8 Blood ClottingAstud(32 1)

8/3/2019 8 Blood ClottingAstud(32 1)

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Blood coagulation

1 Blood Coagulation


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Hemostasis

2 Blood Coagulation


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Blood clotting

The ability of the body to control (stop) the flow ofblood (bleeding) following vascular injury

Is a complex process by which blood forms clots

3 Blood Coagulation


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Hemostasis:

Hemostasis is defined as a property of circulation

whereby blood is maintained within a vessel and the

ability of the system to prevent excessive blood loss

when injured.

Hemostasis is composed of 3 major stages that occur

in a set order following the loss of vascular integrity.

4 Blood Coagulation


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Hemostasis (cont.):

DAMAGE TO BLOOD VESSEL leads to:

1. Vasoconstriction.

2. The formation of platelet plug.

3. The production of a web of fibrin proteins that

penetrates and surrounds the platelet plug.

5 Blood Coagulation


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Steps in Hemostasis

1. Vascular Constriction:

Immediate constriction of blood vessel

Vessel walls pressed togetherbecome sticky/adherent

to each other

Minimize blood loss

6 Blood Coagulation


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Steps in Hemostasis(Cont.):

2. Platelet Plug formation:

PLATELETS attach to exposed collagen with the presence ofvon Willebrand factor (vWF) and Glycoprotein IbIX

Aggregation of platelets causes release of chemical

mediators (ADP, Thromboxane A2)

ADP attracts more platelets

Thromboxane A2(powerful vasoconstrictor)

* promotes aggregation & more ADP

Leads to formation of platelet plug!

7 Blood Coagulation


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Platelet

In adults, platelets originate in the red bone marrow by

fragmentation of the cytoplasm of mature

megakaryocytes(Gr. Megas, big, + karyon, nucleus, + kytos),

which, in turn, arise by differentiation of megakaryoblasts.

Megakaryoblasts. Megakaryocytes Platelets.

Platelets are repelled from each in absence of blood vessel

damage.

8 Blood Coagulation


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Role of platelets in blood clotting:

Platelets (Thrombocytes) have several functions in blood

clotting:

Form platelet plug at the site of injury Sites of activation of some clotting factors (II, X) Provide the surface on which certain clotting

factors bind (Va, Xa, II, Ca2+)

Sources of some clotting factors (XIII, PL)9 Blood Coagulation


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Role of platelets in blood clotting (cont....):

If platelets are not lysed, blood does not clot

Individuals with thrombocytopenia (low platelets), bleeding

for a long time

Platelets deficiency can be due to many agents

(drugs, some infections, ionizing radiation)

10 Blood Coagulation


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Simplified diagram to illustrate platelet production from Megalocytes.



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Hemostasis: Vasoconstriction & Plug Formation



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Platelets adhesion



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Steps in Hemostasis (cont)

3- Blood Coagulation (clot formation):

Final Step in Hemostasis:

Transformation of blood from liquid to solid

Clot reinforces the plug

Multiple cascade steps in clot formation

Fibrinogen FibrinThrombin



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Clotting Cascade

A cascade is a mechanism in which enzymes activate other

enzymes sequentially usually leading to an amplification of

an initial signal.

Participation of 14 different clotting factors (except for

calcium and thromboplastin, clotting factors are proteins)

These factors circulate as inactive zymogens



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Clotting Cascade (cont)

Factors are designated by a Roman numeral (I, II, III, IV,

V, VI, VII, VIII, X, IX, XI, XII, XIII).

Active forms are usually designated by the letter a afterthe Roman numeral and may also have a different name for

example: Ia/ Fibrin).

Cofactors are needed for many reactions in the cascade

example: Calcium, platelet factor 3 (PF3).



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Coagulation Factors (Cont.)

The intrinsic and extrinsic coagulation pathways are a series

of reactions involve coagulation factors known as

1. Enzyme precursors (zymogens)

2. Non-enzymatic (cofactors)

3. Calcium (Ca ++)

4. Phospholipids (PL).

All coagulation factors normally are present in the plasma, with

PL being provided by platelets.



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Zymogens: Factors II, VII, IX, X, XI, XII, and prekallikrein

NO biologic activity until converted by enzymes to activeenzymes called serine proteases

Cofactors Factors V, VIII, tissue factor, and HMWK



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Clotting Cascade (cont)

The last step of the cascade leads to insoluble fibrin as the

end product.

The reactions leading to fibrin formation can be divided into

the extrinsic, intrinsic and common pathways.

Both pathways are initially independent, then they converge

on common pathway leading to the formation of a fibrin

clot !



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Clotting Cascade

Inactive

Active

CLOT !

Ca2+

PL

Ca2+



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Clotting Cascade

A cascadeis a mechanism in which enzymes activate other

enzymes sequentially usually leading to an amplification of

an initial signal.

Each of these pathways leads to the conversion offactor X

(inactive) to factor Xa(active)



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Hemostasis: Coagulation & Clot Stabilization

Prothrombin Ca++

Fibrinogen

Fibrin

Polymerization



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Intrinsic Pathway:

The formation of clot in response to abnormal vessel wall in

absence of tissue injury is the result ofintrinsic pathway

Begins with the activation offactor XII

(Hageman factor)



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Extrinsic Pathway

Fibrin clot formation in response to tissue injury is the

result ofextrinsic pathway

Requires tissue factors external to blood: Factor III

(Tissue Thromboplastin)

Each of these pathways leads to the conversion offactor

X(inactive) to factor Xa(active)



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Intrinsic Clotting Pathway

Under normal physiological conditions, it is less

significant to hemostasis than extrinsic pathway

Under abnormal physiology (hyperlipidemic states;

bacterial infiltration) activation of thrombosis via intrinsic

clotting cascade



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Intrinsic Clotting Pathway (cont)

The intrinsic pathway requires:

1. The factors VIII, IX, X, XI, and XII2. The proteins: Prokallikrein (PK),

High MW Kininogen (HK)

3. Calcium ions

4. PLs from platelets



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Intrinsic Clotting Pathway (cont)

Initiation of the intrinsic pathway occurs when Prokallikrein

(PK), high MW Kininogen (HK), factorXI, and factor XII are

exposed to a negatively charged surface Contact phase.

Contact phaseoccurred as result of interaction with:

o PLs,

o Circulating lipoprotein particles (VLDL, Chylomicrons)

o On the surface of bacteria



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Clotting Cascade

Inactive

Active

CLOT !

Ca2+

PL

Ca2+


Th i i h i i d h i f i j bF t III i f t i th f t VII hi h t l dF t X i th it t hi h th i t i i d t i iTh fi l th f bl d l tti i l


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Inactive

Active

CLOT !

The extrinsic pathway is triggered at the site of injury bytrauma(serious injury or shock to the body), which activates factor VIIand releases a lipoprotein, called tissuefactor (Factor III), fromblood vessels

Factor IIIis a cofactor in the factor VIIa which catalyzedactivation offactor X (inactive) to factor Xa (active)

Factor Xa is the site at which the intrinsic and extrinsiccoagulation cascade converge

The final common pathway of blood clotting involvesactivation of prothrombin into thrombin



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Coagulation cascade



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thrombin

Prothrombin is soluble single chain glycoprotein (72kDa) synthesizedin liver

Thrombin is produced by the enzymatic cleavage of two sites on

prothrombin by activated Factor X (Xa) and generate active 2 chain

thrombin molecule which is then released from platelet surface The A and B chains of thrombin are held together by a dissulfide

bond

A B

S S

+21

Fragment 2 -1 Active thrombin

(34 kDa)

A B21

S S

Xa Xa

Fragment 2 -1 Prethrombin

Prothrombin(72kDa) Converts fibrinogen to fibrin31 Blood Coagulation


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Thrombin in Hemostasis

Factor Xa



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Factor Va is subsequentlyinactivated by further action

of thrombin to limit activation of prothrombin to

thrombin

The activation of prothrombin occurs on the surface

of activated platelets and requires assembly of

prothrombinase complex consisting of platelet

anionic PLs, Ca2+

, factor Xa and prothrombin

This complex is termed factor Vawhich isactivated by

traces of thrombin

Factor Xa produced by either intrinsic or extrinsic

pathway activates prothrombin (factor II) tothrombin

(factor IIa) which converts fibrinogen to fibrin

Blood Coagulation


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Fibrinogen (Factor I):

Fibrinogen is soluble plasma glycoprotein that consists

of 3 non identical pairs of polypeptides chains (A,

B, )2 covalently linked by disulfide bonds.

It has a molecular weight of 340kDa .

Represent the first coagulation factor.



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The A and B portions of the

A and B chains, termed

Fibrinopeptide A (FPA) and

Fibrinopeptide B (FPB)

Release of FBs by thrombin

generate fibrin monomer (weak)

Thrombin

Aggregate spontaneously forming

insoluble fibrin polymer (fibrin clot)

(hard, insoluble)

Conversion of

Fibrinogen to Fibrin



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Controlof thrombin level:

There are 2 principal mechanisms by which thrombin activity isregulated:

1. The predominant form of thrombin in circulation is the

inactive prothrombin, whose activation requires the pathways

of proenzyme activation (coagulation cascade) At each step

in the cascade, feed back mechanism regulate the balance

between active and inactive enzymes

2. Activation of thrombin is also regulated by 4 specific thrombin

inhibitors (natural inhibitors of blood clotting):



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Natural inhibitors of blood clotting

1- Antithrombin III

Is the most important one: contributes 75% of antithrombinactivity in plasma

It is a plasma protein that inactivates thrombin by forming anirreversible complex with it

It resembles 1-antitrypsin except that it inhibits thrombinmuch more strongly than it inhibits elastase

Also, it blocks other serine proteases in the clotting cascadenamely, factors XIIa, XIa, IXa, and Xa


N l i hibi f bl d l i ( )


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Natural inhibitors of blood clotting (cont)

2.Heparin

The inhibitory action of antithrombin III is enhanced by

heparin

It is a negatively charged polysaccharide found in mast cells

near the walls of blood vessels and on the surfaces of

endothelial cells

Heparin acts as an anticoagulant by increasing the rate of

formation of irreversible complexes between antithrombin

III and the serine protease clotting factors



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Natural inhibitors of blood clotting (cont)

3. Alpha 2-macroglobulins:

Contributes most of the remaining (25%) of antithrombin

activity in plasma

4. Alpha 1-antitrypsin:

Acts as a minor inhibitor under physiological conditions, which

normally inhibits elastase

Alpha 1-Antitrypsin activity normally increases markedly afterinjury to counteract excess elastase arising from stimulated

neutrophils

39Blood Coagulation


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Dissolution of fibrin clot : fibrinolysis(The removal of fibrin from the blood)

Clot is slowly dissolved by the fibrin splitting called

Plasmin

Plasmin gets trapped in clot and slowly dissolves it by

breaking down the fibrin meshwork at various places,

leading to the production of circulating fragments that arecleaved by other proteases or by the kidney and liver.

40Blood Coagulation

Di l i f fib i l fib i l i


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Dissolution of fibrin clot : fibrinolysis

(cont)

Plasminogen is the inactive pre-cursor that is activated by

activators in plasma:

1. Tissue plasminogen activator (t-PA)

2. Urokinase (to lesser extend)

Is produced as a precursor prourokinase by epithelial

cells

Its main action is probably in the degradation of

extracellular matrix

41Blood Coagulation


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Dissolution of fibrin clot : fibrinolysis (cont)

Inactive t-PA is released from vascular endothelial cells following

injury

It binds to fibrin and is consequently activated

Active t-PA converts plasminogen into plasmin

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Dissolves the clot

Blood Coagulation


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Coagulation factor disorders

Inherited bleedingdisorders

Hemophilia A and B

Von Willebrand disease

Other factor deficiencies

Acquired bleedingdisorders

Liver diseaseVitamin K deficiency

43Blood Coagulation


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Coagulation factor disorders (cont)

1- Hemophilia A and BAre the best-known coagulation factor disorders

Hemophilia A Hemophilia B

Coagulation factor deficiency Factor VIII Factor IX

Inheritance X-linked X-linked

recessive recessive

Incidence 1/10,000 males 1/50,000 males

44Blood Coagulation


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2- von Willebrand Disease It is the most common hereditary bleeding disorder and is

characterized as being inherited autosomal recessive or

dominant

In this disease there is a defect in von Willebrand factor

(vWF) which:

1. acts as a carrier for factor VIII

2. mediates the binding of glycoprotein Ib (GPIb) to

collagen


45Blood Coagulation


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2- von Willebrand Disease


This binding helps the activation of platelets and formation

of primary hemostasis

vWD is characterized by excessive bleeding in infants

because platelets fail to form hemostatic plug

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3- Deficiency of Vitamin K

Source of vitamin K Green vegetablesSynthesized by intestinal flora

Required for synthesis Factors II, VII, IX ,Xcontribute to bleeding disorders

Causes of deficiency Malnutrition

Biliary obstructionMalabsorption

Antibiotic therapy

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Function of Vitamin K

Vitamin K is essential for the functioning of several proteins

involved in blood clotting (II, VII, IX and X)

These proteins contain a unique modified glutamate residue,

called carboxyglutamate (Gla).

These proteins are synthesized as inactive precursors that are

activated by the vitamin K-dependent carboxylase which

converts glutamate in these proteins to carboxyglutamate

forming mature clotting factors.

1. Formation of carboxyglutamate

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Function of Vitamin K (cont)1.Formation of carboxyglutamate (cont)

-

49Blood Coagulation


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Function of Vitamin K (cont)

Dicumarol,

Warfarin

- (Gla residue)

(mature)

1.Formation of carboxyglutamate (cont)

50Blood Coagulation


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Function of Vitamin K (cont)

The Gla residue of prothrombin is a natural high affinity binder

(chelator) of positively calcium ions, hence the designation of

calcium as a co-factor (factor IV) in the schematic.

The prothrombin-calcium complex is then able to bind to PLs

essential for blood clotting on the surface of platelets.

2. Interaction of prothombin with platelets

Blood Coagulation

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8 Blood ClottingAstud(32 1)