Which structure allows the passage of this oxygen to the environment? ?

The structure allows the passage of this oxygen to the environment is the cell membrane. Cell membrane is the semipermeable membrane surrounding the cytoplasm of a cell. It is also known as  plasma membrane or cytoplasmic membrane that is  a biological membrane that separates the interior of all cells from the outside environment.

Usually this mountain-building process produces majestic peaks and steep slopes. you should look for these along the other features.