Small animals are able to absorb enough oxygen and remove carbon dioxide through the body surface . The larger the organism the more oxygen it needs, and the external surface is too small for gaseous exchange. Larger organisms have developed special gaseous exchange organs to remedy this.
The requirements for an efficient gaseous exchange surface are:
- a large surface area to exchange the maximum amount of gases
- a thin surface to allow gases to diffuse across it quickly
- a good ventilation system to provide oxygen and remove carbon dioxide quickly
- a good transport system to take oxygen from the lungs to the tissues, and return carbon dioxide from the tissues to the lungs
- protective structures like the ribs, as the surface has to be very delicate and needs to be protected from physical damage
Reference:
http://www.learnthings.co.za/content/secondary/Home/Lessons/Science/Biology/Grade10/organisat/gaseousex/thefeatur/default.htm