What makes a good respiratory surface
For gas exchange to be efficient, a respiratory surface needs four features.
Moist and permeable
Gases must dissolve before they can cross.
Thin
A thin surface lets gases diffuse quickly.
Large
A large area exchanges enough gas for the animal's needs.
Well supplied with blood
Blood carries gases away and keeps the difference in concentration high.
How the alveoli fit
- The alveolar walls are thin.
- They stay moist.
- They are covered by a network of capillaries.
- There are huge numbers of air sacs.
Many other animals use their body surface as their respiratory surface, and exchange gases through their skin.