The most common cause of upper gastrointestinal bleeding is likely peptic ulcers, such as stomach ulcers, duodenal bulb ulcers, and compound ulcers. Other conditions, such as acute gastric mucosal lesions caused by medication, can also lead to gastrointestinal bleeding. Gastrointestinal tumors, such as esophageal cancer, stomach cancer, and duodenal cancer, can also invade blood vessels and cause upper gastrointestinal bleeding. Vascular diseases, such as varicose veins of the esophagus and stomach base caused by liver cirrhosis, can rupture and lead to upper gastrointestinal bleeding. Massive bleeding can also occur due to the rupture of the gastric artery. Other conditions, like systemic diseases and blood disorders, can lead to thrombocytopenia or coagulation dysfunction, which can also result in upper gastrointestinal bleeding.