Why alveoli are perfect for gas exchange
Gas exchange means letting oxygen move into the blood and carbon dioxide move out, by diffusion. Diffusion is faster when the surface is large, the distance is short, and the difference in gas amounts (the gradient) is steep. Human alveoli are shaped to do all three.
- Large surface area: millions of tiny alveoli give a huge total area for diffusion.
- Thin walls: the alveolus wall and the capillary wall are each one cell thick, so gases travel only a very short distance.
- Moist surface: a thin film of water lets gases dissolve before they diffuse.
- Rich blood supply: a dense network of capillaries keeps carrying oxygen away, so the concentration gradient stays steep.
Key idea
Large area + short distance + steep gradient + moist surface = fast, efficient gas exchange.
Adaptations in other animals
Different animals have surfaces suited to where they live. Fish use gills with thin filaments to take oxygen from water. Insects use a system of air tubes (tracheae) opening at spiracles. Amphibians such as frogs can also exchange gases through their thin, moist skin. All of these share the same features: large, thin, moist surfaces with a good blood or air supply.
Remember
- At high altitude the air has less oxygen.
- People living high up make more red blood cells to carry enough oxygen.