Alveolar gas exchange
BiologyΒ· 25 min read
1. Structure of Alveoliβ β ββββ± 15 min
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Alveoli are tiny air sacs located at the termini of respiratory bronchioles in mammalian lungs. They form the primary gas exchange surface between inhaled air and the pulmonary circulation.
Alveolus (pl. alveoli)
Thin-walled, elasticated air sac that is the primary site of gas exchange between lungs and blood in mammals
Example:
Human lungs contain ~480 million alveoli, with a total surface area of ~70 mΒ².
Explain how two structural features of an alveolus adapt it for gas exchange.
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First, always link structural features directly to the requirement for rapid diffusion:
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Feature 1: Alveolar walls are only one cell thick, made of flattened squamous epithelial cells.
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This gives a very short diffusion distance, which increases the rate of diffusion of oxygen and carbon dioxide.
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Feature 2: Alveoli have a very large combined total surface area across the lung.
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A larger surface area allows a greater total volume of gas to diffuse per unit time, supporting high metabolic demand.
Exam tip:
CIE exam questions require you to link structure to function, not just list features. Always add the functional outcome.
2. Mechanism of Alveolar Gas Exchangeβ β β βββ± 20 min
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Gas exchange across the alveolar surface is a passive process that occurs by diffusion down partial pressure gradients. Net movement of each gas depends on the difference in its partial pressure between alveolar air and blood.
Partial pressure gradient
A difference in the partial pressure of a specific gas between two regions, which drives net diffusion of the gas from higher to lower pressure.
Alveolar = 100 mmHg, deoxygenated blood = 40 mmHg. Alveolar = 40 mmHg, blood = 45 mmHg. State the direction of net movement of each gas and explain why.
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Net diffusion always moves from higher to lower partial pressure for a given gas:
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For oxygen: is higher in alveoli than in blood, so net movement is:
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For carbon dioxide: is higher in blood than in alveoli, so net movement is:
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This gradient is continuously maintained by ventilation (fresh air flow) and constant blood flow, so diffusion never reaches equilibrium.
3. Diffusion Pathway Adaptationsβ β β βββ± 15 min
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An oxygen molecule must cross multiple tissue layers to move from alveolar air into a red blood cell. All layers are adapted to keep the total diffusion distance extremely short.
Alveolar air space
Squamous epithelial cell of the alveolar wall
Fused basement membrane between alveolar epithelium and capillary endothelium
Endothelial cell of the capillary wall
Blood plasma
Red blood cell membrane
Describe the full diffusion path of an oxygen molecule from inhaled air to binding with haemoglobin.
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- The oxygen molecule starts in the air space inside the alveolus.
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- It diffuses across the thin cytoplasm of the alveolar squamous epithelial cell.
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- It crosses the fused shared basement membrane between the epithelium and capillary.
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- It diffuses through the capillary endothelial cell into the blood plasma.
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- It crosses the red blood cell membrane and binds to haemoglobin.
4. Common Pitfalls
Wrong move:
Claiming alveolar gas exchange occurs via active transport.
Why:
Confusion between cellular respiration (which requires ATP) and the physical process of gas exchange.
Correct move:
Alveolar gas exchange is passive diffusion down a partial pressure gradient; no ATP is required.
Wrong move:
Only listing alveolar adaptations without linking them to function.
Why:
Most CIE questions ask to 'explain' adaptations, which requires connecting structure to diffusion rate.
Correct move:
Always add the functional link e.g. 'one cell thick walls give a short diffusion distance, increasing diffusion rate'.
Wrong move:
Mixing up the direction of oxygen and carbon dioxide movement.
Why:
Forgetting that partial pressure gradients determine direction of diffusion.
Correct move:
Oxygen diffuses from alveoli into blood; carbon dioxide diffuses from blood into alveoli.
Wrong move:
Forgetting to mention what maintains the diffusion gradient.
Why:
Many candidates only name adaptations, not the process that keeps the gradient steep.
Correct move:
State that both constant ventilation (to refresh alveolar air) and continuous blood flow (to remove oxygen, bring carbon dioxide) maintain the gradient.
5. Quick Reference Cheatsheet
Alveolar Adaptation | Function for Gas Exchange |
|---|---|
Large total surface area | Maximises total rate of diffusion |
One cell thick walls | Short diffusion distance, increases rate |
Fused basement membrane | Eliminates extra extracellular space, keeps diffusion path <1ΞΌm |
Dense capillary network | Maintains steep partial pressure gradient |
Elastic fibres | Stretch for inhalation, recoil for exhalation |
Surfactant secretion | Reduces surface tension, prevents alveolar collapse |
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2022 Β· 2
Alveolar adaptations for gas exchange
- 2023 Β· 1
Diffusion path for carbon dioxide
Going deeper
What's Next
Understanding alveolar gas exchange is the foundation for learning how gases are transported in the blood and how respiration is regulated during activity like exercise. The core principle of linking gas exchange surface structure to function applies to all gas exchange systems you will study for CIE 9700, from fish gills to plant leaf stomata. This topic is frequently tested in both multiple choice and extended response questions, so mastering the key links between structure and function is critical for exam success. Follow the links below to build on this knowledge: