To reach cells, substances in the ~blood~ must first pass through the capillary wall into the interstitial fluid surrounding the cell. The walls of |capillaries| are very thin, consisting of only a single layer of endothelial cells, making them highly permeable. These thin, flattened cells permit rapid movement of substances between ~blood~ and interstitial fluid. Unlike |veins| and arteries, |capillaries| have no |smooth muscle| layer, |elastic fibers|, or surrounding coats.
