Lung alveoli. Coloured scanning electron micrograph (SEM) of a freeze-fracture plane through the alveoli of a healthy human lung. A bronchiole, a tube leading to these air passages deep within the lung, can be seen at centre. The air passages end in alveoli, air sacs through which gas exchange with the blood takes place. Red blood cells take up oxygen from the alveoli, releasing carbon dioxide at the same time. The carbon dioxide is exhaled, while the oxygenated blood is pumped around the body. Magnification: x80 at 6x7cm size.