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

9. Haematology 2659. HaematologyHaematology is the study of the cells that are found in blood and the factors thataffect their functioning.Volume of blood in the human bodyAn adult weighing 60kg has about 4.5 litres of blood. There is therefore no dangerinvolved in taking 0.5 litre of blood as a donation for transfusion, and no risk intaking two 10-ml tubes or more for analysis. Make this clear to anxious patientswhen you take their blood.9.1 Types of blood cellThree main classes of blood cell can be distinguished under the microscope: redcells (erythrocytes), white cells (leukocytes) and platelets (thrombocytes).9.1.1 Erythrocytes (Fig. 9.1)Appearance: round or slightly oval cells filled with haemoglobin. After stainingwith a Romanowsky stain (see section 9.10.4), they appear pink with apale central area. From the side erythrocytes look like biconcave discs;they do not contain nuclei.Size: 7–8mm.Number concentration: normally around 4–5 ¥ 10 12 per litre (4–5 ¥ 10 6 permm 3 ) of blood.Erythrocytes carry haemoglobin which combines with and carries oxygen fromthe lungs to the tissues. They also carry carbon dioxide from the tissues to thelungs, thus removing the principal end-product to which most organic substancesare metabolized in the body.9.1.2 Leukocytes (Fig. 9.2)Appearance: round cells containing a nucleus and granules in the cytoplasm.Size: 9–20mm.Number concentration: normally about 8 ¥ 10 9 per litre (8000 per mm 3 ) ofblood.The presence of a nucleus enables leukocytes to be readily distinguished fromerythrocytes under the microscope. There are five types of leukocyte(neutrophils, eosinophils, basophils, lymphocytes and monocytes) which differin size, shape of the nucleus, colour of the granules in the cytoplasm andother factors. They can be identified by microscopy after staining with a Romanowskystain (see section 9.10.4).Leukocytes play an important role in the defence or immune system.Fig. 9.1 ErythrocytesFig. 9.2 Leukocytes265