Coagulation of Blood

Coagulation of Blood

„ DEFINITION

„ FACTORS INVOLVED IN BLOOD CLOTTING

„ SEQUENCE OF CLOTTING MECHANISM

„ BLOOD CLOT

„ ANTICLOTTING MECHANISM IN THE BODY

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„ DEFINITION

Coagulation or clotting is defined as the process in whichblood loses its fluidity and becomes a jelly-like mass few minutes after it is shed out or collected in a container.

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 FACTORS INVOLVED IN BLOOD CLOTTING

Coagulation of blood occurs through a series of reactions due to the activation of a group of substances. Substances necessary for clotting are called clotting factors.

Thirteen clotting factors are identified:

Factor I Fibrinogen

Factor II Prothrombin

Factor III Thromboplastin (Tissue factor)

Factor IV Calcium

Factor V Labile factor (Proaccelerin or accelerator

globulin)

Factor VI Presence has not been proved

Factor VII Stable factor

Factor VIII Antihemophilic factor (Antihemophilic globulin)

Factor IX Christmas factor

Factor X Stuart-Prower factor

Factor XI Plasma thromboplastin antecedent

Factor XII Hageman factor (Contact factor)

Factor XIII Fibrin-stabilizing factor (Fibrinase).

Clotting factors were named after the scientists who discovered them or as per the activity, except factor

IX. Factor IX or Christmas factor was named after the

patient in whom it was discovered.

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SEQUENCE OF CLOTTING MECHANISM

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ENZYME CASCADE THEORY

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 proenzymes must be activated into enzymes to enforce clot formation. It is carried out by a series of proenzyme-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 continues till the final active enzyme thrombin is formed. Enzyme cascade theory explains how various reactions, involved in the conversion of proenzymes to active enzymes take place in the form of a cascade. Cascade refers to a process that occurs through a series of steps, each step initiating the next, until the final step is reached.

Stages 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 produced either within the blood or outside the blood.

Thus, formation of prothrombin activator occurs through two pathways:

i. Intrinsic pathway

ii. Extrinsic pathway.

i. 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

(Fig. 20.1).

Sequence of Events in Intrinsic pathway

i. During the injury, the blood vessel is ruptured.

Endothelium is damaged and collagen beneath

the endothelium is exposed.

ii. When factor XII (Hageman factor) comes

in contact with collagen, it is converted into

activated factor XII in the presence of kallikrein

and high molecular weight (HMW) kinogen.

iii. The activated factor XII converts factor XI into

activated factor XI in the presence of HMW

kinogen.

iv. The activated factor XI activates factor IX in the

presence of factor IV (calcium).

v. Activated factor IX activates factor X in the

presence of factor VIII and calcium.

vi. When platelet comes in contact with collagen

of damaged blood vessel, it gets activated and

releases phospholipids.

vii. Now the activated factor X reacts with platelet

phos pholipid and factor V to form prothrombin

activa tor. This needs the presence of calcium

ions.

viii. Factor V is also activated by positive feedback

effect of thrombin (see below).

ii. 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

i. Tissues that are damaged during injury release tissue thromboplastin (factor III). Thromboplastin contains proteins, phospholipid and glycoprotein, which act as proteolytic enzymes.

ii. Glycoprotein and phospholipid components of thromboplastin convert factor X into activated factor X, in the presence of factor VII.

iii. 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.

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STAGE 2: CONVERSION OF PROTHROMBIN

INTO THROMBIN

Blood clotting is all about thrombin formation. Once thrombin is formed, it definitely leads to clot formation.

Sequence of Events in Stage 2

i. Prothrombin activator that is formed in intrinsic and extrinsic pathways converts prothrombin into thrombin in the presence of calcium (factor

IV).

ii. 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. This effect of thrombin is called positive feedback effect (Fig. 20.1).

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STAGE 3: CONVERSION OF FIBRINOGEN

INTO FIBRIN

The final stage of blood clotting involves the conversion of fibrinogen into fibrin by thrombin.

Sequence of Events in Stage 3

i. Thrombin converts inactive fibrinogen into activated fibrinogen due to loss of 2 pairs of polypeptides from each fibrinogen molecule. The activated fibrinogen is called fibrin monomer.

ii. Fibrin monomer polymerizes with other monomer

molecules and form loosely arranged strands of

fibrin.

iii. Later these loose strands are modified into dense and tight fibrin threads by fibrin-stabilizing factor (factor XIII) in the presence of calcium ions (Fig. 20.1). All the tight fibrin threads are aggregated to form a meshwork of stable clot.

FIGURE 20.1: Stages of blood coagulation. a = Activated, + = Thrombin induces formation of more thrombin

(positive feedback); HMW = High molecular weight.

BLOOD CLOT

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DEFINITION AND COMPOSITION OF CLOT

Blood clot is defined as the mass of coagulated blood which contains RBCs, WBCs and platelets entrapped in fibrin meshwork. RBCs and WBCs are not necessary for clotting process. However, when clot is formed, these cells are trapped in it along with platelets. The trapped RBCs are responsible for the red color of the clot. The external blood clot is also called scab. It adheres to the opening of damaged blood vessel and prevents blood loss.

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 CLOT RETRACTION

After the formation, the blood clot starts contracting. And after about 30 to 45 minutes, the straw-colored serum oozes out of the clot. The process involving the contraction of blood clot and oozing of serum is called clot retraction. Contractile proteins, namely actin, myosin and thrombosthenin in the cytoplasm of platelets are responsible for clot retraction.

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FIBRINOLYSIS

Lysis of blood clot inside the blood vessel is called fibrinolysis. It helps to remove the clot from lumen of the blood vessel. This process requires a substance called plasmin or fibrinolysin.

Formation of Plasmin

Plasmin is formed from inactivated glycoprotein called plasminogen. Plasminogen is synthesized in liver and it is incorporated with other proteins in the blood clot. Plasminogen is converted into plasmin by tissue plasminogen activator (t-PA), lysosomal enzymes and thrombin. The t-PA and lysosomal enzymes are released from damaged tissues and damaged endothelium. Thrombin is derived from blood. The t-PA is always inhibited by a substance called t-PA inhibitor. It is also inhibited by factors V and VIII. Besides t-PA, there is another plasminogen activator called urokinase plasminogen activator (u-PA). It is derived from blood.

Sequence of Events Involved in the

Activation of Plasminogen

1. During intravascular clotting, the endothelium of the blood vessel secretes a thrombin-binding protein, the thrombomodulin. It is secreted by the endothelium of all the blood vessels, except the minute vessels of brain.

2. Thrombomodulin combines with thrombin and forms a thrombomodulin-thrombin complex

3. Thrombomodulin-thrombin complex activates protein C

4. Activated protein C inactivates factor V and VIII in the presence of a cofactor called protein S

5. Protein C also inactivates the t-PA inhibitor

6. Now, the t-PA becomes active

7. Activated t-PA and lysosomal enzymes activate plasminogen to form plasmin. Plasminogen is also activated by thrombin and u-PA (Fig. 20.2).

FIGURE 20.2: Fibrinolysis. t-PA = Tissue plasminogen

activator, u-PA = Urokinase plasminogen activator.

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ANTICLOTTING MECHANISM IN THE BODY

Under physiological conditions, intravascular clotting

does not occur. It is because of the presence of some

physicochemical factors in the body.

1. Physical Factors

i. Continuous circulation of blood.

ii. Smooth endothelial lining of the blood vessels.

2. Chemical Factors – Natural Anticoagulants

i. Presence of natural anticoagulant called heparin that is produced by the liver

ii. Production of thrombomodulin by endothelium of the blood vessels (except in brain capillaries). Thrombomodulin is a thrombin-binding pro tein. It binds with thrombin and forms a thrombomodulin- thrombin complex. This complex activates protein C. Activated protein C along with its cofactor protein S inactivates Factor V and Factor VIII. Inactivation of these two clotting factors prevents clot formation

iii. All the clotting factors are in inactive state.