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Lung airways. Coloured X-ray of the airways (red) of the lungs (not seen). The trachea (windpipe, upper centre) divides into two main bronchi which enter each lung. The bronchi then split further into smaller bronchi and finally bronchioles that terminate at clusters of alveoli (not seen). These air sacs are surrounded by blood vessels, allowing gas exchange to take place. (KEYSTONE/SCIENCE PHOTO LIBRARY/INNERSPACE IMAGING)