MICROSCAPE / SCIENCE PHOTO LIBRARY MICROSCAPE / SCIENCE PHOTO LIBRARY
Capillary. Transmission electron micrograph (TEM) of a section through a capillary, showing two red blood cells (erythrocytes, black) in its interior. Capillaries are the smallest diameter blood vessels (5-8 micrometres) and are just wide enough for red blood cells to pass along them. Their very thin walls are made of endothelial cells, across which gases, nutrients, fluids and cells may pass selectively in either direction. External to the endothelium, most capillaries have another layer of cells called pericytes that may be supportive (fibroblasts) or contractile (myofibroblasts). Magnification: x5,000 when printed 10 centimetres wide.
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