# What are alveoli?

It is called the alveoli. Gray's Anatomy (1918) describes it as tiny air sacs at the end of the smallest airways.

## What it means

- Tiny air sacs at the end of the smallest airways.
- About 300 million in each pair of lungs.
- The site where oxygen enters the blood and carbon dioxide leaves it.

## Action steps

1. Grouped in clusters at the ends of the bronchioles.
2. Each surrounded by a network of pulmonary capillaries.
3. Their walls are only one cell thick.

## Sources

- [Gray's Anatomy of the Human Body (1918)](https://www.bartleby.com/107/)

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