The first section of this model shows a vein cross section with its valves open. Here blood is able to pass through the valves to continue on its way back to the heart. The second section of this model shows a vein cross section with its valves closed. Here blood is prevented from passing through the valve in the incorrect direction. The third section of this model shows a vein cross section with its valves open and the start of blood pooling on the valves. The fourth section of this model shows a vein cross section with its valves open and a severe blood clot.