Being around 10μm in diameter, they let the air to pass between individual alveolus and function as an alternate route for air in case of an obstruction. There are small passages or pores in the alveolar walls to serve as a connection between adjacent alveoli. These capillaries carry the CO 2-rich blood to the lungs to exchange them for O 2. The alveoli are surrounded by a fine network of blood capillaries that are supplied by small pulmonary artery branches. The respiratory membrane, the only barrier between the oxygen-rich air within the alveoli and the carbon dioxide-rich blood carried by the capillaries, is about 0.5μm thick as opposed to the 7.5μm-diameter of blood cells traveling through the capillaries. Respiratory MembraneĪ thin membrane, across which the O 2-CO 2 exchange takes place, the respiratory membrane is composed of the squamous alveolar epithelial cell, and squamous pulmonary capillary endothelial cell, and their fused base membranes.

There is an extracellular matrix, a narrow fluid-filled space, surrounding the alveolar clusters to help the individual sacs to stay in their place and function independently. Even though occupying a small area, type-2 epithelial cells outnumber the squamous cells, accounting for 60% of the total number of alveolar cells, and are capable of mitosis to produce new type-1 cells for replacing any damaged ones. Type-2 great alveolar epithelial cells: The rest of the 5% of the alveolar surface area is covered by large, cuboid to round type-2 epithelial cells, responsible for secreting pulmonary surfactant so the lungs can maintain their elastic recoil property.

Alveolar dead space graph