Blood Lecture Notes: Functions, Components, and Erythrocytes PDF

BLOOD BY DR WAHID SAKA  BLOOD It is a fluid connective tissue which circulates in vascular channels to all tissues of the body. It is composed of the cellular elements which are wbc, rbc & platelets all suspended in the fluid portion called the plasma. Total volume of circula...

BLOOD BY DR WAHID SAKA  BLOOD It is a fluid connective tissue which circulates in vascular channels to all tissues of the body. It is composed of the cellular elements which are wbc, rbc & platelets all suspended in the fluid portion called the plasma. Total volume of circulating blood in a man is about 5.6litters in male and 4.6-6 Litters in female. About 7-8% by weight in a 70kg man. Plasma volume is about 55% accounting for 3L. The formed element accounts for between 43-45% of the total blood volume.  CHARACTERISTICS OF BLOOD 1. Bright red (oxygenated). 2. Dark red/purplish (deoxygenated). 3. Much more dense than pure water. 4. pH range from 7.35 to 7.45 (slightly alkaline). 5. Slightly warmer than body temperature 100.4 o F. 6. Typical volume in adult male 5-6 L. 7. Typical volume in adult female 4-5L. 8. About 8% of body weight.  PRODUCTION OF BLOOD CELLS In fetus: Blood cells are formed in yolk sac and later in the liver and spleen. In infants: Blood cells are produced in bone marrow of all bones. Adults: Blood cells are formed in bone marrow of the long bones i.e. upper humerus and femur. The active marrow responsible for blood cells production is known as red marrow while the inactive one is called yellow marrow.  FUNCTIONS OF BLOOD TRANSPORT FUNCTION Transport of various substance suspended in the blood such as 1. Amino acids 2. Lipids 3. Carbohydrate (glucose) 4. Minerals 5. Vitamins 6. Hormones 7. Oxygen 8. CO2 9. Heat-maintenance of body temperature by distributing heat 10. Excess body water 11. Immunoglobulin 12. Blood clotting factors and Platelets 13. Waste product of excretory organs  HOMEOSTATIC FUNCTIONS  Blood is involved in regulation of the following; Interstitial fluid compartment Body pH- maintenance of acid-base balance with the help of buffers Body temperature Protection against infection Blood clotting  PROTECTION. Blood can clot (become gel-like), which protects against its excessive loss from the cardiovascular system after an injury. White blood cells protect against disease by carrying on phagocytosis. Several types of blood proteins, including antibodies, interferons, and complement help protect against disease in a variety of ways.  CELLULAR COMPONENTS OF THE BLOOD All cellular components of the blood stem from same multipotent uncommitted stem cell. The population of these committed stem cells in the marrow showed that 75% are white blood cells and 25% are red blood cells.  HAEMOPOIESIS  In order to form blood cells, pluripotent stem cells in red bone marrow produce two further types of stem cells, which have the capacity to develop into several types of cells.  These stem cells are called myeloid stem cells and lymphoid stem cells.  Myeloid stem cells begin their development in red bone marrow and give rise to red blood cells, platelets, monocytes, neutrophils, eosinophils, basophils, and mast cells.  Lymphoid stem cells, which give rise to lymphocytes, begin their development in red bone marrow but complete it in lymphatic tissues Lymphoid stem cells also give rise to natural killer (NK) cells.  Although the various stem cells have distinctive cell identity markers in their plasma membranes, they cannot be distinguished histologically and resemble lymphocytes.  COMPONENTS OF BLOOD  Whole blood has two components:  blood plasma, a watery liquid extracellular matrix that contains dissolved substances, and  formed elements, which are cells and cell fragments.  Blood is about 45% formed elements and 55% blood plasma.  Normally, more than 99% of the formed elements are cells named for their red color—red blood cells (RBCs).  Pale, colorless white blood cells (WBCs) and platelets occupy less than 1% of the formed elements. Because they are less dense than red blood cells but more dense than blood plasma, they form a very thin buffy coat layer between the packed RBCs and plasma in centrifuged blood.  ERYTHROCYTE Erythrocytes are also known as red blood cells. Mature red blood cell is non nucleated & has a biconcave shape. It has a diameter of 8.5micrometer, edge thickness of 2.5 & center thickness of 1.5 micrometer. There are about 5 million/mm3of blood with variation between the two sexes. Male 4.5-6 & female 4.3-4.5 million cells/mm3of blood.  The variation is due to the following; 1. Gonadal hormone testosterone in male which is a red blood cell differentiation inducer. 2. Greater mass musculature in male requires more oxygen supply. The shape of the red blood cell provides maximum surface area for the small volume of the cell and large diffusion surface for passage of gases. It also allows for squeezing of red blood cell when passing through narrowed vessels without damage to the integrity of the cell. Glucose-6-phosphate dehydrogenase is present on the membrane helping to utilize glucoce, O2and ATP. Life span is 120 days. The count is higher in new born (6mmillion/mm3 ) than in an adult. Its production is stimulated by hypoxia with the release of erythropoietin (EPO) 85% from kidney % 15% from liver.  ERYTHROPOIETIN (EPO) A glycoprotein Contains 165 amino acid residues & 4 oligosaccharide chains. It is a differentiation inducer Half life is about 5 hours. Production stimulated by hypoxia, but can also be stimulated by cobalt salt and androgens. 85% from kidney, 15% from liver Principal site of inactivation is liver. Chonic renal disease will adversely affect its production. The gene is cloned and recombinant EPO is available for clinical use.  PACKED CELL VOLUME (PCV) If one takes a sample of blood, treats is with an agent to prevent clotting [anticoagulant], and spins it in a centrifuge, the red cells settle to the bottom the white cells settle on top of them forming the “Buffy coat”. The fraction occupied by the red cells is called the haematocrit. Normally it is approximately 45%. (47% in male, 42% in female). Values much lower than this are sign of anemia.  ERYTHROCYTE SEDIMENTATION RATE (ESR) This is the rate of settlement of red blood cell without being centrifuged. It depends on Shape of the cells Concentration of plasma proteins Infection The stacking of red blood cell on one another is termed Rouleux formation.  USES Monitoring recovery from diseases or efficiency of treatment. Varies with ages and sex. Normal values are New born = 2mm/hr Adult male = 3-7mm/hr (5.7) Adult female = 3-15mm/hr (9.5) REQUIREMENTS FOR RBC PRODUCTION A. Erythropoietin:- A glycoprotein Molecular weight of 35kdalton 165 amino acid residue & 4 oligosaccharide chain Produced primarily from kidney (85%) & liver (15%) & other areas such as astrocytes in brain Produced in response to hypoxia as a result of production of REE (renal erythropoietic factor) by the kidney Erythropoietin stimulates red blood cell production from bone within 2 days. B. Iron Absorb from first part of the small intestine by active transport 3× rapidly if in ferrous state (Fe2+), than in the ferric state (Fe3+) Form the core element of heme porphyrin structure. It is able to combine reversibly with oxygen. The amount needed daily = 0.5mg in male and 2mg in menstruating female. Iron is released when red blood cell are broken down, transported by transferrin (β-globulin) to the liver. The liver stores about 60% of the body iron as ferritin. Iron is distributed in the body as follows; Hemoglobin (65%), Myoglobin (4%), Ferritin (15 -30%), Trasferrin (0.1%) C. Vitamin B12 Important for all cell functions and tissue growth and also for conversion of ribose nucleotide to deoxyribonucleotide which is an important component of DNA. Absorption occur at the terminal ileum and this is enhanced by intrinsic factor from parietal cell. Amount needed to maintain normal red blood cell production is about 1ng. Deficiency leads to pernicious anaemia (failure of nuclear maturation and division). Other materials needed 1. Folic acid 2. Lipids 3. Protein 4. Amino acid  HEMOGLOBIN Is the red oxygen carrying pigment in the red blood cell in vertebrates It’s a globular molecule. A protein with molecular weight of 64,450. Has 2 parts, heme portion and globin part Heme part is attached to 4 polypeptide chain which constitute the globin portion of the hemoglobin. The heme has an iron central dormain. A fully saturated hemoglobin can carry 4 molecules of Oxygen. Hemoglobin concentration is about 14g/dl in female and 16g/dl in male  JAUNDICE A clinical condition seen in patients with yellowish discoloration of the sclera of the eyes and other soft tissues of the body It usually occurs when more than 300 - 500 mls of blood is hemolysed [destroyed] in less than a day CAUSES OF JAUNDICE Hemolysis. Infection or toxic effect on liver cells. Obstruction of the bile duct.  WHITE BLOOD CELLS The white blood cells are also known as leucocytes. There are about 4-11 thousand white blood cells/mm of blood. 3 The amount varies with the health state of the subject with increased concentration during infection. They are broadly divide into; 1.GRANULOCYTE :- contains cytoplasmic granules that pick up stains. It includes;  Eosinophils Staining colour-Bright red No of lobes of nucleus. 1-2 Concentration- 150-300 cells/mm blood 3 % of wbc- 1- 4% Half life-12-20 hours  Basophil Staining colour – Blue No of lobes of nucleus – No definite lobe Concentration – 0-100 cell/mm of blood 3 % white blood cell- 1-4% Half life – 12-20 hour  Neutrophil Staining – Neutral No of lobes- 3-5 Concentration - 3000 -6000 cells/mm of 3 blood. Half life – 6 hours. 2. AGRANULOCYTES  Monocytes Has horse shoe shaped nucleus occupying 2/3 of the cytoplasm Concentration is 300-600 cells/mm of 3 blood % white blood cell – 2-8% Half life – 72 hours to months  When these cells are released into the blood from bone marrow, they are still very immature cells. They migrate into the tissues, enlarge up to five times and develop numerous cytoplasmic granules (lysosomes). These cells are called macrophages and they are much more powerful phagocytes than neutrophils. Macrophages have a powerful lysosomal lipase which breaks down the lipid-rich cell memebranes of many bacteria.  Usually, monocytes circulates for about 72 hours in the blood after which they enter the tissue and are transformed into tissue macrophages where they can survive for months. Example of tissue macrophages; 1. Kupffer cell of the liver 2. Pulmonary alveolar macrophages 3. Osteoclast in bones 4. Microglia cells in the brain and nervous system 5. Microphages of the lymph nodes 6. Macrophages of the spleen They are activated by lymphokines from T- lymphocytes and are called histocytes or wandering cells.  LYMPHOCYTES Lymphocytes are produced from lymph nodes, thymus and spleen. The precursors, all came from the bone marrow. Approximately one-fourth of the blood’s circulating leucocytes are lymphocytes. They are actively motile cells second only to the neutrophils in this respect. They are also capable of phagocytosis, but rarely display it. They are of two main types: B- lymphocytes, which are responsible for humoral immunity i.e. they synthesize circulating antibodies. T-lymphocytes, which are processed by or in some way dependent on the thymus gland. They are responsible for cell- mediated immunity i.e. the production of lymphocytes which are sensitized against specific antigens.  They have single large nucleus occupying almost the whole cytoplasm concentration. Concentration- 1500- 4000 cells/mm3of blood. Concentration decreased by glucocorticoids from zonal fasculata of adrenal cortex. % white blood cells: 20-40% Half life – 200 days. Lymphocytes are important component of body immune system Reticulum cell in lymph node can change to lymphoblast which form lymphocyte, plasma blast which form plasma cell (immunoglobulins)  Examples of lymph nodes are: i. Cervical duct lymph node ii. thoracic duct lymph node iii.The axillary duct lymph node (armpit) iv.The inguinal duct lymph node (thigh) White blood cells and all other nucleated cells in the body have proteins, called major histocompatibility (MHC) antigens, protruding from their plasma membrane into the extracellular fluid. These “cell identity markers” are unique for each person (except identical twins). Although RBCs possess blood group antigens, they lack the MHC antigen.  Functions of White Blood Cells In a healthy body, some WBCs, especially lymphocytes, can live for several months or years, but most live only a few days. During a period of infection, phagocytic WBCs may live only a few hours. WBCs are far less numerous than red blood cells; at about 4000–11,000 cells per microliter of blood, they are outnumbered by RBCs by about 700:1. Leukocytosis, an increase in the number of WBCs above 11,000/L, is a normal, protective response to stresses such as invading microbes, strenuous exercise, anesthesia, and surgery. An abnormally low level of white blood cells (below 4000/L) is termed leukopenia. It is never beneficial and may be caused by radiation, shock, and certain chemotherapeutic agents.  RBCs are contained within the bloodstream, but WBCs leave the bloodstream by a process termed emigration, also called diapedesis, in which they roll along the endothelium, stick to it, and then squeeze between endothelial cells. The precise signals that stimulate emigration through a particular blood vessel vary for the different types of WBCs. Molecules known as adhesion molecules help WBCs stick to the endothelium. For example, endothelial cells display adhesion molecules called selectins in response to nearby injury and inflammation. Selectins stick to carbohydrates on the surface of neutrophils, causing them to slow down and roll along the endothelial surface.  On the neutrophil surface are other adhesion molecules called integrins, which tether neutrophils to the endothelium and assist their movement through the blood vessel wall and into the interstitial fluid of the injured tissue. Neutrophils and macrophages are active in phagocytosis; they can ingest bacteria and dispose of dead matter. Several different chemicals released by microbes and inflamed tissues attract phagocytes, a phenomenon called chemotaxis. The substances that provide stimuli for chemotaxis include toxins produced by microbes; kinins, which are specialized products of damaged tissues; and some of the colony- stimulating factors (CSFs).  The CSFs also enhance the phagocytic activity of neutrophils and macrophages. Among WBCs, neutrophils respond most quickly to tissue destruction by bacteria. After engulfing a pathogen during phagocytosis, a neutrophil unleashes several chemicals to destroy the pathogen. These chemicals include the enzyme lysozyme which destroys certain bacteria, and strong oxidants, such as the superoxide anion (O2), hydrogen peroxide (H2 - O2 ), and the hypochlorite anion (OCl ), which is similar to - household bleach. Neutrophils also contain defensins, proteins that exhibit a broad range of antibiotic activity against bacteria and fungi.  Defensins form peptide “spears” that poke holes in microbe membranes; the resulting loss of cellular contents kills the invader. Eosinophils leave the capillaries and enter tissue fluid. They are believed to release enzymes, such as histaminase, that combat the effects of histamine and other substances involved in inflammation during allergic reactions. Eosinophils also phagocytize antigen– antibody complexes and are effective against certain parasitic worms. A high eosinophil count often indicates an allergic condition or a parasitic infection.  At sites of inflammation, basophils leave capillaries, enter tissues and release granules that contain heparin, histamine, and serotonin. These substances intensify the inflammatory reaction and are involved in hypersensitivity (allergic) reactions. Basophils are similar in function to mast cells, connective tissue cells that originate from pluripotent stem cells in red bone marrow. Like basophils, mast cells release substances involved in inflammation, including heparin, histamine, and proteases. Mast cells are widely dispersed in the body, particularly in connective tissues of the skin and mucous membranes of the respiratory and gastrointestinal tracts.  Lymphocytes are the major soldiers in lymphatic system battles. Most lymphocytes continually move among lymphoid tissues, lymph, and blood, spending only a few hours at a time in blood. Thus, only a small proportion of the total lymphocytes are present in the blood at any given time. Three main types of lymphocytes are B cells, T cells, and natural killer (NK) cells. B cells are particularly effective in destroying bacteria and inactivating their toxins. T cells attack viruses, fungi, transplanted cells, cancer cells, and some bacteria, and are responsible for transfusion reactions, allergies, and the rejection of transplanted organs.  Immune responses carried out by both B cells and T cells help combat infection and provide protection against some diseases. Natural killer cells attack a wide variety of infectious microbes and certain spontaneously arising tumor cells. Monocytes take longer to reach a site of infection than neutrophils, but they arrive in larger numbers and destroy more microbes. On their arrival, monocytes enlarge and differentiate into wandering macrophages, which clean up cellular debris and microbes by phagocytosis after an infection.  In conclusion, an increase in the number of circulating WBCs usually indicates inflammation or infection. A physician may order a differential white blood cell count or a count of each of the five types of white blood cells, to detect infection or inflammation, determine the effects of possible poisoning by chemicals or drugs, monitor blood disorders (for example, leukemia) and the effects of chemotherapy, or detect allergic reactions and parasitic infections. Because each type of white blood cell plays a different role, determining the percentage of each type in the blood assists in diagnosing the condition.  PLATELETS Besides the immature cell types that develop into erythrocytes and leukocytes, hemopoietic stem cells also differentiate into cells that produce platelets. Under the influence of the hormone thrombopoietin, myeloid stem cells develop into megakaryocyte colony-forming cells that in turn develop into precursor cells called megakaryoblasts. Megakaryoblasts transform into megakaryocytes, huge cells that splinter into 2000 to 3000 fragments  Each fragment, enclosed by a piece of the plasma membrane, is a platelet. Platelets break off from the megakaryocytes in red bone marrow and then enter the blood circulation. Between 150,000 and 400,000 platelets are present in each microliter of blood. Each is irregularly disc-shaped, 2–4 m in diameter, and has many vesicles but no nucleus. Their granules contain chemicals that, once released, promote blood clotting. Platelets help stop blood loss from damaged blood vessels by forming a platelet plug. Platelets have a short life span, normally just 5 to 9 days. Aged and dead platelets are removed by fixed macrophages in the spleen and liver.  BLOOD GROUPS Blood groups existence is based on the two types of agglutinogens (antigens) on the surface of red blood cells. These agglutinogens are A and B and they are responsible for the four types of blood groups i.e., A, B, AB and O. Blood plasma usually contains antibodies called agglutinins that react with the A or B antigens if the two are mixed. These are the anti-A antibody, which reacts with antigen A, and the anti-B antibody, which reacts with antigen B.  (red cell) Blood group (plasma) Agglutinogen Agglutinin A A β B B α AB AB Nil O Nil α and β  You do not have antibodies that react with the antigens of your own RBCs, but you do have antibodies for any antigens that your RBCs lack. For example, if your blood type is B, you have B antigens on your red blood cells, and you have anti-A antibodies in your blood plasma. The agglutinogens are mucopolysaccarides, which are not only present on the red blood cells, but also, in body secretions, such as, saliva, gastric juice and in tissues of liver, kidney and lungs.  There are at least 24 blood groups and more than 100 antigens that can be detected on the surface of red blood cells but they do not induce any transfusion reactions. Other blood groups include the Lewis, Kell, Kidd, and Duffy systems. For the purpose of blood transfusion, A, B, O and AB groups cross matching is done with donor’s red cell and recipient’s plasma. The red cell surface in 85% of the individuals, show another type of the agglutinogen called Rh agglutinogen.  This is present in addition to the A ,B agglutinogens and such individuals are called Rh.+ The blood group will be called Rh , if the Rh - agglutinogen is absent on the red cell membrane. Normally, blood plasma does not contain anti-Rh antibodies. If an Rh person receives an Rh blood transfusion, - + however, the immune system starts to make anti- Rh antibodies that will remain in the blood. If a second transfusion of Rh blood is given later, + the previously formed anti-Rh antibodies will cause agglutination and hemolysis of the RBCs in the donated blood, and a severe reaction may occur.  Transfusions A transfusion is the transfer of whole blood or blood components (red blood cells only or blood plasma only) into the bloodstream or directly into the red bone marrow. A transfusion is most often given to alleviate anemia, to increase blood volume (for example, after a severe hemorrhage), or to improve immunity. In an incompatible blood transfusion, antibodies in the recipient’s plasma bind to the antigens on the donated RBCs, which causes agglutination or clumping, of the RBCs. Agglutination is an antigen–antibody response in which RBCs become cross-linked to one another. (Note that agglutination is not the same as blood clotting.)  When these antigen–antibody complexes form, they activate plasma proteins of the complement family. In essence, complement molecules make the plasma membrane of the donated RBCs leaky, causing haemolysis or rupture of the RBCs and the release of hemoglobin into the blood plasma. The liberated hemoglobin may cause kidney damage by clogging the filtration membranes.. Consider what happens if a person with type A blood receives a transfusion of type B blood.  People with type AB blood do not have anti-A or anti-B antibodies in their blood plasma are called universal recipients because theoretically they can receive blood from donors of all four blood types. They have no antibodies to attack antigens on donated RBCs. People with type O blood have neither A nor B antigens on their RBCs and are sometimes called universal donors because theoretically they can donate blood to all four ABO blood types. Type O persons requiring blood may receive only type O blood. In practice, use of the terms universal recipient and universal donor is misleading and dangerous. Blood contains antigens and antibodies other than those associated with the ABO system that can cause transfusion problems.  HEMOLYTIC DISEASE OF THE NEWBORN (HDN) The most common problem with Rh incompatibility, hemolytic disease of the newborn (HDN), may arise during pregnancy. Normally, no direct contact occurs between maternal and fetal blood while a woman is pregnant. However, if a small amount of Rh blood leaks from the fetus through the placenta into the bloodstream of an Rh mother, the mother will start to make anti-Rh antibodies. Because the greatest possibility of fetal blood leakage into the maternal circulation occurs at delivery, the firstborn baby usually is not affected.  If the mother becomes pregnant again, however, her anti-Rh antibodies can cross the placenta and enter the bloodstream of the fetus. If the fetus is Rh , there is no problem, because Rh - - blood does not have the Rh antigen. If the fetus is Rh ,+ however, agglutination and hemolysis brought on by fetal–maternal incompatibility may occur in the fetal blood. An injection of anti-Rh antibodies called anti-Rh gamma globulin (RhoGAM®) can be given to prevent HDN. Rh women should receive RhoGAM® before delivery, and soon after every delivery, miscarriage, or abortion. These antibodies bind to and inactivate the fetal Rh antigens before the mother’s immune system can respond to the foreign antigens by producing her own anti-Rh antibodies.  Haemostasis  Haemostasis , not to be confused with the very similar term homeostasis, is a sequence of responses that stops bleeding. When blood vessels are damaged or ruptured, the haemostatic response must be quick, localized to the region of damage, and carefully controlled in order to be effective.  Three mechanisms reduce blood loss: (1) vascular spasm, (2) platelet plug formation, and (3) blood clotting (coagulation). When successful, hemostasis prevents hemorrhage, the loss of a large amount of blood from the vessels. Hemostatic mechanisms can prevent hemorrhage from smaller blood vessels, but extensive hemorrhage from larger vessels usually requires medical intervention.  Vascular Spasm When arteries or arterioles are damaged, the circularly arranged smooth muscle in their walls contracts immediately, a reaction called vascular spasm. This reduces blood loss for several minutes to several hours, during which time the other haemostatic mechanisms go into operation. The spasm is probably caused by damage to the smooth muscle, by substances released from activated platelets, and by reflexes initiated by pain receptors.  Platelet Plug Formation Considering their small size, platelets store an impressive array of chemicals. Within many vesicles are clotting factors, ADP, ATP, Ca , and 2+ serotonin. Also present are enzymes that produce thromboxane A , a prostaglandin; fibrin-stabilizing 2 factor, which helps to strengthen a blood clot; lysosomes; some mitochondria; membrane systems that take up and store calcium and provide channels for release of the contents of granules; and glycogen. Also within platelets is platelet-derived growth factor (PDGF), a hormone that can cause proliferation of vascular endothelial cells, vascular smooth muscle fibers, and fibroblasts to help repair damaged blood vessel walls.  Platelet plug formation occurs as follows: 1. Initially, platelets contact and stick to parts of a damaged blood vessel, such as collagen fibers of the connective tissue underlying the damaged endothelial cells. This process is called platelet adhesion. 2. Due to adhesion, the platelets become activated, and their characteristics change dramatically. They extend many projections that enable them to contact and interact with one another, and they begin to liberate the contents of their vesicles. This phase is called the platelet release reaction. Liberated ADP and thromboxane A2play a major role by activating nearby platelets. Serotonin and thromboxane A2 function as vasoconstrictors, causing and sustaining contraction of vascular smooth muscle, which decreases blood flow through the injured vessel.  A platelet plug is very effective in preventing blood loss in a small vessel. Although initially the platelet plug is loose, it becomes quite tight when reinforced by fibrin threads formed during clotting. A platelet plug can stop blood loss completely if the hole in a blood vessel is not too large.  Blood Clotting Normally, blood remains in its liquid form as long as it stays within its vessels. If it is drawn from the body, however, it thickens and forms a gel. Eventually, the gel separates from the liquid. The straw-colored liquid, called serum, is simply blood plasma minus the clotting proteins.  The gel is called a blood clot. It consists of a network of insoluble protein fibers called fibrin in which the formed elements of blood are trapped. The process of gel formation, called clotting or coagulation, is a series of chemical reactions that culminate in formation of fibrin threads. If blood clots too easily, the result can be thrombosis, a condition of clotting in an undamaged blood vessel. If the blood takes too long to clot, hemorrhage can occur.  Clotting is a complex cascade of enzymatic reactions in which each clotting factor activates many molecules of the next one in a fixed sequence. Finally, a large quantity of product (the insoluble protein fibrin) is formed. Clotting can be divided into three stages 1. Two pathways, called the extrinsic pathway and the intrinsic pathway which will be described shortly, lead to the formation of prothrombinase. Once prothrombinase is formed, the steps involved in the next two stages of clotting are the same for both the extrinsic and intrinsic pathways, and together these two stages are referred to as the common pathway. 2. Prothrombinase converts prothrombin (a plasma protein formed by the liver) into the enzyme thrombin. 3. Thrombin converts soluble fibrinogen (another plasma protein formed by the liver) into insoluble fibrin. Fibrin forms the threads of the clot.  The Extrinsic Pathway The extrinsic pathway of blood clotting has fewer steps than the intrinsic pathway and occurs rapidly—within a matter of seconds if trauma is severe. It is so named because a tissue protein called tissue factor (TF), also known as thromboplastin, leaks into the blood from cells outside (extrinsic to) blood vessels and initiates the formation of prothrombinase. TF is a complex mixture of lipoproteins and phospholipids released from the surfaces of damaged cells. In the presence of Ca , TF begins a sequence of 2 + reactions that ultimately activates clotting factor X. Once factor X is activated, it combines with factor V in the presence of Ca to form the active enzyme 2 + prothrombinase, completing the extrinsic pathway.  The Intrinsic Pathway The intrinsic pathway of blood clotting is more complex than the extrinsic pathway, and it occurs more slowly, usually requiring several minutes. The intrinsic pathway is so named because its activators are either in direct contact with blood or contained within (intrinsic to) the blood; outside tissue damage is not needed. If endothelial cells become roughened or damaged, blood can come in contact with collagen fibers in the connective tissue around the endothelium of the blood vessel. In addition, trauma to endothelial cells causes damage to platelets, resulting in the release of phospholipids by the platelets.  Contact with collagen fibers (or with the glass sides of a blood collection tube) activates clotting factor XII, which begins a sequence of reactions that eventually activates clotting factor X. Platelet phospholipids and Ca2+ can also participate in the activation of factor X. Once factor X is activated, it combines with factor V to form the active enzyme prothrombinase (just as occurs in the extrinsic pathway), completing the intrinsic pathway.  The Common Pathway The formation of prothrombinase marks the beginning of the common pathway. In the second stage of blood clotting prothrombinase and Ca 2+ catalyze the conversion of prothrombin to thrombin. In the third stage, thrombin, in the presence of Ca , 2+ converts fibrinogen, which is soluble, to loose fibrin threads, which are insoluble. Thrombin also activates factor XIII (fibrin stabilizing factor), which strengthens and stabilizes the fibrin threads into a sturdy clot. Plasma contains some factor XIII, which is also released by platelets trapped in the clot. Thrombin has two positive feedback effects. In the first positive feedback loop, which involves factor V, it accelerates the formation of prothrombinase. Prothrombinase in turn accelerates the production of more thrombin, and so on.  In the second positive feedback loop, thrombin activates platelets, which reinforces their aggregation and the release of platelet phospholipids.  Clot Retraction Once a clot is formed, it plugs the ruptured area of the blood vessel and thus stops blood loss. Clot retraction is the consolidation or tightening of the fibrin clot. The fibrin threads attached to the damaged surfaces of the blood vessel gradually contract as platelets pull on them. As the clot retracts, it pulls the edges of the damaged vessel closer together, decreasing the risk of further damage.  During retraction, some serum can escape between the fibrin threads, but the formed elements in blood cannot. Normal retraction depends on an adequate number of platelets in the clot, which release factor XIII and other factors, thereby strengthening and stabilizing the clot. Permanent repair of the blood vessel can then take place. In time, fibroblasts form connective tissue in the ruptured area, and new endothelial cells repair the vessel lining.  Coagulation Blood coagulation pathways in vivo showing the central role played by thrombin Clotting (Coagulation) Factors No NAME(S) SOURCE ACTIVATION I Fibrinogen. Liver. Common. II Prothrombin. Liver. Common. III Tissue factor (thromboplastin). Damaged tissues and activated platelets. IV Calcium ions (Ca 2+ ). Diet, bones, and platelets. All. V Proaccelerin, labile factor, Liver and platelets. or accelerator globulin (AcG). VII Serum prothrombin conversion Liver. Extrinsic. accelerator (SPCA), stable factor, or proconvertin. VIII Antihemophilic factor (AHF), or Liver. Intrinsic. antihemophilic globulin (AHG). IX Christmas factor, plasma thromboplastin Liver. Intrinsic. component (PTC), or antihemophilic factor B. X Stuart factor, Prower factor, Liver. Extrinsic and intrinsic. or thrombokinase. XI Plasma thromboplastin antecedent Liver. Intrinsic. (PTA) or antihemophilic factor C. XII Hageman factor, glass factor, Liver. Intrinsic. contact factor, or antihemophilic factor D. XIII Fibrin-stabilizing factor (FSF). Liver and platelets. Common. *There is no factor VI. Prothrombinase (prothrombin activator) is a combination of activated factors V and X.  Role of Vitamin K in Clotting Normal clotting depends on adequate levels of vitamin K in the body. Although vitamin K is not involved in actual clot formation, it is required for the synthesis of four clotting factors. Normally produced by bacteria that inhabit the large intestine, vitamin K is a fat soluble vitamin that can be absorbed through the lining of the intestine and into the blood if absorption of lipids is normal.  People suffering from disorders that slow absorption of lipids (for example, inadequate release of bile into the small intestine) often experience uncontrolled bleeding as a consequence of vitamin K deficiency. PLASMA Plasma is the fluid portion of the blood. It contains the following;. COMPONENT PERCENTAGE (%) Water 92 Proteins 6-8 Salts 0.8 Lipids 0.6 Glucose (blood glucose) 0.1 Plasma protein concentration varies with health status of individuals with a decrease during starvation, liver damage and renal damage  FUNCTIONS OF PLASMA PROTEIN 1. Contribute to the viscocity of the plasma 2. Responsible for osmotic return of filtered fluid from interstitial fluid compartment. 3. Create suspension stability in blood aiding maintenance of dispersion of materials. 4. Reserve of amino acid for the body. 5. transport CO in blood 2 6. Transport of Hormones, urea, lipids, and glucose.  EXAMPLE OF PLASMA PROTEINS  ALBUMIN Most abundant (55-64%) 4-5g/100mls of blood (3.5-5g/dl) The smallest size with molecular weight of 69000-70,000 Total exchangeable albumin is 4-5g/kg body weight 38.45% intravascular 10% of exchangeable pool degraded daily Replacement comes from hepatic cells producing 200 to 400mg/kg/day Synthesis decreases in fasting and increasse in nephrosis because of loss in urine  GLOBULIN Form about 20% of total plasma protein Amount in circulating blood is about 2 -3g/100ml of blood Occurs in various form - α- globulin (150,000-160,000).Transport retinol and Thyroxine -Y-globulin (150,000 – 900,000). E.g. iron transporting protein called trasferrin. -β- globulin (90,000) e.g. antibodies Albumin-globulin ratio = 2:1  FIBRINOGEN Only soluble plasma protein Molecular weight = 350,000 Amount in circulation = 0.15 – 0.3g/100ml Converted to fibrin as blood coagulates  IMMUNITY Immunity is the abilityof the body to resist invasion by foreign organism or toxins that tend to damage the tissue and organs of the body.  There are 2 types of immunity. A. Innate immunity: Non specific nor directed at a specific disease causing organism includes; a. Phagocytic action of white blood cells b. Action of digestive enzymes in the GIT c. Destruction of swallowed organisms by the acid secretions of the stomach  d. Resistance pose by the skin secretions like tear, mucosa lining the airway etc. These act as barrier by trapping bacteria, which may try to enter the body. e. Presence in the blood of certain chemical compounds that attach to foreign organism or toxins and destroy them.  Some of these compounds are:  Lysozyme, a mucolytic polysaccharide that attacks bacteria and causes them to dissolute.  The compliment complex, a system of 20 proteins that can be activated in various way to destroy bacteria.  Natural killer cell that can recognize and destroy foreign cells, tumour cells and even some infected cells.  Basic polypeptides which react with and inactivate certain types of gram-positive bacteria  B. Acquired immunity: This is developed against a specific invading organism or toxins after first experience  Involves production of specific antibodies  There are two forms of acquired immunity  Humoral immunity: This is immunity produced by circulating antibodies (γ- globulin)  Cellular immunity: This is achieved via production of large number of activated lymphocytes designed to destroy the foreign agent. It is responsible for delayed allergic reaction and rejection of foreign tissue transplant.  Humoral Immunity  B-cell lymphocytes are involved in recognising the antigen and release specific antibodies against it.  This is only possible if the helper T- cell activates B-cell.  There are 2 ways of activating the helper T- cell which in turn stimulate B-cell proliferation: 1, Macrophages, after taken the antigen, combine with the classII major histocompatibility complex (MHC). This combination is then presented to the helper T-cells. Helper T-cells release cytokines (interleukins) which activate B- cells to proliferate and transform into plasma cells. The plasma cells elaborate immunoglobulins  In the second mechanism, the antigen can bind directly to the B-cell surface receptors which also have class II protein (MHC). This combination on the surface of the B-cell results in the activation of helper T-cells. This inturn leads to B-cells transforming in to plasma cells and liberate immunoglobulins. The activated B-cells also form memory B-cells. These cells are specialized to liberate specific antibodies against the specific antigens, when there is subsequent entry of the antigen into the body.