Lung alveoli and bronchus. Coloured Scanning Electron Micrograph (SEM) of lung tissue, showing alveoli, a bronchus, and a pulmonary blood vessel. At right is a bronchus (blue), a branching air tube. Beside it is a blood vessel (red) with smooth endothelial surface through which blood passes. Lung tissue (yellow) has a spongy appearance, and is made up of alveoli air sacs whose interstital walls are lined with blood capillaries. Air passes from the bronchus into alveoli where the gases of carbon