Lung alveoli and blood cells. Coloured transmission electron micrograph (TEM) of a section through lung alveoli and blood cells. Alveoli are the site of gaseous exchange in the lungs, where oxygen is taken up by the red blood cells (two at right) and carbon dioxide released for exhalation out of the body. Red blood cells have a biconcave surface, which produces a double-lobed shape when seen in vertical cross-section. The blood cell at right is a white blood cell, which is both more spherical in shape and more irregular around its edges. White blood cells are part of the body's immune system. Magnification: x4400 when printed at 10 centimetres wide.