Why is it not possible for humans to gather oxygen through diffusion? *

because it's a very slow process , it will take about 3 years for it to fully circulate throughout the whole body and by 3 years your body will be at a state where the blood cannot circulate as fast because as the body grows more cells grow , the more cells the harder it is to get oxygen through diffusion

Explanation:

the duration of a photographic flash is related to an rc time constant, which is 0.100 μs for a certain camera.