Composition and characteristics of blood

Blood plasma protein, serum, characteristics of blood, RBC, Anemia, WBC, lymphocytes Platelets, Blood coagulation

1. Blood system Prepared by Murshida Mollik Lecturer Department of Pharmacy Manarat International University

2. Definition: Blood is a complex, specialized body fluid composed of four main components: plasma, red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes). Plasma, a yellowish liquid, makes up about 55% of the blood volume and contains water, proteins, glucose, and hormones, while the remaining 45% consists of the blood cells, primarily red blood cells.  Composition of blood: Blood is composed of blood cells suspended in  blood plasma . Plasma, which constitutes 55% of blood fluid, is mostly  water  (92% by volume), and contains  proteins ,  glucose , mineral  ions , and  hormones . The blood cells are mainly  red blood cells  (erythrocytes),  white blood cells  (leukocytes), and (in mammals)  platelets  (thrombocytes).

3. Physical properties of blood Color :  The color of blood depends on its oxygen saturation. Bright red:  Seen in arterial blood, where hemoglobin is bound with oxygen. Dark red:  Found in venous blood, where oxygen has been released to the body's tissues. Viscosity:  Blood is about five times thicker and more viscous than water due to the presence of blood cells and plasma proteins. This viscosity is crucial for maintaining proper blood pressure and flow. Temperature:  The normal temperature of blood is slightly higher than core body temperature, around 38°C (100.4°F). Volume:  The average adult has a blood volume of approximately 5 liters, with males typically having 5–6 liters and females 4–5 liters. 

4. Key Chemical Properties and Components: pH: Blood is slightly alkaline, with a normal pH range of 7.35 to 7.45. Viscosity: It is about five times more viscous (thicker) than water, due to the presence of formed elements (cells) and proteins. Temperature: Blood temperature is slightly higher than normal body temperature, around 38°C, which helps in regulating the body's overall temperature. Water: The most abundant component, making up over 90% of plasma. Plasma Proteins: A significant portion of blood plasma consists of proteins like: Albumin: A major protein important for maintaining fluid balance. Globulins: Includes antibodies and other immune proteins.

5. Fibrinogen: A key protein involved in blood clotting. Nutrients: Dissolved sugars (like glucose), amino acids, and fatty acids are transported in the blood. Electrolytes: Ions such as sodium, potassium, and chloride are present. Dissolved Gases: Oxygen, carbon dioxide, and nitrogen are dissolved in the plasma. Hormones: Chemical messengers that are circulated throughout the body. Waste Products: Metabolic wastes such as urea and lactic acid are carried away from tissues by the blood.

6. Functions of blood: Supply of nutrients such as  glucose ,  amino acids , and  fatty acids  (dissolved in the blood or bound to  plasma proteins  (e.g.,  blood lipids )) Supply of  oxygen  to tissues (bound to  hemoglobin , which is carried in red cells) Removal of waste such as  carbon dioxide ,  urea , and  lactic acid Immunological functions, including circulation of  white blood cells , and detection of foreign material by  antibodies Coagulation , the response to a broken blood vessel, the conversion of blood from a liquid to a semisolid gel to stop bleeding Messenger functions, including the transport of  hormones  and the signaling of  tissue  damage Regulation of core  body temperature Hydraulic  functions

7. What is Plasma? Plasma is the pale yellow, liquid portion of your blood, making up about 55% of its volume, and it carries red blood cells, white blood cells, and platelets throughout the body. This vital fluid, composed mainly of water but also containing  proteins ,  nutrients ,  hormones ,  salts , and  waste products , transports essential substances to the body's tissues and removes waste materials, playing a crucial role in maintaining overall health and bodily functions.   Components of Plasma: Water : Plasma is primarily water (around 92%), providing the medium for other components to be transported.  Proteins : It contains various proteins, including albumin for fluid balance,  clotting factors  for blood coagulation, and  immunoglobulins  ( antibodies ) for immune function.  Nutrients : Plasma delivers glucose, fats, and other vital nutrients to the body's cells.  Hormones : It carries  hormones  from endocrine glands to their target tissues.  Salts and Minerals : Electrolytes and other mineral salts are dissolved in plasma, helping to maintain the body's fluid and chemical balance.  Waste Products : Plasma collects metabolic waste products, such as carbon dioxide and  urea , for elimination by organs like the kidneys and lungs. 

8. Plasma volume measurement: It can be measured by using Evans blue dye, which binds to plasma proteins, or by injecting serum albumin labeled with radioactive iodine. The average PV in a normal 70 kg adult is approximately 3.5 L (50–55 mL/kg). Blood volume measurement: Once plasma volume and hematocrit (the percentage of red blood cells in the blood) are known, the total blood volume can be calculated using the formula: Blood Volume = Plasma Volume / (1 – Hematocrit)

9. What is serum: Serum in blood is the clear, liquid portion that remains after red blood cells, white blood cells, platelets, and clotting proteins have been removed. It is essentially blood plasma without fibrinogen and other clotting factors. This straw-colored fluid contains antibodies, antigens, hormones, electrolytes, and other substances, and is crucial for various medical diagnostic tests, tissue culture, and transplant medicine.

10. What Serum Contains? Proteins: All proteins not used in blood clotting. Antibodies: Immune proteins that fight infection. Hormones: Chemical messengers. Electrolytes: Minerals like sodium and potassium. Antigens: Substances that can trigger an immune response. Other substances: Including drugs, microorganisms, and growth factors. Uses of Serum: Diagnostic Tests: Used for various blood tests, such as hormone tests and blood typing.  Tissue Culture: As a nutrient-rich fluid for cell growth and maintenance.  Transplant Medicine: To detect specific antigens in tissue typing.  Biomarker Detection: Used in metabolomics to assess physiological status and detect environmental pollutants. 

11. Differences between plasma and serum

12. Plasma protein: 1. Albumin Making up around 60% of all the protein in the blood, albumin is the most prevalent form of plasma protein.  It is a simple protein made up of just one amino acid chain. Albumin is essential for preserving the blood’s osmotic pressure, which is required to maintain the body’s electrolyte and water balance. Additionally, it aids in the transportation of numerous molecules such as fatty acids, hormones, and enzymes. 2. Globulins Alpha, beta, and gamma globulins are only a few of the many subtypes of globins , a class of proteins that are more complicated than albumin. Alpha Globulins – The group of proteins known as alpha globulins includes the alpha-fetoprotein, which is generated by the foetal liver and is utilised as a marker for certain cancers. Alpha-1-antitrypsin, which aids in lung damage prevention, and alpha-2-macroglobulin, which aids in immune response regulation, are other alpha globulins. Beta globulins – Beta globulins include a variety of proteins, including complement proteins and transferrin, which aid in the movement of iron throughout the body. Immunoglobulins, commonly referred to as gamma globulins, are essential for the immunological response.  They are produced by B cells and help protect the body against infections. 3 . Fibrinogen Fibrinogen is a complex protein that is involved in the blood clotting process. It is synthesized by the liver and is converted to fibrin during the clotting process. Fibrin helps to form a clot, which is necessary to stop bleeding after an injury. In addition to these three main types of plasma proteins, there are also a number of other proteins that are found in smaller quantities in the blood plasma, including lipoproteins, which transport lipids throughout the body, and enzymes, which catalyze biochemical reactions.

13. Functions of plasma protein: Blood osmotic pressure maintenance  – Albumin is the primary protein that aids in blood osmotic pressure maintenance. This pressure is required to maintain the body’s electrolyte and water balance.The maintenance of the body’s general health depends on plasma proteins. They assist in controlling the body’s water balance, moving crucial chemicals around the body, and guarding the body against infections. For instance, albumin maintains the blood’s osmotic pressure, which helps to avoid the buildup of extra fluid in the tissues. Transportation  – Hormones and other molecules are transported by globulins and albumin, which also aid in the movement of enzymes and other molecules. Blood Clot Formation  – The primary protein that aids in the formation of blood clots is fibrinogen. In order to stop excessive bleeding after an injury, they are also essential for the blood clotting process. Immunity –  Some globulins, including immunoglobulins, participate in the immunological response and aid in defending the body against infections.  

14. Red blood cells (RBCs), also called erythrocytes, are crucial for transporting oxygen throughout the body.  1. Composition Hemoglobin: This iron-rich protein gives RBCs their red color and binds to oxygen in the lungs, releasing it in the body tissues where needed. Cell Membrane: A flexible outer layer composed of proteins and lipids that allows RBCs to change shape and navigate narrow blood vessels. Lack of Organelles: Mature mammalian RBCs are unique among cells because they lack a nucleus and other organelles like mitochondria. This maximizes space for hemoglobin, enhancing oxygen-carrying capacity.  2. Count (RBC count) A red blood cell count measures the number of RBCs per microliter of blood.  Normal ranges: These can vary slightly between laboratories. Generally, the normal ranges are: Males: 4.7 to 6.1 million cells/ mcL Females: 4.2 to 5.4 million cells/ mcL Children: 4 to 5.5 million cells/ mcL

15. Erythropoiesis: It is the normal production and formation of red blood cells (erythrocytes) in the bone marrow, which are essential for transporting oxygen from the lungs to the body's tissues. This complex process is stimulated by the hormone erythropoietin (EPO) in response to low oxygen levels and requires specific nutrients like iron, vitamin B12, and folate to proceed correctly.   In adults, almost all red blood cell production occurs in the red bone marrow, located in the flat bones like the pelvis, ribs, and vertebrae. Fetal Development: In early fetal development, erythropoiesis begins in the yolk sac, spleen, and liver before shifting entirely to the bone marrow after birth. The Process 1.  Stimulus: Low oxygen levels in the body, such as during anemia or blood loss, trigger the production of erythropoietin (EPO) by the kidneys.  2.  Activation: Increased EPO levels stimulate erythroid progenitor cells in the bone marrow to proliferate and differentiate into mature red blood cells.  3.  Maturation: The maturing cells accumulate hemoglobin, lose their nuclei and other organelles, and develop into immature reticulocytes.  4.  Release: These reticulocytes are released from the bone marrow into the bloodstream, where they continue to mature into fully functional red blood cells.  5.  Function : The mature red blood cells then circulate in the bloodstream, carrying oxygen to the body's tissues.  Key Factors Required Erythropoietin (EPO): A hormone that signals for red blood cell production.  Iron: A crucial component of hemoglobin, which binds oxygen.  Folate and Vitamin B12: Essential vitamins for the process of cell division and maturation. 

16. Hemoglobin: It is an iron-rich protein found in red blood cells that carries oxygen from the lungs to the body's tissues and transports carbon dioxide back to the lungs. It gives blood its distinctive red color. A hemoglobin test is a common blood test that measures the amount of this protein in your blood, and both low (anemia) and high (polycythemia) levels can indicate an underlying health issue.

17. Composition of Hemoglobin: It is composed of four polypeptide subunits, specifically two alpha chains and two beta chains in adult hemoglobin (HbA), each with a ring-like heme group at its center. Each heme group contains a  ferrous iron  ion (Fe²⁺), which binds to oxygen for transport through the blood.   Components of a Hemoglobin Molecule Globin: This is the protein component of hemoglobin, which consists of polypeptide chains.  Alpha chains (α):  Two of these protein chains are present in adult hemoglobin.  Beta chains (β):  Two beta protein chains are also found in adult hemoglobin.  Other subunits:  Fetal hemoglobin ( HbF ) uses gamma (γ) chains instead of beta chains, and a small amount of adult hemoglobin ( HbA2 ) uses delta (δ) chains.  Heme : This is a non-protein prosthetic group found in each of the four globin subunits.  Protoporphyrin :  A ring-like structure that forms the basis of the heme group.  Iron ion :  A single ferrous (Fe²⁺) iron ion is located at the center of the protoporphyrin ring, and this is where oxygen binds.  Assembly and Function The four subunits (two alpha and two beta) assemble into a tetramer.  Each of the four subunits can bind one oxygen molecule to the iron atom in its heme group, allowing a single hemoglobin molecule to transport four oxygen molecules.  The iron must be in the ferrous (Fe²⁺) state to bind oxygen; the ferric (Fe³⁺) state is called methemoglobin and cannot carry oxygen. 

18. ESR : It stands for Erythrocyte Sedimentation Rate, a blood test that measures how fast red blood cells (erythrocytes) settle in a test tube, with a faster rate indicating more inflammation in the body. It's a non-specific indicator for conditions like  infections ,  autoimmune disorders  (e.g., rheumatoid arthritis),  cancer , or  tissue injury , but cannot diagnose a specific disease on its own. A higher ESR is often a sign of underlying inflammation, while a slower rate suggests less inflammation.  

19. Anemia : It is a condition characterized by a deficiency of healthy red blood cells or hemoglobin, the protein in red blood cells that carries oxygen. This lack of oxygen-carrying capacity results in insufficient oxygen reaching the body's tissues and organs, leading to symptoms like fatigue, weakness, and shortness of breath. Anemia is not a disease itself but a sign of an underlying issue and is diagnosed with a blood test that reveals low hemoglobin or red blood cell levels. There are some of the most common types of anemia: Iron Deficiency Anemia : This is the most common type, occurring when the body doesn't have enough iron to produce hemoglobin, the protein in red blood cells that carries oxygen.  Vitamin Deficiency Anemia : Lack of essential vitamins, particularly B12 and folate (folic acid), leads to the body not being able to produce enough healthy red blood cells, resulting in large, immature cells.  Anemia of Chronic Disease : Also known as anemia of inflammation, this type is linked to ongoing health conditions like kidney disease, cancer, or inflammatory diseases. 

20. Aplastic Anemia: In this rare and serious condition, the bone marrow stops producing enough new blood cells, including red blood cells.  Hemolytic Anemias: This category involves the premature destruction of red blood cells, which can be caused by inherited factors or acquired conditions.  Sickle Cell Anemia: A genetic disorder where red blood cells are crescent-shaped (sickle-shaped), leading to blockages in blood flow and pain.  Thalassemia: Another genetic blood disorder where the body produces less hemoglobin than normal, leading to a reduction in red blood cells. 

21. WBC: A WBC with count, or White Blood Cell count, is a blood test that measures the total number of white blood cells (leukocytes) in your blood. These cells are a crucial part of the immune system that fights infection and disease. A normal count typically falls between 4,500 and 11,000 cells per microliter of blood in adults. Results are interpreted in conjunction with a WBC differential, which provides the percentages of the five main types of WBCs: neutrophils, lymphocytes, monocytes, eosinophils, and basophils.

22. Types of White Blood Cells: There are five different types of white blood cells, which are classified mainly based on the presence and absence of granules. 1. Granulocytes: They are the type of white blood cells, with the presence of granules in their cytoplasm. The granulated cells include: Basophil Eosinophil Neutrophils 2. Agranulocytes: They are the types of white blood cells, with the absence of granules in their cytoplasm. The a-granulated cells include: Monocytes Lymphocytes

23. 1. Antigens: Macromolecules, often proteins or carbohydrates, found on the surface of foreign invaders like viruses, bacteria, parasites, and allergens, but also on cells within the body (autoantigens). Function: They act as markers or "foreign" identifiers, signaling the immune system that something is not "self" and needs to be dealt with. Examples: A virus, a bacterium, a toxin, or a pollen grain can all be antigens. 2. Antibodies: Proteins, also known as immunoglobulins, produced by the immune system's plasma cells. Function: Antibodies specifically bind to unique regions on an antigen called epitopes. This binding can neutralize the antigen's harmful effects or flag it for destruction by other immune cells. How they work: The highly specific binding of an antibody to its corresponding antigen is crucial for the immune response. In Summary Antigen = The Trigger: A substance that prompts an immune response. Antibody = The Defender: A protein produced to fight the specific antigen. This antigen-antibody relationship is key to the development of immunity against infections and is the basis for many diagnostic tests, like those used to detect infections or determine past exposure to certain substances.

24. 3. Lymphocytes: They are a type of white blood cell. They play an important role in your  immune system , which helps your body fight disease and infection. Your immune system is made up of an intricate web of immune cells, lymph nodes, lymph tissue and lymphatic organs. Lymphocytes are a type of immune cell. There are two main types of lymphocytes: immunologically its 2 types T lymphocytes (T cells) : T cells control your body’s immune system response and directly attack and kill infected cells and tumor cells. B lymphocytes (B cells) : B cells make antibodies. Antibodies are proteins that target viruses, bacteria and other foreign invaders.

25. Immunoglobulin, or antibody: It is a Y-shaped protein produced by plasma cells that acts as a key part of the immune system, specifically binding to antigens (like bacteria or viruses) to neutralize them or tag them for destruction by other immune cells. These proteins help fight infection by recognizing foreign substances and initiating an immune response to eliminate them from the body. The five main types of immunoglobulins, or antibodies, are IgG, IgM, IgA, IgE , and IgD . These classes are distinguished by their unique heavy chains, including gamma (γ) for IgG, mu (μ) for IgM, alpha (α) for IgA, epsilon (ε) for IgE , and delta (δ) for IgD . Here is a brief overview of each type: IgG (Immunoglobulin G): The most abundant antibody in the blood, accounting for about 75% of antibodies. It plays a key role in fighting bacterial and viral infections. IgM (Immunoglobulin M): A larger, pentameric antibody (formed from five smaller units) that is the first to appear during an initial infection. IgA (Immunoglobulin A): Found in mucus, tears, and saliva, IgA is crucial for mucosal immunity, providing protection against pathogens at entry points into the body. IgE (Immunoglobulin E): Associated with allergic reactions and defense against parasitic worms. IgD (Immunoglobulin D): Found on the surface of certain immune cells, IgD acts as a receptor and plays a role in B cell activation.

26. Types of immune response via B cells: B cells have receptors on their surfaces where antigens attach. B cells learn to recognize the different antigens and produce specific antibodies to attack each one. The B cells respond to antigens in two ways: Primary immune response : When an antigen attaches to a receptor, your B cells are stimulated. Some B cells change into memory cells. Other B cells change into plasma cells. Plasma cells make an antibody specific to the particular antigen that stimulated it. Production of enough of that specific antibody can take several days. Secondary immune response : If your B cells encounter that antigen again, the memory cells remember it and multiply. They change into plasma cells and quickly produce the correct antibody.

27. Types of T-cells: Your T cells help kill infected cells and control your body’s  immune response  to foreign substances. Most of your T cells need the help of another immune cell to become activated. After your T cells are activated, they multiply and specialize into different types of T cells. These types include: Cytotoxic (killer) T cells : Cytotoxic T cells attach to antigens on infected or abnormal cells. Then, they kill the infected cells by making holes in their cell membranes and inserting enzymes into the cells. Helper T cells : Helper T cells help your other immune cells. Some helper T cells help B cells make antibodies against foreign invaders. Others help activate cytotoxic T cells. Regulatory (suppressor) T cells : Regulatory T cells make substances that help end your immune system’s response to an attack. Sometimes, they prevent harmful responses from occurring.

28. Immunity : Itis the body's defense system that protects it from diseases caused by pathogens, like viruses and bacteria. There are three main types of immunity: innate immunity, which is the general, built-in protection everyone has from birth, like the skin barrier; adaptive (or acquired) immunity, which develops over a lifetime through exposure to diseases or vaccination and provides specific, long-lasting protection; and passive immunity, which is temporary, "borrowed" protection, such as the antibodies a baby receives from its mother's breast milk.   Types of Immunity 1. Innate Immunity: This is your body's first line of defense. It's a general, natural resistance to many infections.  Examples: Physical barriers like the skin and mucous membranes, and general immune cells that recognize and fight off invaders.  2. Adaptive Immunity : Also known as acquired immunity, this type is developed over time as you encounter new germs and diseases.  How it works: When you're exposed to a specific pathogen, your body creates antibodies to that pathogen. This creates a "memory" so that if the same pathogen enters your body again, your immune system can fight it off more quickly and effectively.  How it's acquired: Through natural infection or vaccination.  3.Passive Immunity: This is a temporary immunity that doesn't come from your own body's response but from an external source.  Examples: Antibodies passed from mother to baby through the placenta during pregnancy or through breast milk. 

29. Functions of WBC: Fighting Infections: WBCs are the primary soldiers of your immune system, identifying and attacking harmful microorganisms that enter the body.  Producing Antibodies: Certain WBCs produce antibodies, which are specific proteins that bind to pathogens and flag them for destruction.  Engulfing Pathogens: Other types of WBCs, like phagocytes, directly engulf and "eat" foreign substances, bacteria, and viruses in a process called phagocytosis.  Destroying Abnormal Cells: WBCs are crucial for identifying and destroying abnormal or cancerous cells that develop in the body.  Managing Inflammation: WBCs are involved in initiating and controlling the body's inflammatory response to injuries and infections.  Removing Debris: They also help clear out dead cells and other waste materials from the body, contributing to tissue health and repair.

30. Blood grouping: It determines compatibility for transfusions by identifying inherited antigens (A and B) and the Rh factor (+ or -) on red blood cells, leading to eight primary blood types: A+, A-, B+, B-, AB+, AB-, O+, and O-. A mismatch between a recipient's and a donor's blood can trigger a dangerous immune response where the recipient's body attacks the transfused cells. The ABO System This system classifies blood based on the presence of A and B antigens on the surface of red blood cells: Type A: Has A antigens and produces anti-B antibodies. Type B : Has B antigens and produces anti-A antibodies. Type AB: Has both A and B antigens and produces no anti-A or anti-B antibodies. Type O : Has no A or B antigens and produces both anti-A and anti-B antibodies. The Rh System This system identifies a protein, or Rh factor, on the surface of red blood cells: Rh Positive (+): The Rh factor is present. Rh Negative (-): The Rh factor is absent.

31. Why Blood Grouping is Important? Transfusions: It ensures that donated blood is compatible with the recipient's blood. Mismatched blood can cause severe complications. Pregnancy: It's crucial during pregnancy to prevent issues if the mother is Rh-negative and the baby is Rh-positive, a condition that can be managed with medication. Other Uses: Blood grouping is also used for transplants and other medical procedures. Common Blood Types Combining the ABO and Rh systems, the eight common blood types are: O positive (+), O negative (-), A positive (+), A negative (-), B positive (+), B negative (-), AB positive (+), and AB negative (-).

32. What are platelets? Platelets, also known as thrombocytes, are small, cell-like fragments in your blood that are crucial for forming clots and stopping bleeding. A normal platelet count in adults is between 150,000 and 450,000 platelets per microliter of blood. A platelet count below 150,000 is called thrombocytopenia and increases the risk of bleeding, while a count above 450,000 is called thrombocytosis and can lead to dangerous blood clots. Your platelet level is determined through a complete blood count (CBC) blood tesNormal range: 150,000 to 450,000 platelets per microliter ( mcL ) of blood.

33. What are abnormal platelet counts? Thrombocytopenia (low platelet count): A count less than 150,000 platelets per mcL . This can result from inherited or acquired medical conditions and may cause symptoms like excessive bleeding or bruising. Thrombocytosis (high platelet count): A count over 450,000 platelets per mcL . This can be caused by other conditions, such as infections, inflammation, or certain cancers, and increases the risk of blood clots.

34. Blood coagulation: Blood clotting, is a vital physiological process where liquid blood changes into a semisolid or gel-like state (a clot) to stop bleeding from a damaged blood vessel. This process involves platelets and proteins called clotting factors working together to form a plug over the injury, preventing excessive blood loss and allowing the vessel to heal.

35. Blood Coagulation Cascade The process of coagulation is a cascade of enzyme catalysed reactions wherein the activation of one factor leads to the activation of another factor and so on. The three main steps of the blood coagulation cascade are as follows: Formation of prothrombin activator Conversion of prothrombin to thrombin Conversion of fibrinogen into fibrin 1. Formation of prothrombin activator The formation of a prothrombin activator is the first step in the blood coagulation cascade of secondary haemostasis . It is done by two pathways, viz. extrinsic pathway and intrinsic pathway. Extrinsic Coagulation Pathway It is also known as the tissue factor pathway. It is a shorter pathway. The tissue factors or tissue thromboplastins are released from the damaged vascular wall. The tissue factor activates the factor VII to VIIa . Then the factor VIIa activates the factor X to Xa in the presence of Ca 2+ .

36. Intrinsic Coagulation Pathway It is the longer pathway of secondary haemostasis . The intrinsic pathway begins with the exposure of blood to the collagen from the underlying damaged endothelium. This activates the plasma factor XII to XIIa . XIIa is a serine protease, it activates the factor XI to XIa . The XIa then activates the factor IX to IXa in the presence of Ca 2+  ions. The factor IXa in the presence of factor VIIIa , Ca 2+  and phospholipids activate the factor X to Xa. Common Pathway The factor Xa, factor V, phospholipids and calcium ions form the prothrombin activator. This is the start of the common pathway of both extrinsic and intrinsic pathways leading to coagulation.

37. 2. Conversion of prothrombin to thrombin Prothrombin or factor II is a plasma protein and is the inactive form of the enzyme thrombin. Vitamin K is required for the synthesis of prothrombin in the liver. The prothrombin activator formed above converts prothrombin to thrombin. Thrombin is a proteolytic enzyme. It also stimulates its own formation, i.e. the conversion of prothrombin to thrombin. It promotes the formation of a prothrombin activator by activating factors VIII, V and XIII. 3. Conversion of fibrinogen into fibrin Fibrinogen or factor I is converted to fibrin by thrombin. Thrombin forms fibrin monomers that polymerise to form long fibrin threads. These are stabilised by the factor XIII or fibrin stabilising factor. The fibrin stabilising factor is activated by thrombin to form factor XIIIa . The activated fibrin stabilising factor ( XIIIa ) forms cross-linking between fibrin threads in the presence of Ca2+ and stabilises the fibrin meshwork. The fibrin mesh traps the formed elements to form a solid mass called a clot.

38. Reference Book: CC Chatterjee's Human Physiology Volume 1 Ganong's Review of Medical Physiology

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