Respiration of animals

Question

Oxygen comes from the environment and carbon dioxide in the end returns to the environment. How small animals do solve the problem of taking away and bringing these molecules from/to their cells? Why isn't that solution possible for the larger animals?

Answer 1

Tissues of the Small animals make direct contact or are very close to the environment, just like poriferans and cnidarians, make gas exchange through the diffusion. Larger animals having cells without direct contact with environment or far from it need special gas transportation systems. In these animals respiratory and the circulatory systems play this role.