# Alveoli and Gas Exchange

> The lungs end in millions of tiny air sacs called alveoli. Each is wrapped in capillaries, and the wall between air and blood is so thin that oxygen and carbon dioxide diffuse across in a fraction of a second.

Web page: https://cellnaut.com/topics/alveoli-gas-exchange

![Teaching illustration: Alveoli and Gas Exchange](https://cellnaut.com/texture-references/gpt-image-2-biology-more-teaching-2026-05-31/jpg/alveoli-gas-exchange-teaching.jpg)

## Labelled parts

1. **Alveolar lumen**: The air-filled space where inhaled air meets the wall.
2. **Alveolar wall**: A wall only one cell thick, so gases cross a very short distance.
3. **Type I pneumocyte**: Thin, flat cells that cover most of the surface and form the exchange barrier.
4. **Type II pneumocyte**: Rounder cells that make surfactant.
5. **Surfactant film**: A thin film that lowers surface tension and keeps the alveoli open.
6. **Capillary network**: A dense mesh of tiny vessels wrapped around each alveolus.
7. **Red blood cell**: Carries oxygen on hemoglobin and brings CO₂ back from the tissues.
8. **Gas molecules**: O₂ diffuses into the blood and CO₂ diffuses out.
9. **Blood-air barrier**: The alveolar cell, a shared basement membrane and the capillary cell together form the thin barrier gases cross.

## Key points

- Gases cross by simple diffusion, down their concentration gradients: O₂ into the blood, CO₂ out.
- The blood-air barrier is often only about half a micrometre thick.
- Surfactant from type II cells reduces surface tension so the alveoli do not collapse.

## Check yourself

**1. How do O₂ and CO₂ cross the blood-air barrier?**

- A. Simple diffusion
- B. Active transport by pumps
- C. Endocytosis
- D. Cell division

**Answer:** A. Simple diffusion - Small nonpolar gases slide through, down their concentration gradients.

**2. What is the job of surfactant?**

- A. Carry oxygen
- B. Lower surface tension so alveoli do not collapse
- C. Make the wall thicker
- D. Kill all bacteria

**Answer:** B. Lower surface tension so alveoli do not collapse - Without it the wet inner surface would pull the alveoli shut.

## Related lessons

- [The Plasma Membrane and Transport](https://cellnaut.com/learn/plasma-membrane-and-transport)

## See it in 3D

- [Red Blood Cell](https://cellnaut.com/studio/redBlood)
- [Epithelial Cell](https://cellnaut.com/studio/epithelial)