Which substances are produced by the liver that are essential for blood clotting?

The substances produced by the liver that are essential for blood clotting are prothrombin and fibrinogen. These proteins play critical roles in the coagulation process. Prothrombin is a precursor of thrombin, which is an enzyme that converts fibrinogen into fibrin, the protein that forms the meshwork of blood clots. Fibrinogen itself is a soluble plasma protein that is converted into fibrin during the clotting process. This transformation is crucial for stopping bleeding and initiating wound healing.

In contrast, the other choices consist of substances that have different functions. Insulin and glucagon, for instance, are hormones involved in glucose metabolism rather than clotting. Cholesterol and bilirubin have roles in lipid metabolism and the breakdown of red blood cells, respectively, but they do not contribute to the coagulation cascade. Similarly, bile acids and urea are associated with digestion and the excretion of waste products rather than clotting. Therefore, prothrombin and fibrinogen are correctly identified as the essential substances for blood clotting produced by the liver.