventilation is therefore wasted relative to whole-body exchange.
- Efficient exchange requires matching ventilation and perfusion.