Why do animals with millions of cells need a respiratory system while animals with few cells can do without one?

Animals with millions of cells require a respiratory system to efficiently transport oxygen to their tissues and remove carbon dioxide, as diffusion alone cannot meet the metabolic demands of their larger and more complex bodies. In contrast, smaller animals with fewer cells often have a high surface area-to-volume ratio, allowing them to rely on simple diffusion to exchange gases directly with their environment. This efficient gas exchange is sufficient for their metabolic needs, eliminating the necessity for a specialized respiratory system. As organisms increase in size and complexity, the need for more specialized systems, like respiration, becomes essential for sustaining life.