Prothrombin is a protein produced by the liver that plays a crucial role in blood clotting. It is a precursor to thrombin, an enzyme that converts fibrinogen into fibrin, forming a stable blood clot. Prothrombin is part of the coagulation cascade, which is a series of reactions that help stop bleeding when injuries occur. The levels of prothrombin in the blood can be affected by various factors, including vitamin K intake and certain medical conditions. A deficiency in prothrombin can lead to excessive bleeding, while elevated levels may increase the risk of thrombosis, where blood clots form inappropriately within blood vessels.