1, venous blood flow sluggishness The spinal anesthesia or general anesthesia in surgery leads to the dilatation of peripheral veins and the slowing down of venous flow; the complete paralysis of the lower limb muscles due to the anesthesia during the surgery and the loss of contraction function; after the surgery and bed rest due to incision pain and other reasons, the muscles of the lower limbs are in a relaxed state, resulting in blood flow sluggishness and inducing the formation of deep vein thrombosis in the lower limbs. 2.Venous wall injury (1) chemical injury intravenous injection of various irritant solutions and hypertonic solutions, such as various antibiotics, organic iodine solution, hypertonic glucose solution, etc. can stimulate the venous lining to varying degrees, leading to phlebitis and venous thrombosis. Zhang Rui, Department of Medical Imaging, Sixth Affiliated Hospital of Xinjiang Medical University (2) Mechanical injuries to veins local contusions, lacerations or fracture fragments trauma can cause venous thrombosis. Fractures of the femoral neck can damage the common femoral vein, and pelvic fractures can often damage the common iliac vein or its branches, which can be complicated by iliofemoral vein thrombosis. (3) Infectious injury septic thrombophlebitis caused by perivenous foci of infection, less common, such as infectious endometritis, can cause septic thrombophlebitis of uterine veins. 3, blood hypercoagulable state This is one of the basic factors that cause venous thrombosis. Various major surgeries are caused by the enhanced ability of platelet adhesion in hypercoagulable state; the level of inhibitors of both pre-serum fibrinolytic activator and fibrinolytic enzyme is increased after surgery, thus reducing fibrinolysis. Blood coagulability can be increased after splenectomy due to sudden increase in platelets, and blood concentration due to burns or severe dehydration. Advanced cancer such as lung cancer, pancreatic cancer, others such as ovarian, prostate, stomach or colon cancer, when cancer cells destroy tissues at the same time, they often release many substances, such as mucin coagulant, etc. The activity of certain enzymes increases and reduces the level of antithrombin III, which increases blood coagulation. The application of large doses of hemostatic drugs can also cause the blood to be in a hypercoagulable state. The two main causes of venous thrombosis are stagnant venous blood flow and hypercoagulable blood. A single factor cannot cause the disease independently yet, but often the combination of two or three factors causes deep vein thrombosis. For example, the high incidence of postpartum DVT is the result of a combination of factors. The ability of placental abruption in the uterus after delivery to stop bleeding rapidly in a short period of time without causing postpartum hemorrhage is closely related to the hypercoagulable state of the blood. The placenta produces a large amount of estrogen during pregnancy, peaking at term, and the amount of estriol can increase to 1000 times that of non-pregnancy. Estrogen promotes the production of various coagulation factors by the liver, and at the same time, the fibrinogen in the body also increases greatly at the end of pregnancy, resulting in a hypercoagulable state of blood, which, together with bed rest after delivery, causes blood flow to stagnate in the lower extremities and thus has a tendency to develop deep vein thrombosis. The stagnant blood flow alone is not sufficient to produce the disease, but sometimes there is damage to the vessel wall, such as direct injury, chronic disease, or distant tissue damage, which produces leukocyte tropism factors that cause leukocytes to move to the vessel wall. Similarly, fissures in the endothelial cell layer and the exposure of subendothelial collagen in the basement membrane can cause platelets to move toward the intima, leading to the onset of the coagulation process.