Organisms that have ventilation systems will draw in air (or water) that is rich in oxygen and will draw out air that has carbon dioxide. This is essential for a multicellular organism to thrive because carbon dioxide is harmful for an organism since carbon dioxide (CO2) is a waste product. Such is the system that contain veins, capillaries and arteries. On the other hand, organisms that don’t have ventilation system, can survive without it because they are single-celled and the path for the nutrients and needed oxygen is so short that they can efficiently exchange gases at the surface.
Compare Organisms that do not have ventilation system to ones that have one