A capillary tube is a very thin tube made of a rigid material, such as plastic or glass. The tubes are used to collect samples of liquids, which will flow up into the tubes against the effects of gravity in a process called capillary action. They are commonly used in medical and research fields to extract very small samples of liquids to be analyzed, such as blood samples for diabetics, or to transport them to other devices, such as Petri dishes or microscopes, for further study or experimentation.Microhaematocrit Capillary tubesRed band on?tube?indicates heparinization