Answer :
Ventilation maintains a steep concentration gradient, blood at the exchange surface usually has lower O2 and higher CO2 content than the air (of for fish, the water) at the exchange surface. This is what allows for diffusion of O2 into the blood and CO2 out of the blood. Multicellular organisms have a greater number of cells so will have a higher energy demand than single cellular organisms, this demand can only be met by allowing the cells to respire, a ventilation system brings oxygen molecules which are a raw product of respiration into the hemoglobin of the blood to be transported to tissues. As surface area : volume ratio is very low so if there was no ventilation system and the organism instead relied on exchange of materials through the exposed skin it would be very difficult to met the high energy demand as less O2 and CO2 would be able to be exchanged.
In multicellular organisms, the division of labor provided by these bodies grouped into specialized devices imposes a number of constraints.
One of the constraints is that it is necessary for all cells in the body to be well supplied with both nutrients and oxygen. The transition to multicellularity requires the establishment of a circulatory system and a respiratory system for the supply and purification systems for the disposal of waste.
The amount of oxygen required to breathe depends on:
Ratio surface / volume plus it is small and the intake by the respiratory system must be important.
The depth of the cells, which evolves proportionally with the oxygen supply required.
The number of cells that also evolves proportionally with the oxygen supply required.
