Light micrograph showing blood supply to muscle fibers. The muscle fibers (pale pink) have been teased apart to reveal the capillary bed (red) that has been perfused with dye. Each muscle fiber has its own extensively branched fine capillary network surrounding it. Capillaries are the smallest blood vessels in the body. They have thin walls that allow dissolved nutrients and oxygen to pass into the surrounding muscle cells. Skeletal muscles are under voluntary control and move the bones of the skeleton.