Respiratory system
As the membrane, oxygen can easily diffuse in and out. Oxygen at high concentration in the air sacs diffuses into the blood where oxygen concentration is low, and carbon dioxide at high concentration in the blood diffuses into the air sacs where carbon dioxide concentration is low.  The oxygen in the blood enters the circulatory system and is used by the cells in the body.  The carbon dioxide in the air sacs is exhaled out of the body.