Bleeding constitutes a major concern to surgeons. Problems may arise during operation or postoperatively, and bleeding may be concealed or grossly obvious. The major causes are the following:

1. Physical defect in hemostasis—improper vessel ligation, overlooking a small transected vessel or other failure to achieve adequate hemostasis, or postoperative tissue devitalization and release of a vessel ligature.
2. Unrecognized preoperative bleeding problem— a coagulation defect is present but was not recognized before surgery. This may be congenital (e.g., hemophilias), secondary to a disease that the patient has (e.g., cirrhosis), or due to medications (e.g., aspirin or anticoagulant therapy).
3. Transfusion reactions or complications from massive transfusion.
4. Intraoperative DIC.
5. Unexplained bleeding difficulty.

Unusual bleeding has some correlation with the type (magnitude) of the operative procedure, the length of the operation, and the particular disease involved. The more that any of these parameters is increased, the more likely that excessive bleeding may occur. In most cases, the defect can be traced by means of laboratory tests or, retrospectively, by reestablishing physical hemostasis. In some cases laboratory workup is hindered by transfusions administered before a coagulation problem was considered. Sometimes the source of bleeding is never uncovered, even after thorough investigation. This fact cannot be used as an excuse for inadequate workup, because proper therapy depends on finding the etiology. This is why some knowledge of blood coagulation mechanisms is necessary.