The SI unit of volume is the cubic meter (m 3 ), which is the volume occupied by a cube that measures 1 m on each side. This very large volume is not very convenient for typical use in a chemistry laboratory. A liter (L) is the volume of a cube that measures 10 cm (1 dm) on each side.
The correct word to fill the blank in the sentence is "capillaries". The thinnest blood vessels in your body are the capillaries. These are the blood vessels that blood is able to transfer sugars and oxygen into the tissues.