<P> The alveoli are located in the respiratory zone of the lungs, at the ends of the alveolar ducts and alveolar sac, representing the smallest units in the respiratory tract . They provide total surface area of about 75 m . </P> <P> A typical pair of human lungs contain about 700 million alveoli, producing 70m of surface area . Each alveolus is wrapped in a fine mesh of capillaries covering about 70% of its area . An adult alveolus has an average diameter of 200 micrometres, with an increase in diameter during inhalation . </P> <P> The alveoli consist of an epithelial layer and an extracellular matrix surrounded by small blood vessels called capillaries . In some alveolar walls there are pores between alveoli called Pores of Kohn . The alveoli contain some collagen and elastic fibres . The elastic fibres allow the alveoli to stretch as they are filled with air during inhalation . They then spring back during exhalation in order to expel the carbon dioxide - rich air . </P> <P> There are three major types of cell in the alveolar wall--two types of alveolar cell and a phagocyte . </P>

The walls of the alveoli are mostly made of