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.

1. Rich supply of blood vessels

2. Thin and delicate 

3. Large surface area 

4. Kept well outside to absorb oxygen 

1. Only very few cells are exposed to the environment.

2. They have high  oxygen requirements 

3. Their respiration is complex than inactive animals.