Transmission electron microscope image of a thin section cut through the alveolar sac region of the lung (mouse). The alveolar cell shown has vesicles which contain surfactant, a macroaggregate of phosolipids and proteins which help with breathing. The surfactant is released by exocytosis and forms a thin monolayer on the wall of the alveoli. This layer can't be seen in a TEM image, because no fixative adequately preserves it, but it can be seen inside the surfactant vesicles in the cytoplasm. This layer helps to reduce surface tension, thereby preventing collapse of the alveoli during exhalation and reduces the energy needed during inhalation.

The capillaries contain red blood cells. The capillary lining consists of long, thin endothelial cells, connected by tight junctions.