Ruptured capillary. Coloured scanning electron micrograph (SEM) of a red blood cell squeezing out of a torn capillary. A capillary is the smallest type of blood vessel, often only just large enough for red blood cells to pass through. Red blood cells (erythrocytes) are biconcave, disc-shaped cells that transport oxygen from the lungs to body cells. The oxygen transfer occurs in capillaries, where carbon dioxide is also removed and then taken to the lungs for exhalation. Red blood cells are the most abundant cell in the blood. They have no nucleus and are about 7 micrometres across.