# Alveoli

Tiny air sacs in the lungs where oxygen enters the blood and carbon dioxide leaves it.

Biology (school) · Part 03: The human body · Card 4 of 11 · Last updated 2026-10-19

## Analogy

Millions of tiny balloons, each wrapped in a net of blood vessels.

## In practice

The lungs have about 300 million alveoli, giving a huge area for gas exchange.

## Practice

### Flashcards

- **What is exchanged at the alveoli?** Oxygen goes into the blood. Carbon dioxide comes out.

### Interview questions

**Explain how the lungs are adapted for gas exchange.**

- Millions of alveoli give a large surface area
- Alveoli walls are one cell thick for a short diffusion distance
- A rich blood supply keeps the concentration gradient steep
- Breathing keeps fresh air flowing in

## Questions people ask

### Why does exercise make you breathe faster?

Working muscles respire faster, so they use more oxygen and make more carbon dioxide. Sensors in the body detect the rising carbon dioxide and signal the brain, which increases breathing rate and depth. This brings in more oxygen and clears the extra carbon dioxide.

### How does smoking damage the alveoli?

Smoke contains tar and chemicals that irritate the lungs. Over many years, the alveoli walls can break down, which reduces the surface area for gas exchange. This disease is called emphysema. Sufferers become breathless because less oxygen can reach the blood.

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