Water, carbon dioxide, and oxygen are among the few simple molecules that can cross the cell membrane by diffusion (or a type of diffusion known as osmosis ). Diffusion is one principle method of movement of substances within cells, as well as the method for essential small molecules to cross the cell membrane.
The blood going out from the arteries is full of oxgyen.It delievers that oxgyen to organs,therefore when it returns in the veins it contains carbon dioxide.
