Caption

Respiratory system. Illustration of the outline of a man's upper body with cut-away head and lung revealing his respiratory system. Air is breathed in through the mouth or nose before travelling down into the tubular trachea (at upper centre). The trachea splits into two bronchi, one of which supplies oxygen to each lung. The trachea and bronchi contain hair-like cilia and mucus- producing goblet cells. The mucus traps foreign matter which is transported away from the lungs and up to the throat by rhythmic movement of the cilia. Air travels into the lungs through the tiny bronchioles, ending up at air sacs called alveoli, where gas exchange with the blood takes place.