This is a transmission electron micrograph (TEM) of a thin section through some tissue.
Capillaries are the smallest blood vessels in your body. They may be as small as 0.013 mm (0.0005 inch). This is a transmission electron micrograph of a very thin slice of tissue that shows some red blood cells (red) inside the lumen (greenish) of a capillary made up of endothelial cells (blue), in a block of collagen (orange) wart. The walls of the capillary are very thin and permeable so that nutrients and oxygen from the blood can go out to the tissue, and waste products and carbon dioxide from the tissue can get into the capillary to be taken away. Pretty cool, huh?