Bleeding occurs when there is a break in a blood vessel wall. Control of bleeding (hemostasis) begins when platelets in the blood become activated (change shape and develop spines) and stick to the injured area. The platelets form a mesh with blood cells, collagen, and other proteins. This mesh, strengthened by long strands of insoluble fibrin, entraps more platelets and blood cells, producing a clot that plugs the break. The clot dissolves as the blood vessel heals.