1. Rich supply of blood vessels

2. Thin and delicate 

3. Large surface area 

4. Kept well outside to absorb oxygen 

1. Taking in of O₂ and giving out of CO₂ 

2. Diffusion of O₂ into blood and CO₂ out of blood.

3. Transport of oxygen to the cells.

4. Combination of glucose inside cells  to release energy.