Any surface that promotes the diffusion of gases must have following four basic features :
1. The surface area must have a relatively large surface area. Diffusion is a slow process, and the larger the area, the more is diffusion. Large surface area is maintained by extensive tissue projections and foldings.
2. The surface must be moist at all times, because oxygen and CO2 must be dissolved in water to diffuse across cell membranes. Aquatic organisms easily meet this requirement, but it is difficult for the organisms living on land to maintain a moist surface. Many adaptations in terrestrial organisms minimize but can never totally prevent water loss during respiration.
3. A surface for gas exchange must be close to the active cells of the body or to the fluid that transports gases between these cells and the surface. Diffusion is slow to carry gases very deeply into the body of an organism.
4. The membrane should be thin and permeable to the gases