Lungs. Computer artwork of the air passages of a pair of normal human lungs. The larynx (voicebox) is at top. The trachea (windpipe) runs down from top centre before dividing into two bronchi. These then enter each lung before dividing many more times to form thin bronchioles. The bronchioles terminate in tiny air sacs called alveoli (not visible), which are surrounded by capillaries. The alveoli are the site of gaseous exchange. Oxygen diffuses into the blood and is taken to the body's cells for respiration. The waste product of respiration, carbon dioxide, is returned to the lungs where it diffuses across the surface of the alveoli and is exhaled.