Lung tissue, coloured transmission electron micrograph (TEM). A pulmonary capillary containing two red blood cells (red) and three platelets (blue). A basement membrane (pink) surround the endothelium separating it from the alveolar epithelium. Alveoli are the site of gaseous exchange in the lungs, where oxygen is taken up by the red blood cells and carbon dioxide released for exhalation out of the body. Magnification: x7000 when printed at 10 centimetres wide.