Alveolus. Coloured scanning electron micrograph (SEM) of a freeze fracture of lung to show an alveolus. Alveoli are tiny air sacs that are the site of gas exchange in the lungs, where oxygen from inhaled air is taken up by the blood and carbon dioxide is released to be exhaled. Fractured capillaries containing red blood cells surround the alveolus. Magnification: x3000 when printed 10 centimetres wide.