BLOOD CLOTTINGBLOOD CLOTTING

When blood is shed out or collected in a When blood is shed out or collected in a container, it looses its fluidity & becomes a container, it looses its fluidity & becomes a jelly like mass after few minutes. This process jelly like mass after few minutes. This process is called coagulation or clotting of blood. The is called coagulation or clotting of blood. The clot is a mesh of thin fibrils entangling the clot is a mesh of thin fibrils entangling the blood cells. These fibrils consist of fibrin. The blood cells. These fibrils consist of fibrin. The fibrin is formed from fibrinogen. fibrin is formed from fibrinogen.

FACTORS INVOLVED IN FACTORS INVOLVED IN BLOOD CLOTTINGBLOOD CLOTTING

Coagulation of blood occurs through a Coagulation of blood occurs through a series of reactions due to the activation of series of reactions due to the activation of a variety of substances. Those substances a variety of substances. Those substances necessary for clotting are called clotting necessary for clotting are called clotting factors. The clotting factors are : factors. The clotting factors are :

Factor I : FibrinogenFactor I : Fibrinogen Factor II : ProthrombinFactor II : Prothrombin Factor III: ThromboplastinFactor III: Thromboplastin Factor IV: CalciumFactor IV: Calcium Factor V: Labile factor(Proaccelerin or Factor V: Labile factor(Proaccelerin or

accelerator globulin)accelerator globulin) Factor VII: Stable factor Factor VII: Stable factor Factor VIII: Antihemophilic factorFactor VIII: Antihemophilic factor Factor IX: Christmas factorFactor IX: Christmas factor Factor X: Stuart – Prower factorFactor X: Stuart – Prower factor Factor XI: Plasma thromboplastin antecedentFactor XI: Plasma thromboplastin antecedent Factor XII: Hegman factorFactor XII: Hegman factor Factor XIII: Fibrin stabilizing factor Factor XIII: Fibrin stabilizing factor

SEQUENCE OF CLOTTING SEQUENCE OF CLOTTING MECHANISMMECHANISM

Normally during circulation, the blood Normally during circulation, the blood does not clot, because the enzymes does not clot, because the enzymes involved in clotting are in inactive form. involved in clotting are in inactive form. Slight initial activation causes clotting in Slight initial activation causes clotting in which each enzyme activates another one which each enzyme activates another one in a sequential manner till the conversion in a sequential manner till the conversion of fibrinogen into fibrin. In general, of fibrinogen into fibrin. In general, clotting occurs in 3 stages namely:clotting occurs in 3 stages namely:

Formation of prothrombin activator.Formation of prothrombin activator. Conversion of prothrombin into thrombin.Conversion of prothrombin into thrombin. Conversion of fibrinogen into fibrin.Conversion of fibrinogen into fibrin.

During the process of blood clotting, the clotting During the process of blood clotting, the clotting factors, which are in inactive forms, are factors, which are in inactive forms, are converted into active forms. And their converted into active forms. And their enzymatic actions produce the successive enzymatic actions produce the successive reactions one after another in a cascading reactions one after another in a cascading manner.manner.

Thus, the various reactions involved in blood Thus, the various reactions involved in blood clotting are explained by Enzyme cascade clotting are explained by Enzyme cascade theory. theory.

Stage 1 : Formation of Stage 1 : Formation of prothrombin activatorprothrombin activator

Prothrombin activator is formed in two ways Prothrombin activator is formed in two ways namely: namely:

1.1. Extrinsic pathway:Extrinsic pathway: In this, the formation of In this, the formation of prothrombin activator is initiated by the prothrombin activator is initiated by the tissue thromboplastin.tissue thromboplastin.

2.2. Intrinsic pathwayIntrinsic pathway: In this, the formation of : In this, the formation of prothrombin activator is initiated by prothrombin activator is initiated by platelets, which are within the blood itself.platelets, which are within the blood itself.

EXTRINSIC PATHWAYEXTRINSIC PATHWAY

Tissue traumaTissue trauma↓ ↓

Tissue thromboplastinTissue thromboplastin(Protein, Glycoprotein, Phospholipid)(Protein, Glycoprotein, Phospholipid)

Factor VII→ ↓Factor VII→ ↓ Factor X → Activated Factor XFactor X → Activated Factor X

Calcium→ ↓ ← Factor V←ThrombinCalcium→ ↓ ← Factor V←Thrombin

Prothrombin ActivatorProthrombin Activator

INTRINSIC PATHWAYINTRINSIC PATHWAYTissue traumaTissue trauma

↓↓

Factor XII → Activated Factor XIIFactor XII → Activated Factor XII

kininogen→ ↓ ←Prekallkreinkininogen→ ↓ ←Prekallkrein

Factor XI →Activated Factor XIFactor XI →Activated Factor XI

Calcium → ↓Calcium → ↓

Factor IX → Activated Factor IX Factor IX → Activated Factor IX

Calcium → ↓ ←Factor VIIICalcium → ↓ ←Factor VIII

Factor X → Activated Factor XFactor X → Activated Factor X

Calcium → ↓ ← Factor V← Thrombin Calcium → ↓ ← Factor V← Thrombin

↓ ↓

Prothrombin ActivatorProthrombin Activator

Stage 2 : Conversion of Stage 2 : Conversion of prothrombin into prothrombin into

thrombinthrombin Prothrombin activator converts Prothrombin Prothrombin activator converts Prothrombin

into Thrombin in the presence of Calcium. into Thrombin in the presence of Calcium. Thrombin itself can accelerate this reaction Thrombin itself can accelerate this reaction by positive feedback mechanism. That is, the by positive feedback mechanism. That is, the initial Thrombin activates Factor V. This in initial Thrombin activates Factor V. This in turn accelerates the formation of both turn accelerates the formation of both Extrinsic & Intrinsic Prothrombin activator.Extrinsic & Intrinsic Prothrombin activator.

Stage 3 : Conversion of Stage 3 : Conversion of Fibrinogen into FibrinFibrinogen into Fibrin

During this, the soluble Fibrinogen is converted During this, the soluble Fibrinogen is converted

into insoluble Fibrin by Thrombin. Initially, the into insoluble Fibrin by Thrombin. Initially, the fibrinogen is converted into activated fibrinogen, fibrinogen is converted into activated fibrinogen, which is called fibrin monomer. This polymerizes which is called fibrin monomer. This polymerizes with other monomer molecules to form Fibrin.with other monomer molecules to form Fibrin.

The first formed fibrin contains loosely arranged The first formed fibrin contains loosely arranged strands. This is modified later into a dense tight strands. This is modified later into a dense tight aggregate by fibrin stabilizing factor (Factor XII) & aggregate by fibrin stabilizing factor (Factor XII) & this reaction requires the presence of calcium this reaction requires the presence of calcium ions. ions.

FIBRINOGENFIBRINOGEN

Thrombin → ↓Thrombin → ↓

ACTIVATED FIBRINOGENACTIVATED FIBRINOGEN

↓ ↓ polymerizationpolymerization

FIBRIN – loose strandsFIBRIN – loose strands

Factor XIII→ ↓ ←CalciumFactor XIII→ ↓ ←Calcium

FIBRIN – tight clotFIBRIN – tight clot

ANTICOAGULANTSANTICOAGULANTS

The substances, which prevent or postpone The substances, which prevent or postpone coagulation of blood, are called coagulation of blood, are called anticoagulants. Some substances are used anticoagulants. Some substances are used to prevent clotting inside the body i.e., in to prevent clotting inside the body i.e., in vivo & some substances used to prevent vivo & some substances used to prevent clotting outside the body i.e., in vitro.clotting outside the body i.e., in vitro.

The various anticoagulants The various anticoagulants are as follows:are as follows:

HeparinHeparin DicoumoralDicoumoral WarfarinWarfarin EDTA(ethylenediaminetetraacetic acid)EDTA(ethylenediaminetetraacetic acid) Oxalate compoundsOxalate compounds Citrates(sodium, ammonium or potassium Citrates(sodium, ammonium or potassium

citrate)citrate)

Anticoagulants are used:Anticoagulants are used: For the prophylaxis or treatment of For the prophylaxis or treatment of

thromboembolic disorders.thromboembolic disorders. For preservation of stored whole blood & For preservation of stored whole blood &

blood fractions (heparin & acid citrate blood fractions (heparin & acid citrate dextrose).dextrose).

To keep laboratory blood specimens from To keep laboratory blood specimens from clotting (EDTA, citrate, oxalate & fluoride). clotting (EDTA, citrate, oxalate & fluoride).

Tests for Clotting:Tests for Clotting:

1.1. Bleeding TimeBleeding Time

2.2. Clotting TimeClotting Time

3.3. Prothrombin TimeProthrombin Time

MICROVASCULATUREMICROVASCULATURE The blood vessels are closed circuits for the The blood vessels are closed circuits for the

transport of blood from the left heart to the transport of blood from the left heart to the metabolizing cells, & then back to the right metabolizing cells, & then back to the right heart. The blood containing oxygen, heart. The blood containing oxygen, nutrients & metabolites is routed through nutrients & metabolites is routed through ARTERIESARTERIES, , ARTERIOLESARTERIOLES, , CAPILLARIESCAPILLARIES, , VENULESVENULES & & VEINSVEINS. These blood vessels . These blood vessels differ from each other in their structure & differ from each other in their structure & function. function.

Blood flows from the heart to arteries, which Blood flows from the heart to arteries, which branch & narrow into arterioles. The arterioles branch & narrow into arterioles. The arterioles give rise to metarterioles, which give rise to give rise to metarterioles, which give rise to capillaries. After the tissue has been perfused, capillaries. After the tissue has been perfused, capillaries drain into venules , which are then capillaries drain into venules , which are then widen more to become veins, & which return widen more to become veins, & which return blood to the heart. blood to the heart.

The microcirculation of each organ is The microcirculation of each organ is specially organized to serve that organ’s own specially organized to serve that organ’s own special needs – i.e., transport of nutrients to special needs – i.e., transport of nutrients to the tissues & removal of cellular excreta.the tissues & removal of cellular excreta.

The “capillary bed” is the network of The “capillary bed” is the network of capillaries supplying an organ.capillaries supplying an organ.

The capillaries are extremely thin The capillaries are extremely thin structures with walls of a single layer of structures with walls of a single layer of highly permeable endothelial cells. Here highly permeable endothelial cells. Here interchange of nutrients and cellular interchange of nutrients and cellular excreta occurs between the tissues & the excreta occurs between the tissues & the circulating blood.circulating blood.

The walls of the arteries, arterioles & The walls of the arteries, arterioles & small venules contain relatively large small venules contain relatively large amounts of smooth muscle. The openings amounts of smooth muscle. The openings of the capillaries are guarded by muscular of the capillaries are guarded by muscular precapillary sphincters. precapillary sphincters.