# Alveolar gas exchange

> Biology · CIE A-Level
> Source: https://www.owlsprep.com/study/cie-9700-u9-alveolar-gas-exchange/

This module covers the structure and function of alveoli as the mammalian respiratory gas exchange surface. You will learn how structural adaptations enable efficient diffusion of oxygen and carbon dioxide between alveolar air and blood.

**Prerequisites:** [Structure of the mammalian respiratory system](https://www.owlsprep.com/study/cie-9700-u9-gas-exchange-system-structure/); Diffusion and partial pressure of gases

## Learning objectives

- Describe the structure of the alveolar gas exchange surface
- Explain how partial pressure gradients drive gas exchange between alveoli and blood
- Relate alveolar structural adaptations to efficient gas exchange function

## Structure of Alveoli

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 m².

**Worked example:** Explain how two structural features of an alveolus adapt it for gas exchange.

1. First, always link structural features directly to the requirement for rapid diffusion:
2. Feature 1: Alveolar walls are only one cell thick, made of flattened squamous epithelial cells.
3. This gives a very short diffusion distance, which increases the rate of diffusion of oxygen and carbon dioxide.
4. Feature 2: Alveoli have a very large combined total surface area across the lung.
5. 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.

*Calculator:* forbidden

## Mechanism of Alveolar Gas Exchange

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.

**Worked example:** Alveolar $pO_2$ = 100 mmHg, deoxygenated blood $pO_2$ = 40 mmHg. Alveolar $pCO_2$ = 40 mmHg, blood $pCO_2$ = 45 mmHg. State the direction of net movement of each gas and explain why.

1. Net diffusion always moves from higher to lower partial pressure for a given gas:
2. For oxygen: $pO_2$ is higher in alveoli than in blood, so net movement is:
3. $$O_2: \text{Alveoli} \rightarrow \text{Blood}$$
4. For carbon dioxide: $pCO_2$ is higher in blood than in alveoli, so net movement is:
5. $$CO_2: \text{Blood} \rightarrow \text{Alveoli}$$
6. This gradient is continuously maintained by ventilation (fresh air flow) and constant blood flow, so diffusion never reaches equilibrium.

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## Diffusion Pathway Adaptations

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.

1. Alveolar air space
2. Squamous epithelial cell of the alveolar wall
3. Fused basement membrane between alveolar epithelium and capillary endothelium
4. Endothelial cell of the capillary wall
5. Blood plasma
6. Red blood cell membrane

> **tip**
>
> The shared fused basement membrane eliminates any connective tissue space between the alveolus and capillary, keeping the total diffusion distance under 1 μm, one of the most critical adaptations for rapid diffusion.

**Worked example:** Describe the full diffusion path of an oxygen molecule from inhaled air to binding with haemoglobin.

1. 1. The oxygen molecule starts in the air space inside the alveolus.
2. 2. It diffuses across the thin cytoplasm of the alveolar squamous epithelial cell.
3. 3. It crosses the fused shared basement membrane between the epithelium and capillary.
4. 4. It diffuses through the capillary endothelial cell into the blood plasma.
5. 5. It crosses the red blood cell membrane and binds to haemoglobin.

*Calculator:* forbidden

## Common pitfalls

- **Wrong:** Claiming alveolar gas exchange occurs via active transport.
  - Why it fails: Confusion between cellular respiration (which requires ATP) and the physical process of gas exchange.
  - Correct: Alveolar gas exchange is passive diffusion down a partial pressure gradient; no ATP is required.
- **Wrong:** Only listing alveolar adaptations without linking them to function.
  - Why it fails: Most CIE questions ask to 'explain' adaptations, which requires connecting structure to diffusion rate.
  - Correct: Always add the functional link e.g. 'one cell thick walls give a short diffusion distance, increasing diffusion rate'.
- **Wrong:** Mixing up the direction of oxygen and carbon dioxide movement.
  - Why it fails: Forgetting that partial pressure gradients determine direction of diffusion.
  - Correct: Oxygen diffuses from alveoli into blood; carbon dioxide diffuses from blood into alveoli.
- **Wrong:** Forgetting to mention what maintains the diffusion gradient.
  - Why it fails: Many candidates only name adaptations, not the process that keeps the gradient steep.
  - Correct: State that both constant ventilation (to refresh alveolar air) and continuous blood flow (to remove oxygen, bring carbon dioxide) maintain the gradient.

## 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 <1μm |
| Dense capillary network | Maintains steep partial pressure gradient |
| Elastic fibres | Stretch for inhalation, recoil for exhalation |
| Surfactant secretion | Reduces surface tension, prevents alveolar collapse |

## 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:

- [Mammalian Ventilation Mechanism](https://www.owlsprep.com/study/cie-9700-u9-ventilation-mechanism/)
- [Infectious Disease](https://www.owlsprep.com/study/cie-9700-u10-overview/)
- [Pathogens and transmission routes](https://www.owlsprep.com/study/cie-9700-u10-pathogens-and-transmission-routes/)

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