Explain how the structure of alveoli makes them well adapted as an exchange surface for gases in the lungs.

Alveoli are tiny, sac‑shaped structures that provide a very large surface area for gas exchange. The large surface area increases the rate of diffusion of oxygen and carbon dioxide.

The walls of alveoli are only one cell thick, which reduces the diffusion distance and further speeds diffusion.

Alveoli are surrounded by a dense network of capillaries. The rich blood supply keeps a steep concentration gradient for O₂ and CO₂, allowing continuous diffusion across the alveolar wall.

These features – large surface area, thin walls and a rich capillary network – make alveoli highly efficient as an exchange surface for gases in the lungs.