Note: Circulating blood is a heterogeneous fluid consisting of a clear, slightly yellow liquid (native plasma) and three groups of suspended, formed elements: red blood cells, or erythrocytes (about \(\pu{99\%}\) of the blood cells), white blood cells, or leukocytes, and blood platelets, or thrombocytes. When no anticoagulant is added, normal blood withdrawn from the circulation forms a clot due the polymerization of fibrinogen to fibrin. On standing, the clot retracts, expressing serum which differs from plasma chiefly in that it contains no fibrinogen.