Platelets are very small 1.575? 10^–3 inches (0.004 millimeters) in diameter and 3.9×10^–5 inches (0.001 millimeter) thick. The functions of platelets include: 1) transport of enzyme and proteins critical to clotting; 2) formation of a platelet plug to slow blood loss; and 3) contraction of a clot after it has formed, which then reduces the size of the vessel break.