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Lungs, computer artwork. Both the bronchial tree, the network of airways serving both lungs, and the overall shape of the lungs, is shown here. The trunk of the bronchial tree is the trachea (windpipe, top centre), which branches into the left and right bronchi. Further divisions of the bronchi then takes place, with the smallest branches, the bronchioles, terminating in alveoli (air sacs), where gaseous exchange with the blood circulation takes place. (KEYSTONE/SCIENCE PHOTO LIBRARY/PASIEKA)