第三章 血液
2011-06-08 18:14:09   来源：   作者：  评论：0 点击：

Summary
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 crystal osmotic pressure and plasma protein for colloid osmotic pressure). Blood accounts for about 7%~8% of the weight of the body in adults, which corresponds to a blood volume of 4~6 liters. The blood functions are transportation,amortization, 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 to maintain a supply of these. 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 include multipotential progenitor cells and committed progenitor cell. The third phase is called precursors 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/carbon dioxide and buffer. 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~l0.0)×109/L leukocytes. The leukocytes are not a homogeneous population of cells. Three major groups, which are the granulocytes, monocytes (to form macrophages in tissue), and lymphocytes (for specific immune response), 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 ameboid movement, which permits them to emigrate through the walls of blood vessels (this process is also called diapedesis). They are 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. 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 (don't 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, (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 effector 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, when two such 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 which are called agglutinins. The ABO and Rh systems are of the greatest significances 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 complememt of each individal (genotype); they determine the blood-group phenotype together. Erythrocytes containing D antigen 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 experiments,which are blood typing, determination of antibodies, and cross-match test.
（中山大学中山医学院 张志方；福建医科大学 王晶）
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复习思考题
1.试述血浆蛋白的种类及其生理功能。
2.血液有哪些功能？
3.血浆渗透压如何形成？有何生理意义？
4.试述造血干细胞和造血祖细胞的鉴别方法。
5.造血干细胞有哪些基本特征？在临床治疗学上有什么意义？
6.试述红细胞的生理特性。
7.缺乏铁、维生素B12和叶酸引起的贫血有何不同？为什么？
8.试述EPO促进红细胞生成的作用及其调节机理。
9.简述各类白细胞的功能。
10.试述血小板在生理性止血中的作用。
11.外源性凝血系统和内源性凝血系统有何异同点？
12.血液中有哪些抗凝因素？它们如何发挥作用？
13.简述纤溶系统的组成及其作用。
14.简述ABO血型系统的分型特点。
15.简述Rh血型的特点及临床意义。
16.简述输血的原则。