CBSE Class 10 Answered

Lower groups of animals have a high surface area to volume ratio which enables them to meet all their metabolic gas needs by diffusion through the skin.

Large animals with complex body structure need to transport gases between the internal tissues and a diffusion surface exposed to the external medium (water or air). This is achieved with the help of well-developed respiratory system. The respiratory system exchanges oxygen and carbon dioxide between the external environment and the internal environment.
A respiratory system has three important structures:

• A large respiratory surface area (gills or lungs)
• A method for ventilating the respiratory surface
• A pump and circulatory system (heart and blood vessels) to distribute the gases 

Such a system enables smooth exchange of gases between the body and the environment.