The volume of blood in an adult is around 5 litres and is normally composed of red blood cells (RBCs: erythrocytes) for gas transport (Chapter 25), white blood cells (WBCs) (neutrophils, eosinophils, basophils, monocytes and lymphocytes) for defence/immunology and platelets (for clotting). The remainder is plasma, a solution of electrolytes, glucose, lipids, metabolites, gases, hormones, drugs and plasma proteins. Serum is coagulated plasma.
Control of blood production
Production of blood cells (haemopoiesis) is a tightly regulated process in which a pluripotent stem cell is committed to one cell line and undergoes many cell divisions, leading to clonal expansion. RBC production is stimulated by the polypeptide erythropoietin (EPO), which is released by renal peritubular cells in response to hypoxia. Colony-stimulating factors (CSFs), e.g. G-CSF (granulocyte CSF), act on the neutrophil cell line and are increased in infections to increase the WBC count (Figure 33.1).
Blood counts
The full blood count is the number of blood cells per volume of blood and is differentiated into RBCs, WBC types and platelets.
The amount of haemoglobin (Hb) in blood is expressed as (g) per 100 mL of blood and used to identify anaemias.
Haematocrit (packed cell volume, PCV) is the percentage of blood that is made up of RBCs. It is expressed as either a percentage (typically 40–45%) or as a fraction (0.40–0.45).
Mean corpuscular volume (MCV) is the volume (fl) of individual RBCs [normal range = 80–95 fl]. Increased MCV is described as macrocytosis and a decreased MCV as microcytosis.
Blood groups
These are determined by antigens (glycoproteins) expressed on RBC membranes and antibodies to other antigens may be in plasma. There are more than 400 groups, but ABO and Rhesus are the clinically important ones. Group A (A antigens and b-antibodies), Group B (B antigens and a-antibodies), Group AB (A and B antigens and no antibodies), and Group O (a and b antibodies).
- The Rhesus D (RhD) antigen confers the Rhesus status (positive).
Patients are identified as being A, B, AB, or O, and are either Rh+ve or Rh–ve. This system is essential to ensure correct cross-matching of blood for transfusions, as exposure to an antigen from a different group leads to life-threatening transfusion reactions. This involves antibodies being raised against the foreign antigen, leading to a severe immune response with haemolysis of RBCs.
Blood from the O Rh–ve group is regarded as a universal donor as it lacks antigens and as such will not provoke an immune response. O–ve blood is therefore used in dire emergencies for transfusions until cross-matched blood is available.
Haemolytic disease of the newborn
Approximately 85% of the population are Rh+ve, consequently mothers who are Rh–ve are likely to carry to a Rh+ve fetus. In a first pregnancy this is unlikely to lead to complications, but at birth the transfer of fetal blood leads to the mother raising antibodies against the D-antigen. These antibodies would then attack a subsequent fetus that was Rh+ve, potentially leading to fetal death. To eliminate this risk the mother receives anti-D immunisation after the first birth to bind any D-antigens and so prevent the production of antibodies.
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