Lung. Coloured bronchography (X-ray) of a healthy human lung. A contrast medium has been added to show the network of airways (red) in the right lung. The heart is the orange mass at centre. The trachea (wind pipe) enters the lungs and splits into the two bronchi (one seen, centre). The bronchi then further divide into bronchioles, which continue to divide and decrease in diameter. The bronchioles eventually end in groups of tiny air sacs known as alveoli (not seen). Alveoli are the site of gaseous exchange between the lungs and blood; oxygen enters the blood and carbon dioxide leaves.