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Зміст13.3 The formed elements of the blood
238 13 The properties of blood
Red blood cells have a life span of about 120 days
Erythropoiesis is regulated by the hormone erythropoietin
13.5 Iron metabolism
Maturation of monocytes
Maturation of lymphocytes
244 13 The properties of blood
R-nhcoo- + h
13.7 Major disorders of the red and white blood
Proliferative disorders of the white blood cells
The properties of blood
The blood is a vital vehicle of communication between the tissues of multi-cellular organisms. Its numerous functions include the following:
Blood consists of a fluid called plasma in which are suspended the so-called formed elements—the red cells (erythrocytes), white cells (leukocytes), and platelets (thrombocytes). It is possible to demonstrate the nature of the suspension by centrifuging a sample of whole blood in a test tube for a short time at low speed. After such treatment, the heavier red cells are packed at the bottom of the tube while the plasma can be seen as a clear, pale-yellow fluid above them, as illustrated in Fig. 13.1. A thin
Fig. 13.1 Separation of blood into cells and plasma by centrifugation. On the left is a diagrammatic view of the appearance of whole blood, showing red cells, leukocytes, and platelets.
layer of white cells and platelets (the 'buffy coat') separates the packed red cells from the plasma.
The circulating blood volume is about 7—8 per cent of body weight, so that for a 70 kg man blood volume will be around 5 liters, but for a newborn baby weighing 3.2 kg (7 lb), blood volume will only be around 250 ml—an important point to remember when considering a blood transfusion on a small baby. At any one time, assuming a blood volume of 5 liters, about 1 liter will be in the lungs, about 3 liters in the systemic venous circulation and the remaining liter in the heart, systemic arteries, arterioles, and capillaries (Chapter 15).
13.2 The physical and chemical characteristics of plasma
The total blood volume and the plasma volume may be measured using the dilution techniques described in Chapter 28. Normal adults have 35-45 ml of plasma per kilogram body weight so the plasma volume is 2.8—3.0 liters in men and around 2.4 liters in women.
The plasma accounts for about 4 per cent of body weight in both sexes. It consists of 95 per cent water, the remaining 5 per cent being made up by a variety of substances in solution and suspension. These include mineral ions (e.g. sodium, potassium, calcium, and chloride), small organic molecules (e.g. amino acids, fatty acids, and glucose), and plasma proteins (e.g. albumin). Typical values for a number of important constituents of the plasma are given in Table 13.1. The major constituents of the plasma, which include the inorganic ions and the plasma proteins, are normally present at roughly constant levels. However, the plasma also contains a number of substances which are in transit between different cells of the body and may be present in varying concentrations according to their rates of removal or supply from various organs. Such substances include enzymes, hormones, vitamins, products of digestion (e.g. glucose), and dissolved excretory products.
The composition of the plasma is normally kept within biologically safe limits by a variety of homeostatic mechanisms. However, this balance may be disturbed by a variety of disorders, particularly those involving the kidneys, liver, lungs, cardiovascular system, or endocrine organs. For this reason, accurate
13 The properties of blood
Table 13-1 Principal constituents of the plasma
Note that these values are approximate mean values and that even in health there is considerable individual variation.
analysis of plasma levels of a host of variables forms an essential part of diagnosis and treatment.
The ionic constituents of plasma
The chief inorganic cation of plasma is sodium (see Table 13-1), which has a concentration of 140—145 mmoles I"1. There are much smaller amounts of potassium, calcium, magnesium, and hydrogen ions. Chloride is the principal anion of plasma (around 100 mmoles I"1); electroneutrality is achieved by the presence of other anions including bicarbonate, phosphate, sulfate, protein, and organic anions.
The ionic components of the plasma maintain both its osmo-lality (280-300 mOsm per kg water) and its pH (7.35-7.45) within physiological limits. Further information concerning the homeostatic mechanisms responsible for the regulation of plasma pH, volume, and osmolality may be found in Chapters 17, 28, and 29.
In normal healthy individuals 7-9 per cent of the plasma is made up of plasma proteins. There are a great many dif-
ferent proteins in plasma but the principal proteins can be divided into three categories: the albumins, the globulins, and the clotting factors including fibrinogen and prothrombin.
The albumins are the smallest and the most abundant, accounting for about 60 per cent of the total plasma protein. They are produced by the liver and serve as transport proteins for lipids and steroid hormones. They are also important in body fluid balance since they provide most of the colloid osmotic pressure (the oncotic pressure) that regulates the passage of water and solutes through the capillaries (Chapters 15, 17, and 28).
Globulins account for about 40 per cent of total plasma protein and may be further subdivided into alpha (a), beta (f3), and gamma (у) globulins. The a- and /3-globulins are made in the liver and they transport lipids and fat-soluble vitamins in the blood. The y-globulins are antibodies produced by lymphocytes in response to exposure to antigens (agents usually foreign to the body that evoke the formation of specific antibodies). They are crucial in defending the body against infection.
Fibrinogen is an important clotting factor produced by the liver (Section 13.8). It accounts for about 2—4 per cent of the total plasma protein and is generally grouped with the globulins.
13.3 The formed elements of the blood
The formed elements of blood include red cells, five classes of white cells (recognized according to their morphology and staining reactions), and platelets (see Figs 13.1 and 13.2). Of these, the red cells are by far the most numerous. Table 13.2 lists the cellular components and their concentration in whole blood.
The hematocrit ratio describes the proportion of the total blood volume occupied by the erythrocytes. For any blood sample, the hematocrit may be obtained by centrifuging a small volume of blood in a capillary tube until the cellular components become
packed at the bottom of the tube (see Fig. 13.1). For this reason the hematocrit is also known as the packed cell volume. By measuring the height of the column of red cells relative to the total height of the column of blood and correcting for the plasma which remains trapped between the packed red cells, it is possible to determine the volume occupied by the packed red cells as a percentage of the total blood volume. In adult males the average hematocrit determined in rhis way from a sample of venous blood is around 0.47 11"1 of whole blood (it ranges from 0.4 to 0.54 11"1), while in females it is closer to 0.42 11"1 (normal range 0.37-0.47 11"1). However, the hematocrit is not uniform throughout the body. In the capillaries, arrerioles, and other small vessels, the hematocrit is lower than in the larger arteries and veins as a result of axial streaming of blood cells in vessels (see Chapter 15). This is the tendency for red cells not to flow near to the walls of vessels but to remain near the center. In large vessels the wall surface area to volume ratio is smaller than in the tiny vessels and so the former contain relatively more cells. Consequently, they have a higher hematocrit.
Red blood cells—erythrocytes
The red cells (also called erythrocytes) are the most numerous cell type in the blood—each liter of normal blood containing 4.5-6.5 X 1012 red cells. Their chief function is to transport the respiratory gases oxygen and carbon dioxide around the body. The red cells are small, circular, biconcave discs of 7—8 yarn diameter and they do not possess a nucleus. They are very thin and flexible and can squeeze through the narrow bore of the capillaries which have internal diameters of only 5-8 /xm. As a
Table 13.2 The cellular elements of whole blood
Note that, while mean values are given, these are subject to considerable individual variation. The approximate percentage of individual types of leukocyte are given after the number per litre—this is called the differential white cell count.
13 The properties of blood
result of their shape, red cells have a large surface area to volume ratio, permitting efficient gaseous exchange. In a mature ery-throcyte, the principal protein constituent of the cytoplasm is hemoglobin—an oxygen-binding protein which is synthesized by the red cell precursors in the bone marrow.
White blood cells—leukocytes
Leukocytes are larger than the red blood cells, possess a nucleus, and are present in smaller numbers—normal blood contains 4_10 X 109 white cells I"1 (Table 13.2). These cells have a vital role in the protection of the body against disease—they are the mobile units of the body's protective system, being transported rapidly to specific areas of inflammation to give powerful defense against invading organisms. They possess several characteristics which enhance their efficacy as part of the body's defense system. They are able to pass through the walls of capillaries, to enter the tissue spaces in accordance with the local needs. This process is known as diapedesis. Once within the tissue spaces, the leukocytes (particularly the polymorphonucleocytes) have the ability to move through the tissues by an ameboid motion at speeds of up to 40 fjbm min"1. Furthermore, they seem to be attracted by certain chemical substances released by bacteria or by inflamed tissues (chemotaxis). For more information concerning the immune system see Chapter 14.
There are three major categories of white blood cells:
The monocytes and the lymphocytes are sometimes also referred to collectively as agranulocytes or mononuclear leukocytes. The granulocytes are further subdivided into neutrophils, eosinophils, and basophils, according to their staining reactions.
Although all the white blood cells are concerned with defending the tissues against disease-producing agents, each class of cell has a slightly different role to play. Consider first the granulocytes, which account for around 70 per cent of the total number of white cells in the blood.
Neutrophils are by far the most numerous of the granulocytes. They are phagocytes which are able to enter the intercellular spaces by diapedesis to engulf and destroy disease-producing bacteria. Enzymes within the cytoplasmic granules then digest the phagocytosed particles. As a result of this action, the neutrophils form the first line of defense against infection. They are so named because their cytoplasm does not stain with eosin or basophilic dyes such as methylene blue.
Eosinophils are so called because their granules stain red in the presence of the dye eosin. Normally they represent only about 1.5 per cent of the total number of white blood cells but in people with allergic conditions, such as asthma or hay-fever,
their population greatly increases. These cells have antihistamine properties and they congregate around sites of inflammation. Their life span is very short (12-20 hours).
Basophils possess granules that stain blue in the presence of basic dyes such as methylene blue. They represent only about 0.5 per cent of the white-cell population and are considered to be circulating mast cells. They produce heparin and histamine and are responsible for some of the phenomena associated with local immunological reactions, such as local vasodilatation and increased permeability of blood vessels, resulting in local edema. They are stimulated by certain antigen complexes bound to immunoglobulin E (IgE).
Monocytes are larger than the other classes of white cells, having a diameter of 15—20 д-m. Their nuclei are kidney-shaped. They are formed in the bone marrow where they mature before being released into the circulation. Within 2 days they have migrated to tissues such as the spleen, liver, lungs, and lymph nodes. These cells are macrophages and act in much the same way as the neutrophils ingesting bacteria and other large particles. They also participate in immune responses both by presenting antigens so that they will be recognized by the lymphocytes, and by stimulating the production of lymphocytes (Chapter 14).
Lymphocytes represent around 25 per cent of the total white cell population in adults (although in children they are much more numerous) and vary from 6 to 20 /xm in diameter. They are of two types, the so-called В lymphocytes, which are made in the lymphoid tissue such as the lymph nodes, tonsils, and spleen, and to a lesser extent in the bone marrow, and T lymphocytes which are formed in the thymus. В cells have a very short life in the circulation (a few hours) but T cells can live for 200 days or more. Each has a very important part to play in the protection of the body against infection, either by producing antibodies (B cells) or by participating in cell-mediated immune responses of other kinds (T cells). The functions of the lymphocytes will be discussed in detail in Chapter 14.
Strictly speaking, platelets are not cells at all. They are irregularly shaped, membrane-bound cell fragments, which are formed in the bone marrow by budding off from the cytoplasm of large, polyploid cells called megakaryocytes which are themselves derived from primitive hematopoietic stem cells (see Fig. 13.3 and Section 13.4). They rarely possess a nucleus, are 2-4/xm in diameter and have a life span in the blood of around 10 days. Normal blood contains 150-400 X 109 platelets per liter. Platelets have an important role in the control of bleeding (hemostasis; Section 13.8) and in the maintenance of integrity of the vascular endothelium.
13.4 Hematopoiesis—the formation of blood cells
13.4 Hematopoiesis—the formation of blood cells
Mature blood cells have a relatively short life span in the bloodstream and must therefore be renewed continuously. This is achieved by a process known as hematopoiesis. The term erythro-poiesis refers to the formation of erythrocytes (red blood cells), and leukopoiesis to the formation of leukocytes (white blood cells).
Pluripotent stem cells give rise to all the blood cell types
Despite the fact that the blood contains many different cells with a variety of functions, they are all generated ultimately from a common population of stem cells present within the hematopoietic tissue of the bone marrow. These cells are said to be pluripotent (having the potential to differentiate into any kind of blood cell), and give rise to all the differentiated types of blood cell through a series of cell divisions, which are shown diagrammatically in Fig. 13.2.
The pluripotent stem cells are particularly abundant in the bone marrow of the pelvis, ribs, sternum, vertebrae, clavicles, scapulae, and skull. They proliferate to form two distinct cell lines, the lymphoid cells and the myeloid cells. The lymphoid cells migrate to the lymph nodes, spleen, and thymus, where they differentiate to become lymphocytes. The myeloid cells remain within the bone marrow to develop as granulocytes, monocytes, erythrocytes, and megakaryocytes.
As may be seen from Fig. 13.2, in general terms, the pluripotent stem cells divide (infrequently under normal conditions) to give rise to more stem cells as well as various types of 'com-
mitted' cells, each capable of giving rise to one or a few types of blood cell. These are called progenitor cells. These cells in turn, generate precursor cells in which the morphological characteristics of the mature cell are evident for the first time. Full differentiation and maturation of the blood cells then occurs as a result of a further series of cell divisions.
The maturation of erythrocytes
The precursor cells that are committed to becoming red blood cells are called erythroblasts. These subsequently undergo a further series of cell divisions, each of which produces a smaller cell, as they mature into erythrocytes. During these divisions the cells synthesize hemoglobin. Finally, they lose their nuclei to become reticulocytes. Development from erythroblast to reticulocyte normally takes around 7 days.
Most red cells are released into the circulation as reticulocytes and mature further over the next day or so to become erythrocytes. During this transition they lose their mitochondria and ribosomes. Consequently they also lose the ability to synthesize hemoglobin and carry out oxidative metabolism. The mature red blood cell relies on glucose and the glycolytic pathway for its metabolic needs, including the production of large amounts of 2,3-diphosphoglycerate (2,3-DPG) which reduces the affinity of the hemoglobin for oxygen, thereby facilitating the release of oxygen at the tissues (Section 13.6).
Once it has entered the general circulation, the average life span of a red blood cell is around 120 days, after which time it is destroyed in the spleen, the liver, or lymph nodes by large phagocytic cells known as macrophages. The protein portion of the erythrocyte is broken down into its constituent amino acids. The iron in the heme group is stored in the liver as ferritin and may be reused later (Section 13.5), while the remainder of the heme group is broken down into the two bile pigments bilirubin and biliverdin, both of which are eventually excreted into the gut by way of the bile. If red cell destruction, and therefore bilirubin production, are excessive, unconjugated bilirubin may build up in the blood and give a yellow color to the skin—hemolytic jaundice. This condition can arise following a hemolytic blood transfusion reaction (Section 13-9), in hemolytic disease of the newborn, or in genetic disorders such as hereditary spherocytosis in which the membrane of the red blood cells is defective.
Each liter of blood contains around 5 million million erythrocytes (5 X 1012), although this figure varies according to the age, sex, and state of health of the individual. Because most red cells enter the circulation as reticulocytes, the rate of red cell production is indicated by the relative numbers of reticulocytes in the circulation (normally 1—1.5 per cent). The rate of erythropoiesis is closely matched to the requirement for new red cells
13 The properties of blood
Fig. 13.2 An outline of the cellular differentiation that occurs during hematopoeisis to give rise to the cellular elements of the blood. The appearance of the different types of mature blood cell after staining is given at the foot of the figure.
within the circulation and is controlled by a glycoprotein hormone, erythropoietin, which is secreted mainly by the kidneys (probably by cells in the endothelium of the peritubular capillaries). This hormone acts by accelerating the differentiation of stem cells in the marrow to form erythroblasts. In addition to erythropoietin, iron, folic acid, and vitamin B12 are also essential for normal red blood cell production. Vitamin B12 is absorbed from the small intestine in combination with intrinsic factor which is secreted by cells in the gastric mucosa (vitamin B12 was
formerly known as extrinsic factor). If the diet is deficient in B12 or there is a lack of intrinsic factor, red cell development is impaired, resulting in pernicious anemia (Section 13.7).
A variety of stimuli may cause an increase in the rate of production of new erythrocytes, including loss of red cells through hemorrhage or donation of blood, and chronic hypoxia, such as that experienced while living at high altitude. In all cases it appears that the secretion of erythropoietin is stimulated by a fall in tissue PO2.
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