Lung tissue. Coloured scanning electron micrograph (SEM) of a section through a lung, showing a blood vessel (smooth, right) and an alveolus (left). Alveoli are the site of gaseous exchange in the lungs, where oxygen is taken up by the red blood cells and carbon dioxide released for exhalation out of the body. The alveolar walls are lined with a one-cell thick layer of epithelium that oxygen must pass through to reach the blood. Here a layer of collagen (fibres, diagonal) can be seen in the wall of the blood vessel.