Lung alveolar cells. Coloured Scanning Electron Micrograph (SEM) of epithelial cells lining an alveolus (air sac) of the human lung. At lower frame are smooth alveolar cells type I which cover 90% of the air sac surface, and function in gas exchange. Oxygen and carbon dioxide pass through these cells, to and from the bloodstream. At centre left & top are two alveolar cells type II. They are covered in fine microvilli and secrete surfactant, a substance that reduces surface tension in the air sac and prevents it from collapsing. At centre right is a brush cell with thick microvilli, whose function is unknown. Magnification: x5,100 at 6x4.5cm size.