Blood is a connective tissue composed of 55 % blood plasma (a liquid extracellular matrix) and 45 % formed elements (which are blood cells and cell fragments). Blood plasma consists of proteins like fibrinogen, globulin, albumin, in addition to some minerals like sodium, calcium, magnesium, bicarbonate, glucose, amino acids, lipids, etc. The formed elements of the blood include three principal components: erythrocytes (red blood cells, RBCs), leukocytes (white blood cells, WBCs) and platelets. RBCs and WBCs are whole cells, while platelets are cell fragments.
The clotting or coagulation of blood involves a series of chemical reactions that culminate in the formation of insoluble fibrin threads from the soluble plasma protein, fibrinogen. Injured tissue release thromboplastin and blood platelets disintegrate to release platelet factor-3, both of which combine with calcium ions to form prothrombinase. Prothrombinase inactivates heparin and catalyses the formation of active thrombin from inactive prothrombin. Thrombin acts as an enzyme to convert fibrinogen to fibrin, which combines with blood cells to form the clot.
The clotting cascade involves a plethora of factors like several inactive enzymes synthesised by hepatocytes, various molecules associated with platelets, or released by damaged tissues, all of which are essential for the cascade to progress.