Blood consists of plasma containing cells and cell fragments. Three groups
of blood cells can be identified, namely (a) red blood cells or erythrocytes, (b)
white blood cells or leucocytes and (c) platelets or thrombocytes. The white blood
cells can be separated into neutrophilic-, eosinophilic- and basophilic granulocytes,
lymphocytes and monocytes. The life time of these cells is limited. For example,
in man life time for normal erythrocytes is about 120 days and for granulocytes a
few days. The daily production of these cells required to maintain normal numbers
in blood can be calculated from their life span, their concentration in the blood
and the total blood volume. Thus, an adult human male produces approximately 2
x 10¹¹ erythrocytes and 1 x 10¹⁰ granulocytes every day. These amounts are
necessary for normal maintenance but the production of blood cells can be increased
after stress, for example, infection or blood los