Lungs. Computer illustration of a human figure with healthy lungs (white) superimposed. The figure is composed of contour lines. At upper center, the trachea (windpipe, dark blue), which introduces air from the throat into the respiratory system, is partly seen. It branches into two bronchi each of which enters one of the lungs. Within the lungs the bronchi divide into an internal network of smaller bronchi and then bronchioles. The bronchioles end in tiny air sacs called alveoli (not seen). Exchange of gases occurs in the alveoli as inhaled oxygen enters the blood and carbon dioxide escapes into the alveoli to be exhaled.