This picture is a clot or plug inside of an artificial heart. The long strings are fibrin, and they attach to each other to form the clot. Fibrin is a protein that is formed when blood clots. This protein makes a mesh that traps blood cells and hence stops the blood vessels from bleeding.

The round structures on the surface and between some fibrin strands are white blood cells, which always attach to the surface of a fresh clot. Red cells, also an important cellular component of clots, are not seen on this surface view, they are hidden below the surface of the meshwork. You may discover a few tiny, disc-shaped cells attached to the fibrin strands, these are blood platelets, which usually begin the entire clotting process. While clotting is not desirable inside an artificial heart, clotting is a very important early response to injury: it prevents bleeding. People who don't have certain clotting factors (hemophiliacs), or do not have enough platelets or fibrinogen, may bleed excessively after only minor injuries or at surgery. The American Red Cross prepares such clotting components from your blood donations when they are not immediately used for blood transfusions.