|Clusters of alveoli, forming alveolar sacs, are connected
by ducts to the smallest air pathways of the lung: the respiratory
bronchioles. Several sacs and the network of capillaries that surrounds
them are illustrated here. Seen in cross section is a single alveolus
with its lining of three distinctive kinds of cell. Cells labeled
Type II secrete lipids that reduce surface tension during exhalation
and keep the lung from deflating completely.
An alveolus is a minute air-filled sac in the vertebrate lung.
Pulmonary alveoli are thin-walled and surrounded by blood vessels. There
are large numbers of alveoli in each lung,
and it is through their surfaces that the respiratory exchange of oxygen and carbon dioxide occurs. In
most vertebrates they connect with the mouth by a system of ramifying
air-tubes – the bronchi and bronchioles.
|Simplified diagram of alveoli
- The term alveolus may also mean an expanded sac of secretory epithelium,
which forms from an internal termination of each duct in many glands (e.g. the mammary glands).
- A dental alveolus is a cavity in the jaw-bone into which a tooth fits.