Caption

Respiratory system. Illustration of a healthy human respiratory system. Air is breathed in through the mouth or nose (at upper left) before travelling down the trachea (white, at centre), with its rings of cartilage. The trachea splits into two bronchi (only one shown, at lower centre), which branch into the many smaller bronchi shown in the lung at left. Finally air travels along tiny bronchioles, ending up at air sacs called alveoli (not seen), where gas exchange with the blood takes place. Air is sucked into the body by contracting the diaphragm muscle (red, at lower centre) and raising the rib cage (not seen) thereby expanding the lungs.