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why do we need a ventilation system in our body

Oxygen is required for cell respiration and CO2 is the waste product. Therefore, organisms must take in oxygen from their surroundings and release CO2. Unicellular organisms are able to get sufficient oxygen through diffusion across the membrane. Large animals also depend on diffusion to get their oxygen but they require a specialized ventilation system, which functions to: 1) maintain a concentration gradient, which is necessary for diffusion and 2) increase surface area, which speeds up gas exchange.

In humans, air enters the body through the nose or mouth. From there it passes the trachea, into the bronchi which branch into many smaller bronchioles. Finally the air ends up tiny membranous sacs called alveoli. O2 diffuses from the air in the alveoli to the blood, and CO2 diffuses from the blood to the air in the alveoli. Human Ventilation System
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.

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