The alveoli contain some collagen and elastic fibres. The elastic fibers allow the alveoli to stretch as they fill with air when breathing in. They then spring back during breathing out in order to expel the carbon dioxide-rich air. Each human lung contains about 150 million alveoli, making about 300 million total in the body. Each alveolus is wrapped in a fine mesh of covering about 70% of its area. An adult alveolus has an average diameter of 200 to 300 micrometres, with an increase in diameter during inhalation.
