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

2. They have high  oxygen requirements 

3. Their respiration is complex than inactive animals.

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.