# Oxygen diffuses from the alveoli in the lungs into the surrounding blood capillaries. Explain why oxygen moves in this direction and describe two features of the alveoli that increase the rate of this diffusion.

> Cambridge International IGCSE Biology (0610) — 3.1 Diffusion · Explain · 4 marks

## Mark scheme (4 marks)

1. Oxygen moves from the alveoli into the blood because there is a higher concentration of oxygen in the alveoli than in the blood capillaries (concentration gradient exists).
2. The large surface area of the alveoli increases the rate of diffusion because there is more space available for molecules to diffuse through.
3. The thin walls of the alveoli (short diffusion distance) increase the rate of diffusion.
4. A steep / maintained concentration gradient is kept by the blood flow (or continuous removal of oxygen by blood / continuous supply of air) which maintains the rate of diffusion.

## Key terms

- [diffusion](https://www.gradenine.co.uk/glossary/diffusion)

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