In the lungs, the bronchi branch out into smaller tubes known as bronchioles. The air in the bronchiole then pass into million of tiny air sacs called alveoli which are present at the end. In the alveoli diffusion of gases takes place. There is more oxygen present in the alveoli than in the blood of the capillaries. Therefore, the oxygen move from the air to the blood through diffusion. Since there is more carbon dioxide in the blood than in the air, Carbon dioxide diffuses from the blood to the air in the alveoli. It is sent out of the body when the air is exhaled.