A capillary tube uses a liquid’s properties of cohesion, adhesion and surface tension to draw the liquid up the interior walls of a narrow tube. This allows a liquid, such as blood or water, to climb up the tube even against the forces of gravity.
Capillary tubes are often used in doctors' offices to obtain blood samples. Blood is made primarily of water. Water has inherent physical properties that make it stick to itself (cohesion) as well as other surfaces (adhesion). When the narrow tube is placed in contact with the water, a few molecules attach themselves to the inner part of the tube. Next, the cohesion and surface tension of water causes other water molecules to follow as the first ones rise up the tube. This action is often called “climbing” or “wicking.”
The size of the hole inside the capillary tube determines how far up the liquid can climb; the smaller the hole, the further the liquid can rise against the force of gravity. If the capillary tube is held vertically, the liquid eventually stops climbing, with the forces balanced between gravity and the climbing action. The material the capillary tube is made from may also impact how far the liquid rises, due to different surface friction forces.