In the course of DVT, platelets first adhere to and accumulate locally in the intima, releasing certain active substances, and at the same time causing further platelet aggregation. As the disease progresses, platelet accumulation gradually increases, forming many coral-like platelet trabeculae, slowing down blood flow, gradually increasing the number of activated clotting factors, increasing fibrin formation and deposition, and netting blood cells, eventually becoming thrombus. In the early stage of thrombus formation, only the origin is attached to the vessel wall, almost floating, and can be easily dislodged. Subsequently, the thrombus shrinks and extrudes the serum, and the thrombus becomes a relatively dry, solid structure. At first the thrombus attaches to the vessel wall only by fibrin, but the endothelial fibroblasts of the intima rapidly invade and fix and mechanize the thrombus. Because there are activated clotting factors and thrombin in the serum extruded from the thrombus, under certain conditions, it is easy to have fresh clots deposited on the thrombus that is being mechanized or even has been mechanized, so that the thrombus continues to expand and extend, eventually blocking the venous lumen. The thrombus can also extend retrogradely and lead to thrombosis of the whole lower extremity deep vein trunk. The thrombus spreads and grows and can terminate at any time or can develop continuously in a prograde or retrograde fashion. After thrombosis, secondary thrombi that expand proximally and multiply distally gradually adhere to the vessel wall, stimulating an inflammatory response in the vein wall and perivenous area, and then stop expanding, multiply, and undergo fibroformative mechanization. The new granulation tissue grows from the vessel wall into the thrombus and gradually dissolves and absorbs the thrombotic components such as fibrin and tissue fragments, which are eventually replaced by the mechanized connective tissue. Mechanization usually begins 1 to 2 d after formation and adheres more firmly to the vessel wall in 3 to 4 d. For larger thrombi, it usually takes 2 weeks to fully mechanize. The mechanized thrombus fuses closely with the vessel wall and is no longer at risk of dislodging. Clots that block veins have a strong ability to re-tubulate. During the process of thrombus mechanization, the thrombus contracts, causing a gap between the thrombus and the vessel wall or the thrombus itself to autolyze and become fissured. These fissures are gradually covered by the growing endothelial cells, forming one or several lumens that are connected to the vessel at both ends, thus allowing some of the blood flow to return, i.e., re-tubulation. Extensive revascularization can occur approximately 5 to 12 weeks after venous thrombosis. During the process of mechanization and revascularization, the venous valves are destroyed and lose their normal function, resulting in blood reflux.