Adaptations of Exchange Surfaces
There are four general adaptations:
1) large surface area
2) short diffusion pathway
3) rich blood supply (animals)
4) steep concentration gradient.
5) mitochondria if active transport taking place
These can be applied to all exchange surfaces:
1) 60million alveoli/lung; millions of villi; internal air spaces in leaves; root hair cells on roots
2) Thin alveoli & capillary walls; thin capillary & villi walls; thin leaves
3) capillary network in alveoli; capillary network in villi
4) blood flow & breathing maintain it between alveoli & blood; blood flow maintains it between lumen & blood
5) mitochondria in root hair cells
Always look for factors that increase the above if you are talking about why rate of exchange increases:
E.g. Transpiration increases when it is windy because the wind maintains a steep concentration gradient between the leaf and the air.