The smallest cell in our body is the red blood cell. These cells are only about 7 micrometers in diameter. Thatâ€™s smaller than a grain of salt! Red blood cells are so small because they need to be able to travel through the tiny capillaries in our bodies. If they were any larger, they would get stuck and wouldnâ€™t be able to do their job of carrying oxygen to all of our tissues.