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Blood - a versatile fluid
| What are the components of blood and what are their functions? Blood is the source of life and of components that are essential to life. Thousands of years ago, long before the development of medical science had begun, people already realised that blood is a vital fluid. Since loss of blood could be fatal, it is not entirely illogical to suppose that the administration of blood may have the reverse effect. In the epic of the Greek poet Homer, the hero Odysseus uses the blood of a ram to temporarily bring the dead back to life. This demonstrates that the life-giving qualities of blood were recognised as early as the ninth century BC. Traditionally regarded as the ‘breath of life’ for organs and tissues, blood had a magical and symbolic association. This manifested itself most poignantly in blood sacrifices to appease the gods. In traditional cultures, blood is still used for special therapies and is drunk to cure diseases.
Fluid of life Blood is a versatile fluid. It has many different functions and it is therefore not surprising that it was once called the ‘fluid of life’. The key function of blood is to absorb oxygen in the lungs and transport it to all parts of the body. However, blood has many more functions, including the transportation of nutrients, warmth, antibodies and hormones. Moreover, waste products from the body are removed via the blood.
The body of an adult human being contains five to six litres of blood, which is usually around 7.5% of the total body weight. For example, a person weighing 75 kg has approximately 5.6 litres of blood (one litre of blood weighs roughly one kilogram). Blood consists of cells and fluid (plasma). There are three types of blood cells, namely red blood cells (erythrocytes), white blood cells (leukocytes) and platelets (thrombocytes). Blood cells are produced in the red bone marrow, from where they are released into the blood stream. I the fluid part of the blood, called plasma, blood cells are transported throughout the body. Plasma contains a great variety of unique constituents. Top Red blood cells (erythrocytes) Seen through a microscope, red blood cells resemble red discs (with a diameter of about 0.007 mm). They give the blood its red colour. Erythrocytes have no cell nucleus and, accordingly, contain no genetic material (DNA). New, immature red blood cells do contain DNA, but they lose this when they become mature erythrocytes. A microlitre (0.001 millilitre) of blood from men and women contains on average 5.4 and 4.8 million erythrocytes, respectively. Consequently, the total blood volume of an adult man contains approximately 30 x 1012 (30 trillion) erythrocytes. Red blood cells generally have a life span of 120 days. The key function of the red blood cells is to absorb oxygen in the lungs and transport it to all the tissues of the body. To this end, red blood cells contain haemoglobin, which also gives blood its characteristic red colour. It is a complex of four proteins, each containing an iron atom, which binds oxygen. When the blood comes into contact with oxygen in the lungs, the iron atoms in the red blood cells bind oxygen molecules. The red blood cells then release the oxygen molecules in the organs and tissues. Here the oxygen is replaced by carbon dioxide, a waste product of cells in the organs and tissues. Red blood cells transport the carbon dioxide to the lungs, where it is removed from the body by exhalation. Frequency of blood groups of the AB0 blood group system and Rhesus D factor in the Western European population (Source: Overbeeke, M.A.M., Engelfriet, C.P.: Bloedgroepenonderzoek, Theorie en praktijk (Blood Group Research: Theory and Practice). Second, revised edition. Bohn Stafleu Van Loghum, 1994).
Membrane structures are found on the cell wall of red blood cells, particularly glycoproteins (proteins combined with sugars). These structures may differ from individual to individual and they are said to carry blood groups. More than 250 blood groups have been identified. The best known blood groups are A, B, AB and 0 of the AB0 blood group system and the Rhesus D factor of the Rhesus blood group system. The table above shows the frequency of these blood groups in the Western European population. Subjects with blood group A have naturally occurring antibodies against group B, while subjects with blood group B have naturally occurring antibodies against blood group A. People with blood group AB do not have antibodies to A or B, while someone with blood group 0 has antibodies against both A and B. If patients receive red blood cells against which they have antibodies, it may have serious consequences. For this reason, it is essential to determine the AB0 blood group of the recipient and the donor (see illustration below) prior to a blood transfusion.
In addition to the AB0 blood groups, the Rhesus D factor is of great importance since antibodies against the Rhesus D factor, too, may cause serious transfusion reactions. Most Western Europeans (84%) have this blood group (‘Rhesus D positive’). Unlike blood group A, B or 0 individuals, Rhesus D negative individuals do not have naturally occurring antibodies against the Rhesus D factor. Rhesus D negative individuals nearly always develop antibodies against the Rhesus D factor if they come into contact with this blood group. This occurs, for example, as a result of a transfusion with Rhesus D-positive blood or because the blood of a Rhesus D-positive foetus enters the blood of a Rhesus D-negative mother during pregnancy or childbirth. Therefore Rhesus-D-negative patients are only transfused with Rhesus D-negative red blood cells. As well as Rhesus D, there are many other blood groups on red blood cells to which patients who receive a blood transfusion or pregnant women can develop antibodies, if they do not have these blood groups themselves. If such patients receive a transfusion of red blood cells, which carry the corresponding blood group, a more or less severe transfusion reaction may develop. In order to prevent this, a compatibility test between the serum of the patient and the red blood cells of the donor is required before a transfusion is given. The test is performed to detect antibodies against these other blood groups. If antibodies are present, additional tests will determine against which blood group(s) the antibodies are directed. The patient will then receive red blood cells that do not carry these blood groups. Thus a transfusion reaction will be prevented. Top White blood cells (leukocytes) White blood cells are colourless and larger than red blood cells. Because leukocytes have a cell nucleus, they contain genetic material. One microlitre of blood contains approximately 5,000 to 10,000 leukocytes. The term leukocyte actually is a collective term for a wide range of cells, which can be divided into three classes:
Lymphocytes are produced by bone marrow cells, but mature in other areas of the body, including the lymph nodes, the thymus and the spleen. There are two different functional types: B-lymphocytes (B-cells) and T-lymphocytes (T-cells). B-lymphocytes produce antibodies, while T-lymphocytes are responsible for cell-mediated immunity and they play a regulatory role in the immune system. Lymphocytes have a diameter of 0.006 – 0.009 mm. Every microlitre of blood contains roughly 200 – 800 lymphocytes. Monocytes play an important role in the clearance of cells, micro-organisms and foreign particles that have been coated with antibodies. They remove these intruders by phagocytosis, a process whereby microbes and other foreign particles are ingested and subsequently destroyed. Monocytes have a diameter of 0.016 – 0.020 mm. There are about 1,500 – 4,000 monocytes in every microlitre of blood. Granulocytes are leukocytes whose cytoplasm contains granules. They can be classified as neutrophils, eosinophils and basophils, depending on the staining characteristics of the granules and the shape of their nuclei. The majority of granulocytes are neutrophils. Granulocytes are the first line of (active) defence against invading micro-organisms. Their motility allows them to move from the capillary circulation into the tissues, where they ingest microbes and other foreign particles by phagocytosis. Granulocytes have a diameter of 0.012 – 0.014 mm. Every microlitre of blood contains approximately 2,000 – 7,500 granulocytes. Top Platelets (thrombocytes) Platelets are separated parts of very large cells (megakaryocytes). The thrombocytes are the smallest cells in the blood (much smaller than red and white blood cells) and, like erythrocytes, have no cell nucleus. One microlitre of blood contains 150,000 to 400,000 platelets, which in healthy people have a life span – the period during which they survive in the circulation – of seven to ten days. Platelets play a vital role in blood coagulation (hemostasis). They have a complex structure and contain all kinds of substances. Following a spontaneous or traumatic break in the endothelial lining of a blood vessel, platelets attach themselves to collagen, a protein in the vessel wall that is exposed to the blood in a wound. As a result of this attachment, platelets release a number of substances that cause adjacent platelets to bind to adhering platelets and to each other, a process called platelet aggregation. This clump of platelets (the thrombocyte plug) is stabilised by fribrin strands generated by the sequential activation of protein clotting factors in the plasma. This finally results in the formation of a fibrin mesh around the platelet plug. This process takes 5 to 20 minutes. Top Plasma
In terms of quantity, albumin is the most important plasma protein. It helps to maintain the oncotic pressure in the blood vessels and has various transport functions. Albumin is produced by the liver and helps to maintain a constant plasma volume. The high concentration of albumin in plasma exerts a significant oncotic pressure that keeps the water and solutes within the vessels. The oncotic pressure counterbalances intravascular hydrostatic pressure that tends to force fluids out of the vessels. Low levels of albumin cause excessive leakage of fluid into the tissue, a phenomenon known as oedema. Albumin also acts as a carrier for various enzymes, hormones and metabolites. (see the Albumin section on the page about plasma fractionation). Another significant group of proteins in the plasma are the immunoglobulins (antibodies). These proteins are produced by B-lymphocytes and protect the body against infectious diseases by attaching themselves to micro-organisms that have entered the body, thus facilitating their removal by macrophages. Consequently, the immune system is able to destroy the intruders. Immunoglobulins are found in plasma, in secretions such as saliva or respiratory mucus and on the surface of certain cells. Also involved in this defence system are the so-called complement factors in the plasma. If foreign organisms enter the body, the complement system may be activated, which leads to the formation of biologically active protein complexes, which attach themselves to bacteria and certain other cells, stimulating the lysis of these intruders. In addition, plasma also contains clotting factors. There are several clotting factors, including the factors VIII and IX. Together with the platelets, these proteins play a key role in blood coagulation, limiting the loss of blood when blood vessels are damaged. The clotting factors circulate in a non-active form, and following activation they sequentially catalyse each other in a cascade of reactions. All clotting factors are present in trace amounts except fibrinogen, which is found in high concentrations. If, in response to tissue damage, platelets and clotting factors are activated, a network of fibrin is eventually created, which replaces the thrombocyte plug. A fourth group of proteins in blood plasma are the protease inhibitors. These proteins ensure that natural reactions do not continue uninhibited. They play a particularly significant role in inhibiting blood coagulation and complement reactions. Top |