Manual of basic techniques for a health laboratory - libdoc.who.int

266 Manual of basic techniques for a health laboratory9.1.3 Thrombocytes (Fig. 9.3)Thrombocytes or platelets are fragments of megakaryocytes that are found in theperipheral blood, where they are involved in clot formation.Size: 2–5mm.Cytoplasm: very little visible, contains granules.In healthy adults, the blood contains about 150–300 ¥ 10 9 thrombocytes per litre(150000–300000 per mm 3 ).Fig. 9.3 ThrombocytesClotting of blood 1When blood is collected in a glass tube it solidifies within 5–10 minutes forming aclot; it has coagulated.Clotted blood separates into two components (Fig. 9.4):— the serum, a yellow liquid;— the clot, a solid red mass.If an anticoagulant is added to the blood as soon as it is collected, clotting is preventedand the blood remains fluid. Examples of anticoagulants include: fluorideoxalate (reagent no. 26), trisodium citrate, 3.2% solution (reagent no. 60) and EDTAdipotassium salt, 10% solution (reagent no. 22).Blood treated with an anticoagulant separates into two liquid components (Fig.9.5):— the plasma, a yellow liquid;— the blood cells, which sediment over time or following centrifugation to forma thin layer of leukocytes over a deposit of erythrocytes.Difference between plasma and serum● Plasma contains a soluble protein called fibrinogen in addition to a large numberof other proteins.● Serum does not contain fibrinogen, but all the other proteins are present. Thefibrinogen is changed into insoluble fibrin, which together with the erythrocytesforms the clot.Fig. 9.4 Clotted bloodFig. 9.5 Blood treated with ananticoagulant1See section 9.9.