Lung airways. Coloured chest X-ray showing the branching airways (white/brown) in a healthy right lung. The trachea (windpipe, top right) divides into two main bronchi, one of which enters each lung. Each bronchus then splits further into smaller bronchi and finally bronchioles that terminate in clusters of alveoli (air sacs, not seen). The alveoli are the site of gas exchange between inhaled air and the blood. They are surrounded by thin-walled blood vessels, ensuring that the exchange of oxygen and carbon dioxide is rapid and complete.