Oxygen comes from the environment and carbon dioxide in the end returns to the environment. How do small animals solve the problem of taking away and bringing these molecules from/to their cells? Why isn't that solution possible for larger animals?
Small animals whose tissues make direct contact or are very close to the environment, like cnidarians and poriferans, make gas exchange by diffusion. Larger animals with cells without direct contact with the environment or far from it need special gas transportation systems. In these animals the respiratory and the circulatory systems play this role.
