Lung alveoli. Coloured scanning electron micrograph (SEM) of a freeze-fractured sample of lung tissue, showing alveoli and capillaries with red blood cells. Pneumatocytes (yellow) are also seen. The air passages in the lungs 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. These alveoli are from the lungs of a mouse. Magnification: x600 when printed at 10 centimetres across.