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Mechanism of Blood Coagulation

Hemostasis occurs in three stages: vasoconstriction, platelet plug formation, and blood coagulation. Vasoconstriction decreases blood flow from damaged vessels. Platelets adhere to collagen at injury sites, forming a temporary platelet plug. Coagulation involves thrombin converting fibrinogen to fibrin, forming fibrin threads that strengthen the platelet plug and complete hemostasis. Blood coagulation is initiated through intrinsic and extrinsic pathways activating a cascade of clotting factors that ultimately generate thrombin. Thrombin then converts fibrinogen to fibrin to form a blood clot.

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DEFINITION Hemostasis is defined as arrest or stoppage of bleeding. Stages of Hemostasis When a blood vessel is injured, the injury initiates a series of reactions, resulting in hemostasis. It occurs in three stages.
STAGES OF HEMOSTASIS  Vasoconstriction  Platelet plug formation  Coagulation of blood.
VASOCONSTRICTION Immediately after injury, the blood vessel constricts and decreases the loss of blood from the damages portion. When the blood vessels are cut, the endothelium is damaged and the collagen is exposed. Platelets adhere to this collagen and get activated. The activated platelets secrete serotonin and other vasoconstrictor substances which cause constriction of the blood vessels. Adherence of the platelets to the collagen is accelerated by Von Willebrand factor. This factor acts as a bridge between a specific glycoprotein present on the surface of platelet and collagen fibrils.
PLATELET PLUG FORMATION Platelets get adhered to the collagen of ruptured blood vessel and secrete adenosine di phosphate (ADP) and thromboxane – A. These two substances attract more and more paltelets and activate them. All these platelet aggregate together and form a loose temporary platelet plug or temporary hemostatic plug, which close the ruptured vessel and prevent further blood loss. Platelet aggregation is accelerated by Platelet- Activating Factor (PAF).
COAGULATION OF BLOOD During this process , the fibrinogen is converted into fibrin. Fibrin threads get attached to the loose platelet plug, which plugs the ruptured part of blood vessels and prevents further blood loss completely. Mechanism of blood coagulation is explained further in the coming slides.
DEFINITION Coagulation or clotting is defined as the process in which blood losses its fluid and becomes a jelly like mass few minutes after it is shed out ot collected in a container.
FACTORS INVOLVED IN BLOOD CLOTTING Coagulation of blood occurs through a series of reactions due to the activation of a group of substances necessary for clotting are called clotting factors. Thirteen clotting factors are identified. 1. Factor – 1: Fibrinogen 2. Factor – II: Prothrombin 3. Factor – III:Thromboplastin (Tissue Factor) 4. Factor – IV: Calcium 5. Factor – V: Labile Factor
FACTORS INVOLVED IN BLOOD CLOTTING 6. Factor – VI: Presence has not been proved 7. Factor – VII: Stable factor 8. Factor – VIII: Antihemophillic factor ( Antihemophillic globulin) 9. Factor – IX:Christmas Factor 10. Factor – X: Stuart Prower factor 11. Factor – XI: Plasma Thromboplastin Antecedent 12. Factor – XII: Hageman factor (Contact factor) 13. Factor – XIII: Fibrin stabilizing Factor (Fibrinase)
SEQUENCE OF CLOTTING MECHANISM Most of the clotting factors are proteins in the form of enzymes. Normally , all the factors are present in the form of inactive proenzyme. These pro-enzyme must be activated into enzymes to enforce clot formation. It is carried out by a series of pro-enzyme - enzyme conversion reactions. First one of the series is converted into an active enzyme that activates the second one, which activates the third one:; this continue till the final active enzyme thrombin is formed.
STEPS OF BLOOD CLOTTING In general blood clotting occurs in three stages. 1)Formation of Prothrombin activator 2)Conversion of prothrombin into thrombin 3)Conversion of fibrinogen into fibrin
STAGE – 1: FORMATION OF PROTHROMBIN ACTIVATOR Blood clotting commences with the formation of a substance called prothrombin activator, which converts prothrombin into thrombin. Its formation is initiated by substances either within the blood or outside the blood. Thus, formation of prothrombin activator occurs through two pathways: 1. Intrinsic Pathway 2. Extrinsic Pathway
1. INTRINSIC PATHWAY FOR THE FORMATION OF PROTHROMBIN ACTIVATOR In this pathway, the formation of prothrombin activator is initiated by platelets, which are within the blood itself. Sequence of Events in Intrinsic Pathway During the injury, the blood vessel is ruptured. Endothelium is damaged and collagen beneath the endothelium is exposed. When factor XII (Hageman factor) comes in contact with collagen, it is converted into activated factor XII in the presence of Kallikrein and Kinogen. The activated factor XII Converts factor XI into activated XI in the presence of HMW Kinogen. The activated factor XI activates factor IX in the presence of factor IV (Calcium).
1. INTRINSIC PATHWAY FOR THE FORMATION OF PROTHROMBIN ACTIVATOR Sequence of Events in Intrinsic Pathway (Cont) Activated factor IX activates factor X in the presence of factor VIII and Calcium. When platelet comes in contact with collagen of damaged blood vessel, it gets activated and releases phospholipids. Now the activated factor X reacts with platelet phospholipid and factor V to form Prothrombin activator. This needs the presence of calcium ions, Factor V is also activated by the positive feedback effect of thrombin.
2. EXTRINSIC PATHWAY FOR THE FORMATION OF PROTHROMBIN ACTIVATOR In this pathway, the formation of prothrombin activator is initiated by the tissue thromboplastin, which is formed from the injured tissues. Sequence of Events in Extrinsic Pathway Tissues that are damaged during injury release tissue thromboplastin (Factor – III). Thromboplastin contains proteins, phospholipid and glycoprotein, which act as proteolytic enzymes. Glycoprotein and phospholipid components of thromboplastin convert factor X into activated factor X, in the presence of factor VII. Activated factor X reacts with factor V and phospholipid component of tissue thromboplastin to form prothrombin activator. This reaction requires the presence of calcium ions.
STAGE – 2: CONVERSION OF PROTHROMBIN INTO THROMBIN Blood clot is all about thrombin formation. Once thrombin is formed definitely it leads to clot formation. Prothrombin activator that is formed in intrinsic and extrinsic pathways converts prothrombin into thrombin. In the presence of calcium (factor IV). Once formed thrombin initiates the formation of more thrombin molecules. The initially formed thrombin activates factor V. Factor V in turn accelerates formation of both extrinsic and intrinsic prothrombin activator, which converts prothrombin into thrombin. The effect of thrombin is called positive feedback effect.
STAGE – 3: FIBRINOGEN INTO FIBRIN Sequence of events in stage 3 are as follows. 1. Thrombin converts inactive fibrinogen into activated fibrinogen .the activated fibrinogen is called fibrin monomer. 2. Fibrin monomer polymerizes with other monomer molecule and form loosely arranged strands of fibrin. 3. Later these loose strands are modified into dense and tight fibrin threads by fibrin – stabilizing factor (Factor VIII) in the presence of calcium ions. 4. All the tight fibrin threads are aggregated to form a meshwork of stable clot.
Mechanism of Blood Coagulation
Mechanism of Blood Coagulation