Capillary Action
As mentioned on other pages, blood has adhesive and cohesive qualities. Adhesive qualities refer to the attraction of different molecules, whereas cohesive qualities refer to molecules of the same likeness. Blood sticking to a surface is an example of adhesion. The fact that a blood drop forms a sphere (the smallest shape it can acquire) is a result of its internal molecules and their attraction to one another. Surface tension is a result of these cohesive forces at work on the surface of the blood drop. These cohesive forces are greater on the surface as opposed to the center. The pictures below demonstrate what is known as capillary action. Capillary action is when a source of blood's adhesive attraction towards another object (unlike molecules) exceeds its own cohesive forces.
Here, the attraction of blood to the inside walls of the capillary tube demonstrate the adhesive attraction overriding the bloods own cohesive forces.
Understanding the qualities of blood is important to the Bloodstain Analyst. Notice the stain pattern created on the rolled paper towel by dipping it into a blood source. It appears to me that the type of connecting material (In this case a rolled paper towel) and surface texture thereof, plays an important part in the pattern produced. Sounds like a future experiment to me.
Another example of blood's adhesive qualities and capillary action, whereas the blood is trying to stay connected.
A bloody handprint. Notice the areas that are the darkest giving a possible indication as to how the hand lifted off the paper. As the hand lifts off the surface, the blood rushes to the last points of contact.