Lung blood vessel. Coloured scanning electron micrograph (SEM) a blood vessel around alveoli (air spaces, dark areas) in the lungs. Erythrocytes (red blood cells) are seen in the blood vessel. Alveoli are the smallest divisions of the airways in the lungs, and are the sites of gaseous exchange. Here, oxygen is taken up by the blood from inhaled air, and carbon dioxide is released to be exhaled. Magnification: x180 when printed 10 centimetres wide.