Why multicellular organisms require ventilation ? can i get a answer in depth i need to get distinctions or my assignment

Multicellular organism require ventilation because large organisms have a relatively small surface area to volume ratio and a large diffusion distance. Small organisms breath through their skin. The oxygen diffuses into their skin and the carbon dioxide diffuses out. ... Resulting in us needing a ventilation system to effective transport oxygen to all parts of our body.

Interphaseinterphase

Gay-lussac's law i think.