Chapter5

　Blood is composed of cells and plasma. The blood cells are the erythrocytes (red blood cells), the leukocytes (white blood cells), and the platelets (thrombocytes), which are not complete cells but cell fragments. The hematocrit is defined as the percentage of blood volume that is occupied by blood cells. Plasma, the liquid portion of the blood, consists of a large number of organic and inorganic substances dissolved in water, which can be expressed by the osmotic pressure (crystalloid substance for crystalloid osmotic pressure and plasma protein for colloid osmotic pressure). Blood accounts for about 7%~8% of the body weight in adults, which corresponds to a blood volume of 4~6 liters. The functions of blood are transport, buffer function, regulation of body temperature, physiological hemostasis and protection against foreign substances and organisms.
　All blood cells are derived from cells called hematopoietic stem cells. As these cells reproduce, a small portion of them remains exactly like the original hematopoietic stem cells and is retained in the bone marrow. Most of the reproduced hematopoietic stem cells differentiate to form the other cells. The second phase of genesis of blood cells is called progenitor cell phase, which includes multipotential progenitor cell and committed progenitor cell. The third phase is called precursor cell phase.
　In normal men, the average number of erythrocytes is 5×1012/L and in normal women, 4.2×1012/L. Normal erythrocytes are biconcave discs having a mean diameter of about 7.8 micrometers and a thickness at the thickest point of 2.5 micrometers and in the center of 1 micrometer. The average volume of the erythrocytes is 90 cubic micrometers. The physiological characteristics of erythrocytes include membrane permeability, plastical deformability, suspension stability, and osmotic fragility. The functions of erythrocytes are the carriage of oxygen and carbon dioxide and the buffering of pH. Production of normal erythrocytes requires protein, iron, folic acid, and vitamin B12. Burst promoting activator and erythropoietin accelerate erythropoiesis.
　The blood of a healthy person contains 4.0~l0×109/L leukocytes. The leukocytes are not a homogeneous population of cells but consist of three major groups: the granulocytes, the monocytes (to form macrophages in tissue), and the lymphocytes (for specific immune responses). These cell types, are distinguished on the basis of morphology, function,and site of origin. According to the staining properties of the granules, the granulocytes are classified as neutropils (for phagocytosis), basophils (for anaphylactic reaction), and eosinophils (to attack parasites and appose anaphylactic reaction). All leukocytes are capable of amoeboid movement, which permits them to emigrate through the walls of blood vessels (this process is also called diapedesis). They are also attracted (chemotaxis) by bacterial toxins, the products of decomposition of bacteria or body cells, and antigen-antibody complexes; they can surround foreign bodies and take them into the cytoplasm (phagocytosis).
　Healthy adults are found to have 100-300×109/L thrombocytes or platelets. They are produced in the bone marrow by the shedding of cytoplasmic buds of megakaryocytes. Thrombocyte formation is regulated by a glycoprotein hormone, thrombopoietin. The physiological characteristics of platelets include adhesion, aggregation, secretion reactions, absorption, contraction and repair. The main function of thrombocytes is hemostasis.
　The stoppage of bleeding is known as hemostasis (one should not confuse this word with homeostasis). Whenever a vessel is severed or ruptured, hemostasis is achieved by several mechanisms: (1) vascular spasm, (2) formation of a platelet plug,and (3) formation of a blood clot as a result of blood coagulation. Coagulation can be brought about by an extrinsic (tissue-based) pathway or intrinsic (plasma-based) pathway, each of which is made up of many steps involving clotting factors. The result of either extrinsic pathway or intrinsic pathway is the formation of a complex of activated substances collectively called prothrombin activator, which catalyzes the conversion of prothrombin into thrombin. The thrombin acts as an enzyme to convert fibrinogen into fibrin fibers that enmesh blood cells and plasma to form the clot. There are four plasma anti-clotting substances that oppose clot formation to limit this process and prevent it from spreading excessively. They are serine protease inhibitor, tissue factor pathway inhibitor, protein C system and heparin. A fibrin clot is a transitory device until permanent repair of the vessel occurs. The fibrinolytic (or thrombolytic) system is the principal effecter of clot removal. It constitutes a plasma proenzyme, plasminogen, which can be activated to the active enzyme plasmin by plasminogen activators. Once formed, plasmin digests fibrin, thereby dissolving the clot.
　Agglutination would occur in the circulatory system following blood transfusion, if two incompatible types of blood came into contact. The cause of agglutination is an antigen-antibody reaction. The erythrocyte membrane includes specific glycolipids which are called agglutinogens. The specific antibodies that react with these agglutinogens of the erythrocyte membrane are dissolved in the plasma and called agglutinins. The ABO and Rh systems are of the greatest significance in clinical practice. In the ABO system, group O blood, although containing no agglutinogens, does contain anti-AB (anti-A and anti-B) agglutinins. Group A blood contains type A agglutinogens and anti-B agglutinins. Group B blood contains type B agglutinogens and anti-A agglutinins. Group AB blood contains both A and B agglutinogens but no agglutinins. Two of the three alleles A, B, O (H) are found in the diploid chromosome complement of each individual (genotype); together they determine the blood-group phenotype. Blood containing D erythrocytes are therefore, for the sake of simplicity, called Rh-positive, and those lacking the D property are called Rh-negative. One difference between the Rh and the ABO systems is that the agglutinins of the ABO system are always present after the first few months of life, whereas anti-D antibodies do not appear unless the carrier has been exposed to Rh antigens. Another difference between the two systems lies in the fact that most of the antibodies of the Rh system are incomplete IgG antibodies, which, in contrast to the complete IgM antibodies of ABO agglutinins, are small enough to pass the placental barrier. Before giving a transfusion to a person, it is necessary to do three tests to determine the blood type, the antibodies and a cross-matched test.