Anatomy and Physiology PDF

Communication Blood is a connective tissue. It provides one of the means of communication between the cells of different parts of the body and the external environment, e.g. it carries: oxygen from the lungs to the tissues and carbon dioxide from the tissues to the lung...

Communication Blood is a connective tissue. It provides one of the means of communication between the cells of different parts of the body and the external environment, e.g. it carries: oxygen from the lungs to the tissues and carbon dioxide from the tissues to the lungs for excretion nutrients from the alimentary tract to the tissues and cell wastes to the excretory organs, principally the kidneys hormones secreted by endocrine glands to their target glands and tissues heat produced in active tissues to other less active tissues protective substances, e.g. antibodies, to areas of infection clotting factors that coagulate blood, minimising its Figure 4.1 A. The proportions of blood cells and plasma in whole loss from ruptured blood vessels. blood separated by gravity. B. A blood clot in serum. Blood makes up about 7% of body weight (about 5.6 litres in a 70 kg man). This proportion is less in women Plasma and considerably greater in children, gradually decreasing until the adult level is reached. The constituents of plasma are water (90 to 92%) and Blood in the blood vessels is always in motion. The dissolved substances, including: continual flow maintains a fairly constant environment for the body cells. plasma proteins: albumins, globulins (including Blood volume and the concentration of its many con- antibodies), fibrinogen, clotting factors stituents are kept within narrow limits by homeostatic inorganic salts (mineral salts): sodium chloride, mechanisms. sodium bicarbonate, potassium, magnesium, 60 phosphate, iron, calcium, copper, iodine, cobalt nutrients, principally from digested foods, e.g. monosaccharides (mainly glucose), amino acids, fatty acids, glycerol and vitamins COMPOSITION OF BLOOD organic waste materials, e.g. urea, uric acid, creatinine hormones enzymes, e.g. certain clotting factors Learning outcomes gases, e.g. oxygen, carbon dioxide, nitrogen. After studying this section, you should be able to: Plasma proteins describe the chemical composition of plasma Plasma proteins, which make up about 7% of plasma, are normally retained within the blood, because they are too discuss the structure, function and formation of big to escape through the capillary pores into the tissues. red blood cells, including the systems used in They are largely responsible for creating the osmotic medicine to classify the different types pressure of blood (normally 25 mmHg or 3.3 kPa*), which discuss the functions and formation of the keeps plasma fluid within the circulation. If plasma pro- different types of white blood cell tein levels fall, because of either reduced production or outline the role of platelets in blood clotting. loss from the blood vessels, osmotic pressure is also reduced, and fluid moves into the tissues (oedema) and body cavities. Blood is composed of a straw-coloured transparent fluid, plasma, in which different types of cells are suspended. Plasma constitutes about 55% and cells about 45% of *1 kilopascal (kPa) = 7.5 millimetres of mercury (mmHg) blood volume (Fig. 4.1 A). 1 mmHg = 133.3 Pa = 0.133 kPa  The blood Albumins. These are formed in the liver. They are the most Hormones (Ch. 8) abundant plasma proteins and their main function is to These are chemical compounds synthesised by endocrine maintain a normal plasma osmotic pressure. Albumins also glands. Hormones pass directly from the cells of the act as carrier molecules for lipids and steroid hormones. glands into the blood which transports them to their tar- get tissues and organs elsewhere in the body, where they Globulins. Most are formed in the liver and the remainder influence cellular activity. in lymphoid tissue. Their main functions are: Gases as antibodies (immunoglobulins), which are complex Oxygen, carbon dioxide and nitrogen are transported proteins produced by lymphocytes that play an round the body in solution in plasma. Oxygen and important part in immunity. They bind to, and carbon dioxide are also transported in combination with neutralise, foreign materials (antigens) such as haemoglobin in red blood cells (p. 256). Most oxygen micro-organisms (see also p. 380). is carried in combination with haemoglobin and most transportation of some hormones and mineral salts; carbon dioxide as bicarbonate ions dissolved in plasma. e.g. thyroglobulin carries the hormone thyroxine and Atmospheric nitrogen enters the body in the same way transferrin carries the mineral iron as other gases and is present in plasma but it has no inhibition of some proteolytic enzymes, e.g. 2 physiological function (p. 255). macroglobulin inhibits trypsin activity. Clotting factors. These are substances essential for coagulation of blood (p. 67). Serum is plasma from which Cellular content of blood clotting factors have been removed (Fig. 4.1B). Fibrinogen. This is synthesised in the liver and is There are three types of blood cells (see Fig. 1.5, p. 8). essential for blood coagulation. erythrocytes or red cells Plasma viscosity (thickness) is due to plasma proteins, thrombocytes or platelets mainly albumin and fibrinogen. Viscosity is used as a leukocytes or white cells. measure of the body's response to some diseases. All blood cells originate from pluripotent stem cells and go 61 Inorganic salts (mineral salts) through several developmental stages before entering These are involved in a wide variety of activities, includ- the blood. Different types of blood cells follow separate ing cell formation, contraction of muscles, transmission lines of development. The process of blood cell formation of nerve impulses, formation of secretions and mainte- is called haemopoiesis (Fig. 4.2) and takes place within red nance of the balance between acids and alkalis. In health bone marrow. For the first few years of life, red marrow the blood is slightly alkaline. Alkalinity and acidity are occupies the entire bone capacity and, over the next 20 expressed in terms of pH, which is a measure of hydro- years, is gradually replaced by fatty yellow marrow that gen ion concentration, or [H+] (p. 21 and Fig. 2.6). has no erythropoietic function. In adults, erythropoiesis The pH of blood is maintained between 7.35 and 7.45 by is confined to flat bones, irregular bones and the ends an ongoing complicated series of chemical activities, (epiphyses) of long bones, the main sites being the sternum, involving buffering systems. ribs, pelvis and skull. Nutrients Food is digested in the alimentary tract and the resultant Erythrocytes (red blood cells) nutrients are absorbed, e.g. monosaccharides, amino These are circular biconcave non-nucleated discs with acids, fatty acids, glycerol and vitamins. Together with a diameter of about 7 microns. Measurements of red cell mineral salts they are required by all body cells to provide numbers, volume and haemoglobin content are routine energy, heat, materials for repair and replacement, and and useful assessments made in clinical practice (Table 4.1). for the synthesis of other blood components and body The symbols in brackets are the abbreviations commonly secretions. used in laboratory reports. Organic waste products Erythrocyte count. This is the number of erythrocytes Urea, creatinine and uric acid are the waste products of pro- per litre (1) or per cubic millimetre (mm3) of blood. tein metabolism. They are formed in the liver and conveyed in blood to the kidneys for excretion. Carbon dioxide, Packed cell volume or haematocrit. This is the volume released by all cells, is conveyed to the lungs for excretion. of red cells in 1 litre or 1 mm3 of whole blood.  Communication Figure 4.2 Haemopoiesis: stages in the development of blood cells. 62 Mean cell volume. This is the average volume of cells, measured in femtolitres (fl =101-15litre). Table 4.1 Erythrocytes - normal values Measure Normal values Haemoglobin. This is the weight of haemoglobin in whole blood, measured in grams per 100 ml. Erythrocyte count Male 4.5 x 1012/l to 6.5 x 1012/l (4.5 to 6.5 million/mm3) Mean cell haemoglobin. This is the average amount Female 4.5 x 1012/l to 5 x 10'2/l of haemoglobin in each cell, measured in picograms (4.5 to 5 million/mm3) (pg =101-12gram). Packed cell volume (PCV) 0.4 to 0.5 I/I (40 to 50/mm3) Mean cell haemoglobin concentration. This is the Mean cell volume (MCV) 80 to 96 fl amount of haemoglobin in 100 ml of red cells. Haemoglobin (Hb) Male 13 to 18 g/100 ml Development and life span of erythrocytes Female 11.5 to16.5 g/100 ml Erythrocytes are formed in red bone marrow, which is Mean cell haemoglobin (MCH) 27 to 32 pg/cell present in the ends of long bones and in flat and irregular bones. They pass through several stages of development Mean cell haemoglobin before entering the blood. Their life span in the circulation concentration (MCHC) 30 to 35 g/100 ml of cells is about 120 days.  The blood Figure 4.3 Maturation of the erythrocyte. Figure 4.4 Control of erythropoiesis: the role of erythropoietin. The process of development of red blood cells from Haemoglobin in mature erythrocytes combines with pluripotent stem cells takes about 7 days and is called oxygen to form oxyhaemoglobin, giving arterial blood its erythropoiesis (Fig. 4.2). It is characterised by two main characteristic red colour. In this way the bulk of oxygen features: absorbed from the lungs is transported around the body to maintain a continuous oxygen supply to all cells. maturation of the cell Haemoglobin is also involved, to a lesser extent, in the formation of haemoglobin inside the cell (Fig. 4.3). transport of carbon dioxide from the body cells to the Maturation of the cell. During this process the cell lungs for excretion. 63 decreases in size and loses its nucleus. These changes Each haemoglobin molecule contains four atoms of depend on a number of factors, especially the presence of iron. Each atom can carry one molecule of oxygen, there- vitamin B12 and folic acid. These are present in sufficient fore one haemoglobin molecule can carry up to four mole- quantity in a normal diet containing dairy products, meat cules of oxygen. Haemoglobin is said to be saturated when and green vegetables. If the diet contains more than is all its available binding sites for oxygen are filled. When needed, they are stored in the liver. Absorption of vita- oxygen levels are low, only partial saturation is possible. min B12 depends on a glycoprotein called intrinsic factor secreted by parietal cells in the gastric glands. Together Control of erythropoiesis they form the intrinsic factor-vitamin Bu complex (IF-B12). The number of red cells remains fairly constant, which During its passage through the intestines, the bound vita- means that the bone marrow produces erythrocytes at min is protected from enzymatic digestion, and is the rate at which they are destroyed. This is due to a absorbed in the terminal ileum. homeostatic negative feedback mechanism (Fig. 4.4). The effects of deficient intake of vitamin B12 do not The primary stimulus to increased erythropoiesis is appear for several years because there are large stores in hypoxia, i.e. deficient oxygen supply to body cells. This the liver. occurs when: Folic acid is absorbed in the duodenum and jejunum the oxygen-carrying power of blood is reduced by where it undergoes change before entering the blood. e.g. haemorrhage or excessive erythrocyte breakdown Signs of deficiency are apparent within a few months. (haemolysis) due to disease Deficiency of either vitamin B12 or folic acid leads to the oxygen tension in the air is reduced, as at high impaired red cell production. altitudes. Formation of haemoglobin. Haemoglobin is a complex Hypoxia increases erythrocyte formation by stimulating protein, consisting of globin and an iron-containing sub- the production of the hormone erythropoietin, mainly stance called haem, and is synthesised inside developing by the kidneys. Erythropoietin stimulates an increase erythrocytes in red bone marrow. in the production of proerythroblasts and the release of  Communication increased numbers of reticulocytes into the blood. The ABO system These changes increase the oxygen-carrying capacity About 55% of the population has either A-type antigens of the blood and reverse tissue hypoxia, the original (blood group A), B-type antigens (blood group B) or both stimulus. When the tissue hypoxia is overcome, erythro- (blood group AB) on their red cell surface. The remaining poietin production declines (Fig. 4.4). When erythropoietin 45% have neither A nor B type antigens (blood group O). levels are low, red cell formation does not take place even The corresponding antibodies are called anti-A and anti- in the presence of hypoxia, and anaemia (the inability of the B. Blood group A individuals cannot make anti-A (and blood to carry adequate oxygen for body needs) develops. therefore do not have these antibodies in their plasma), It is believed that erythropoietin regulates normal red cell since otherwise a reaction to their own cells would occur; replacement, i.e. in the absence of hypoxia. they do, however, make anti-B. Blood group B individuals, for the same reasons, make only anti-A. Blood group AB Destruction of erythrocytes make neither, and blood group O make both anti-A and The life span of erythrocytes is about 120 days and their anti-B (Fig. 4.5). breakdown, or haemolysis, is carried out by phagocytic retic- Because blood group AB people make neither anti-A uloendothelial cells. These cells are found in many tissues nor anti-B antibodies, they are known as universal recipi- but the main sites of haemolysis are the spleen, bone mar- ents: transfusion of either type A or type B blood into row and liver. As erythrocytes age, changes in their cell these individuals is safe, since there are no antibodies membranes make them more susceptible to haemolysis. to react with them. Conversely, group O people have Iron released by haemolysis is retained in the body and neither A nor B antigens on their red cell membranes, reused in the bone marrow to form haemoglobin (Fig. and their blood may be safely transfused into A, B, AB 4.3). Biliverdin is formed from the protein part of the ery- or O types; group O is known as the universal donor. throcytes. It is almost completely reduced to the yellow pigment bilirubin, before it is bound to plasma globulin The Rhesus system and transported to the liver (see Fig. 12.41, p. 310). In the The red blood cell membrane antigen important here is liver it is changed from a fat-soluble to a water-soluble the Rhesus (Rh) antigen, or Rhesus factor. About 85% of form before it is excreted as a constituent of bile. people have this antigen; they are Rhesus positive (Rh+) and do not therefore make anti-Rhesus antibodies. The 64 remaining 15% have no Rhesus antigen (they are Rhesus Blood groups negative, or Rh~). Rh~ individuals are capable of making Individuals have different types of antigen on the sur- anti-Rhesus antibodies, but are stimulated to do so only faces of their red blood cells. These antigens, which are in certain circumstances, e.g. in pregnancy (p. 71), or as inherited, determine the individual's blood group. In addi- the result of an incompatible blood transfusion. tion, individuals make antibodies to these antigens, but not to their own type of antigen, since if they did the anti- gens and antibodies would react causing a transfusion Leukocytes (white blood cells) reaction. The main signs are clumping of red blood cells, These cells have an important function in defending the haemolysis, shock and kidney failure. These antibodies body against microbes and other foreign materials. circulate in the bloodstream and the ability to make Leukocytes are the largest blood cells and they account them, like the antigens, is genetically determined and not for about 1% of the blood volume. They contain nuclei associated with acquired immunity (see also Ch. 15). and some have granules in their cytoplasm. There are If individuals are transfused with blood of the same two main types (Table 4.2): group, i.e. possessing the same antigens on the surface of the cells, their immune system will not recognise them as granulocytes (polymorphonuclear leukocytes) foreign and will not reject them. However, if they are — neutrophils, eosinophils and basophils given blood from an individual of a different blood type, agranulocytes i.e. with a different type of antigen on the red cells, their — monocytes and lymphocytes. immune system will mount an attack upon them and destroy the transfused cells. This is the basis of the trans- Granulocytes (polymorphonuclear fusion reaction; the two blood types, the donor and the recipient, are incompatible. leukocytes) There are many different collections of red cell surface During their formation, granulopoiesis, they follow a com- antigens, but the most important are the ABO and the mon line of development through myeloblast to myelocyte Rhesus systems. before differentiating into the three types (Figs 4.2 and  The blood Figure 4.5 The ABO system of blood grouping: antigens, antibodies and compatibility. 65 4.6). All granulocytes have multilobed nuclei in their cyto- Table 4.2 Numbers of different types of leukocyte in plasm. Their names represent the dyes they take up when adult blood stained in the laboratory. Eosinophils take up the red acid dye, eosin; basophils take up alkaline methylene blue; and Type of cell Number x 109/l Percentage of total neutrophils are purple because they take up both dyes. Granulocytes Neutrophils 2.5 to 7.5 40 to 75 Neutrophils Eosinophils 0.04 to 0.44 1 to 6 Their main function is to protect against any foreign mate- Basophils 0.015 to 0.1