Pulmonary embolism can vary in severity, with serious cases primarily causing unstable vital signs. The disease primarily arises from venous thrombi in the venous system, which travel with the venous blood back to the right heart, and then with the blood flow from the right heart back to the aorta. If the thrombus is relatively large, it can easily become lodged in the pulmonary artery, forming a pulmonary embolism. The symptoms of pulmonary embolism can range from mild to severe. Based on the patient's symptoms and vital signs, pulmonary embolism is categorized into low risk, intermediate risk, and high risk. Low and intermediate risk typically indicate that the patient's vital signs are relatively stable, with normal blood pressure, heart rate, and pulse oximetry saturation. In severe cases, patients may exhibit noticeable symptoms such as coughing up blood, difficulty breathing, low blood pressure, shock, and oliguria. These represent the more severe cases of pulmonary embolism.