Respiratory system
The inhaled air goes down to the air sacs at the end of each bronchiole.  The air sacs are called alveoli — they have a large surface area and are moist, thin, and close to a blood supply.  The inhaled air has a much greater concentration of oxygen than carbon dioxide whilst the blood flowing to the lungs has more carbon dioxide than oxygen.  This creates a concentration gradient between the air in the air sacs and the blood, meaning there is more oxygen in the air than the blood.