Q. 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 extremely close to the environment, like poriferans and cnidarians, making 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.