False-colour transmission electron micrograph (TEM) of human lung tissue, showing an alveolar septum, the partition between the alveoli (air sacs) where gas exchange takes place. Blood vessels (capillaries) extend across the entire septum, coloured yellow in this image & filled with red blood cells (dark red). The purple- coloured cell in centre of image with a large nucleus is one of the two alveolar cell types or pneumocytes. Type II cells are believed to secrete surfactant, a substance that reduces alveolar surface tension & prevents the air sacs from collapsing. Magnification: x2,200 at 6x7cm size.