The tiny vesicles or sacs in the vertebrate lungs in which the exchange of oxygen and carbon dioxide takes place. It is a process of gas diffusion by which oxygen is transported from air in the alveoli (during inspiration), while carbon dioxide diffuses out of the blood into the air in the alveoli (during expiration), as shown in first figure below. To aid this process, they have very thin walls that are folded in order to increase their surface area so that rapid gas exchange can take place, and they are composed of squamous cells and those producing surfactant (see other figure below). A lack of surfactant in infants born before term age can give rise to respiratory distress syndrome. Smoking and exposure to other pollutants (e.g., asbestos) can lead to the loss the folds, and thereby create problems with breathing and giving rise to emphysema.
