Your blood flowing. Light microscopy footage of blood cells flowing through capillaries in human tissue. Capillaries are the smallest blood vessels, only a few micrometres across, just large enough for the passage of red blood cells. Red blood cells carry oxygen to body tissues and carry away carbon dioxide (the waste product of respiration). The thin walls of the capillaries allow dissolved blood gases and nutrients to diffuse to and from the capillaries into and from the body tissues. Credit: