Normally blood remains in its fluid form as long as it stays within vessels. If it is comes out side from the body, it thickens and forms a gel. Eventually, the blood in the form of gel get separated from the rest of the fluid. The straw-colored liquid, called serum is simply blood plasma minus the clotting proteins. The gel is called a Clot which gets hardend with the exposure to air.It consists of a network of insoluble proteins fibers called fibrin in which the red blood corpuscles of blood are trapped.
The process of gel formation, called clotting or coagulation, is a series of chemical reactions that culminates in the formation of network of fibrin threads. Blood clotting involves several substances known as clotting factors. These factors include calcium ions, several inactive enzymes that are synthesized by hepatocytes (Liver cells) and released into bloodstreams and various molecules associated with platelets released by damaged tissues. The fundamental reaction in the clotting of blood is a conversion of the soluble plasma proteins fibrinogen to insoluble fibrin as shown in the diagram below.