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Haemostasis
h s nagaraja
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Learning outcomes
State hemostasis
Describe hemostatic response
Describe the events in hemostasis Explain the structure and function of platelets
Describe the process of activation of platelets
Describe the intrinsic and extrinsic coagulationmechanism
Explain the anti-clotting systems
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A 35-year old female with
ecchymosis
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A 40-year old male with bleeding
spots
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Inherited Bleeding Disorders
Hemophilia A, B and Von Willebrand Disease
World Hemophilia Day falls
on 17thApril every year and is dedicated
to stepping up awareness of hemophilia
and other bleeding disorders.
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Haemostasis
The process of blood clotting and then the subsequentdissolution of the clot, following repair of the injuredtissue, is termed haemostasis
Haemostasis Normal physiologic process
Maintain blood in fluid, clot free state
Rapid, localized haemostatic plug
At the site of vessel injury
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H
aemostatic response: Primary Phase Blood vessel and platelets
Secondary Phase Coagulation Factors
Good haemostatic response :
i) quick,ii) carefully controlled
iii) localized to damaged region
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Hemostasis, composed of 4 major events that occur
in a set order following the loss of vascular integrity:
1. Vascular Constriction - The initial phase of the
process. This limits the flow of blood to the area ofinjury
2. Platelet plug formation - platelets become activated
by thrombin and aggregate at the site of injury,
forming a temporary, loose platelet plug
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3. Clot Formation - To insure stability of the initially loose
platelet plug, a fibrin mesh (also called the clot) forms
and entraps the plug
If the plug contains only platelets it is termed a white thrombus; if red bloodcells are present it is called a red thrombus
4. Fibrinolysis - The clot must be dissolved in order for
normal blood flow to resume following tissue repair
The dissolution of the clot occurs through the action ofplasmin
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Vascular Spasm
Damage to blood vessel stimulates pain receptors
Reflex contraction of smooth muscle of small blood vessels
Local reflex mechanism
Can reduce blood loss for several hours until other
mechanisms can take over
Vasoconstrictors released from platelets
Only for small blood vessel or arteriole
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PLATELETSSmallest of Formed Elements of Blood
Biconvex Disc; No nucleus; Life span 7-10 DaysNormal Count: 250,000/cu mm
Formed from megakaryocytes in bone marrow
Platelet Factors:
Glycoproteins, phospholipids
Actin, Myosin, Thrombasthenin
Von Willebrand Factor, Factor VIII, Factor V
Thromboxane A2, ADP, PDGF, Fibrinogen , serotonin
2 types of granules; i) Alpha granules contains fibrinogen, thrombospondin, clotting
factors), ii) Dense granules contains calcium, serotonin, ADP, ATP).
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PLATELETS
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Functions of Platelets:
1. Hemostasis Vasoconstriction and Platelet Plug
2. Blood Coagulation Phospholipids
3. For Clot Retraction Contractile Proteins
4. Repairof Ruptured Blood Vessels Growth Factor [PDGF]
5. Release Granules Chemotactic for Neutrophils
6. Defense Mechanism Phagocytic Activity
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The protein fibrinogen is primarily responsible for
stimulating platelet clumping
Platelets clump by binding to collagen that becomes
exposed following rupture of the endothelial lining of
vessels
Upon activation, platelets release adenosine-5'-diphosphate,
ADP and TXA2 (which activate additional platelets),
serotonin, phospholipids, lipoproteins, and other proteinsimportant for the coagulation cascade
In addition to induced secretion, activated platelets change
their shape to accommodate the formation of the plug
PLATELETPLUG
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1. Platelet Adhesion
Platelets stick to exposed collagen underlying damagedendothelial cells in vessel wall
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2. Platelet Release Reaction Platelets activated by adhesion
Extend projections to make contact with each other
Release thromboxane A2, serotonin & ADP activating other platelets
Serotonin & thromboxane A2 are vasoconstrictors decreasing blood flow
through the injured vessel. ADP causes stickiness
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3. Platelet Aggregation Activated platelets stick together and activate new platelets to form a mass
called a platelet plug
Plug reinforced by fibrin threads formed during clotting process
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Platelet Activation
In order for hemostasis to occur, platelets must adhere to
exposed collagen, release the contents of their granules,
and aggregate
The adhesion of platelets to the collagen exposed on
endothelial cell surfaces is mediated by von Willebrand
factor (vWF)
Inherited deficiencies of vWF are the causes of von
Willebrand disease, (vWD)
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The McGraw-Hill
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Shape of platelets:i) stimulated form discoid shape ( 2- 4 Qm)ii) unstimulated state spiny spheric shape
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HAEMOSTATICPLATELET REACTION
VascularInjury
Sub endothelial Collagen
Adhesion
Shape Change
Release Reaction
Vasoconstriction Aggregation
Thrombin
Coagulation
Haemostatic Plug
Fibrin
TXA2, 5HT
TXA2 ADP
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Platelet Aggregation:Membrane Phospholipids
Arachidonic Acid
PGH2
Thrmobaxane A2
Prostacyclin (PGI2)
AGGREGATION
cAMP cytosolic Ca 2+ cAMP cytosolic Ca 2+
Platelets Endothelium
Cyclo oxygenase 1 / 2
Phospholipase C / A2
++
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Blood Clotting
Two pathways
Extrinsic and Intrinsic
Initiated by two distinct mechanisms
Converge on a Common Pathway
Formation of a clot in response to an abnormal vessel
wall in the absence of tissue injury is the result of the
intrinsic pathway
Clot formation in response to tissue injury is the result
of the extrinsic pathway
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Extrinsic Pathway:
A Cellular element outside the blood is needed
Begins with activation of tissue factor
Tissue factor In the walls of blood vessels
Tissue factor binds to factor VII and activates it
Intrinsic Pathway:
Everything necessary for it is in the blood
Activated when plasma comes in contact with constituents ofsub endothelial tissues
Collagen fibrils, negatively charged surfaces
Factor XII, Factor XI, Pre Kalikrein and HMWK are involved
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Factor Trivial Name(s) Pathway
Prekallikrein
(PK) Fletcher factor Intrinsic
High molecular
weight
kininogen
(HMWK)
contact activation cofactor Intrinsic
I Fibrinogen Both
II Prothrombin Both
III Tissue Factor Extrinsic
IV Calcium Both
V Proaccelerin, labile factor Both
VI (same as
Va)Accelerin Both
VII Proconvertin, serum prothrombin conversion accelerator (SPCA) Extrinsic
VIII Antihemophiliac factorA Intrinsic
IX Christmas Factor, antihemophilic factor Intrinsic
X Stuart-ProwerFactor Both
XI Plasma thromboplastin antecedent (PTA) Intrinsic
XII HagemanFactor Intrinsic
XIII Protransglutaminase, fibrin stabilizing factor (FSF), fibrinoligase Both
BLOOD CLOTTINGFACTORS
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Regulatory/Other
ProteinsActivities
vonW
illebrand factor
associated with subendothelial connective tissue; serves as a
bridge between platelet glycoproteinGPIb/IX and collagen
ProteinCactivated to proteinCa by thrombin bound to thrombomodulin; then
degrades factors VIIIa and Va
Protein S acts as a cofactor of proteinC
Thrombomodulinprotein on the surface of endothelial cells; binds thrombin,which
then activates proteinC
AntithrombinIIImost important coagulation inhibitor, controls activities of
thrombin, and factors IXa,Xa,XIa and XIIa
Regulatory Proteins
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ClottingCascade
The intrinsic cascade (which has less in vivo significance than the extrinsic
cascade) is initiated when contact is made between blood and
exposed negatively charged surfaces
The extrinsic pathway is initiated upon vascular injury which
leads to exposure of tissue factor (TF- also identified as factorIII),
a subendothelial cell-surface glycoprotein that binds
phospholipid
The two pathways converge at the activation of factor X to
Xa
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Contact Tissue Factor + VII
XIIIXIIIaa
XIIIXIII
ThrombinThrombin
FibrinFibrin
(strong)(strong)
FibrinogenFibrinogen FibrinFibrin
(weak)(weak)
IXIX
XIXI
XIXIaa
IXIXaa
XXaa
VVaa
XIIXIIaa
ProthrombinProthrombin
TF-VIIa
PLPL
PLPLVIIIVIIIaa
PLPL
XX
Intrinsic Pathway
HKHKaa
Extrinsic Pathway
Common Pathway
TF Pathway
Coagulation PathwaysCoagulation Pathways
Protein C, Protein S,
Antithrombin III
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Intrinsic Pathway
Less significant to hemostasis under normal physiological
conditions than is the extrinsic pathway
Pathway requires the clotting factors VIII, IX, X, XI, and XII
Also required are the proteins prekallikrein (PK) and high-
molecular-weight kininogen (HK orHMWK), as well as
calcium ions and phospholipids secreted from platelets
Initiation of the intrinsic pathway occurs when prekallikrein,high-molecular-weight kininogen, factor XI and factor XII are
exposed to a negatively charged surface - termed the
contact phase
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What is the significance of intrinsic
pathway?
The intrinsic pathway has low significance under normalphysiological conditions
Most significant clinically is the activation of the intrinsic
pathway by contact of the vessel wall with lipoproteinparticles, VLDLs and chylomicrons
This process clearly demonstrates the role ofhyperlipidemiain the generation of atherosclerosis
The intrinsic pathway can also be activated by vessel wallcontact with bacteria
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The McGraw-Hill
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Intrinsic Pathway-
- Is switched on when endothelial cells (i.e. inner-most
lining) of blood vessels are damaged. And this results
in exposure of;
a) Collagen
b) damaged platelets causing the release of
phospholipids
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Extrinsic Clotting Cascade
Activated factor Xa is the site at which the intrinsic and
extrinsic coagulation cascades converge
Pathway is initiated at the site of injury in response to the
release of tissue factor (factorIII) and thus, is also known as
the tissue factor pathway
Tissue factoris a cofactor in the factor VIIa-catalyzed
activation of factor X
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The McGraw-Hill
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Factor VIIa, a serine protease, cleaves factor X to factor Xa in
a manner identical to that of factor IXa of the intrinsic
pathway
The activation of factor VII occurs through the action of
thrombin or factor Xa A major mechanism for the inhibition of the extrinsic
pathway occurs at the tissue factor-factor VIIa-Ca2+-Xa
complex The protein, TFPI and was formerly named anticonvertin,
specifically binds to this complex
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Factor Xa has a role in the further activation of factor VII to
VIIa
Active factor Xa hydrolyzes and activates prothrombin to
thrombin.
Thrombin can then activate factors XI, VIII and V furthering
the cascade
Ultimately the role of thrombin is to convert
fribrinogen to fibrin and to activate factor XIII
toXIIIa
Factor XIIIa (also termed transglutaminase) cross-links fibrin
polymers solidifying the clot
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The common point in both pathways is the activation
of factor X to factor Xa
Factor Xa activates prothrombin (factorII) to
thrombin (factorIIa)
Thrombin, in turn, converts fibrinogen to fibrin
The activation of thrombin occurs on the surface of
activated platelets and requires formation of a
prothrombinase complex [prothrombin activator]
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Final Common Pathway
Prothrombinase and Ca+2
catalyze the conversion of prothrombin to thrombin
Thrombin
in the presence of Ca+2 converts soluble fibrinogen to
insoluble fibrin threads
activates fibrin stabilizing factor XIII
positive feedback cycle
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Clot Retraction & Blood Vessel Repair
Clot plugs ruptured area ofblood vessel
Platelets pull on fibrin threadscausing clot retraction andexpelling serum
Edges of damaged vessel are
pulled together Fibroblasts & endothelial cells
repair the blood vessel
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Role of Vitamin K in Clotting
Normal clotting requires adequate vitamin K
fat soluble vitamin absorbed if lipids are present
absorption slowed if bile release is insufficient
Required for synthesis of 4 clotting factors by
hepatocytes
Produced by bacteria in large intestine
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ANTI - CLOTTING SYSTEMS
Limits clot formation and dissolves clot
Natural anti coagulant mechanisms
Defect in these mechanisms high incidence of
thrombosis [hyper coagulability]
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THROMBIN
PROTEINC ACTIVATOR
THROMBOMODULIN
PROTEINC ACTIVATED PROTEINC
FACTOR Va & VIIIa INACTIVATED
Endothelium
Receptor
1.
PROTEIN S
INACTIVATES INHIBITORS
OFTISSUEPLASMINOGEN
ACTIVATOR [t-PA]
Factor Xa
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2. Anti-thrombinMechanism:
Antithrombin III Plasma and endothelium
Inactivates Thrombin and other clotting factorsHeparin binding to Antithrombin III activation
Prevents spread of clot
FIBRINOLYTIC SYSTEM:
Dissolves clot
Fibrin broken down to fibrin degradation products by Plasmin
Checks and balances coagulation system Prevents intravascular clotting [Thrombosis]
Plasmin formed from inactive precursor Plasminogen
PLASMINOGEN ACTIVATORS
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PLASMINOGEN PLASMIN
FIBRINOGEN FIBRIN
DEGREDATIONPRODUCTSFIBRIN DEGRADATION PRODUCTS
PLASMINOGEN ACTIVATORS
Intrinsic activators:
Kallikrein
XIIa
Extrinsic Activators:
Tissue PlasminogenActivator [t-PA]
Urokinase Type PA [u-PA]
Streptokinase
++
+
+
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Clinical Application
Thrombocytopenia bleeding time increased
Hemophilia hereditary clotting disorder
increased clotting time
Disseminated intravascular clotting
increased bleeding due to depletion of
coagulation factors
Thrombophilia - hypercoagulability
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NORMAL HEMOSTASISNORMAL HEMOSTASIS
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BLOOD VESSELINJURY
VASOCONSTRICTIONPLATELETS
INTRINSIC CLOTTING
EXTRINSIC CLOTTING
ADHESION
AGGREGATION
PLATELET PLUG
HEMOSTATICPLUG
THROMBIN
FIBRIN
PLASMIN
PLASMINOGEN
ANTIPLASMINS
ANTITHROMBINS
NORMAL HEMOSTASISNORMAL HEMOSTASIS
summary
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