Red blood passes through the smallest what in single file?

Red blood cells pass through the smallest capillaries in single file. Capillaries are the smallest type of blood vessel, and they have a diameter of about 5-10 micrometers. Red blood cells are about 7-8 micrometers in diameter, so they can just barely squeeze through capillaries. This is important because it allows red blood cells to deliver oxygen and nutrients to all of the cells in the body.