Circulatory Disturbances 4: Hemostasis

Shannon Martinson, Feb 2016 http://people.upei.ca/smartinson/ VPM 152 General Pathology

HEMOSTASIS

Learning Objectives

• Understand normal hemostasis

• What are the 3 main components of hemostasis?

• Outline the formation (sequence) of primary and secondary hemostatic plugs

• What are the antithrombotic factors produced by endothelial cells?

• Describe the resolution of hemostatic plugs

HEMOSTASIS

• Refers to the arrest of bleeding Hemostasis

Hemostasis

• Well regulated process

• Keeps blood fluid (clot free) within the blood vessels

• Rapid clot formation (hemostatic plug) occurs with vessel injury

Clotting

• Hemostatic clot is normal in cases of vessel injury

• Thrombosis refers to an inappropriate activation of the hemostatic process

HEMOSTASIS

Three components of hemostasis

Vascular wall / Endothelium

Platelets

Coagulation Cascade

HEMOSTASIS

1

• Arteriolar vasoconstriction

• Endothelium

2

• Primary hemostasis

• Platelets

3

• Secondary hemostasis

• Coagulation cascade

4 • Antithrombotic counter-regulation

Sequence of events in normal hemostasis following vascular injury

HEMOSTASIS

1

• Arteriolar vasoconstriction

• Endothelium • Reflex neurogenic mechanism

• Transient

• Local secretion of endothelin from endothelial cells

HEMOSTASIS

2

• Primary hemostasis

• Platelets • Platelets respond to exposure of subendothelial ECM

• Result = Formation of a primary hemostatic plug

HEMOSTASIS

3

• Secondary hemostasis

• Coagulation cascade

• Coagulation results in formation of a secondary hemostatic plug

HEMOSTASIS

4 • Antithrombotic counter-regulation

• Factors are released to limit the size of the hemostatic plug

HEMOSTASIS

4 • Antithrombotic counter-regulation

• Factors are released to limit the size of the hemostatic plug

Endothelial Cells

HEMOSTASIS

Endothelium modulates hemostasis

Injury is the major initiating event for

thrombosis and coagulation

Anti-thrombotic properties

Pro-thrombotic properties

HEMOSTASIS

Endothelial cells

Anti-thrombotic properties

Antiplatelet

Anticoagulant

Fibrinolytic

• Acts as a barrier

• PGI2

• ADPase

• Heparin-like molecules

• Thrombomodulin

• Tissue factor pathway inhibitor (TFPI)

• Tissue plasminogen activator (tPA)

HEMOSTASIS

ECM

PGI2

ADPase

Antiplatelet

• Barrier to ECM

• Prostacyclin I2

• Adenosine diphosphatase

Inhibit platelet aggregation

Endothelial cells

Anti-thrombotic properties

HEMOSTASIS

Anticoagulant

• Heparin-like molecules

• Bind ATIII

• Thrombomodulin

• Tissue Factor pathway inhibitor

Thrombin

Clotting factors

Heparin like molecules Antithrombin III

Endothelial cells

Anti-thrombotic properties

HEMOSTASIS

Anticoagulant

• Heparin-like molecules

• Bind ATIII

• Thrombomodulin

• Tissue Factor pathway inhibitor

Thrombin

Activates protein C and causes proteolysis of clotting factors

Thrombomodulin

Endothelial cells

Anti-thrombotic properties

HEMOSTASIS

Anticoagulant

• Heparin-like molecules

• Bind ATIII

• Thrombomodulin

• Tissue Factor pathway inhibitor

Tissue factor pathway inhibitor

Complexes and inactivates Tissue Factor 7a and 10a

Endothelial cells

Anti-thrombotic properties

HEMOSTASIS

Fibrinolytic

•Plasminogen activator (tPA)

tPA

Fibrinolytic cascade

Endothelial cells

Anti-thrombotic properties

Activates plasmin

Activated by infectious agents, bacterial endotoxins, plasma mediators, cytokines & direct injury

HEMOSTASIS

Endothelial cells

Pro-thrombotic properties

von Willebrand Factor (vWF)

Tissue Factor (TF)

Plasminogen activator inhibitors (PAIs)

• Synthesize

• Store

• Release

• Secreted by injured endothelium

• ↓Fibrinolysis

• Counteract Plasminogen activators

Activated by infectious agents, bacterial endotoxins, plasma mediators, cytokines & direct injury

• Von Willebrand factor (vWF)

• Produced by endothelium

• Bind surface of platelets

• Activates them → helps them aggregate

• Allows platelets to stick to exposed collagen

• Tissue factor (TF)

• Injured endothelium secrete TF

• Activates extrinsic coagulation cascade

HEMOSTASIS

Endothelial cells

Pro-thrombotic properties

Activated by infectious agents, bacterial endotoxins, plasma mediators, cytokines & direct injury

• Plasminogen activator inhibitors (PAI)

• ↓ fibrinolysis (counteract plasminogen activators → tPA)

HEMOSTASIS

Endothelial cells

Pro-thrombotic properties

HEMOSTASIS

Endothelial Cells

• Stimulates smooth muscles and fibroblasts

Platelet Derived Growth Factor (PDGF)

• Stimulates fibroblasts and angiogenesis

Fibroblast Growth Factor (FGF)

• Modulates vascular (& fibrous) repair

Transforming Growth Factor β (TGF- β)

Also play a role in vascular repair – numerous growth factors are secreted by endothelial cells

• Derived from megakaryocytes; circulate as round, smooth discs

• Play central role in hemostasis

• Form the 1o hemostatic plug covers and seals a small damaged area

• Granules contain mostly pro-coagulants

HEMOSTASIS

Platelets

Platelet granule content – For your information only

HEMOSTASIS

Platelets

Procoagulant Anticoagulant

Thromboxane Adenosine Triphosphate

Phospholipids Protease Nexin II

Adenosine diphosphate Tissue Factor Pathway Inhibitor

Calcium Protein S

Platelet Factor 4

Thrombospondin Vascular repair

Fibrinogen Platelet Derived Growth Factor

Factors V, XI, and XIII β Thromboglobulin

Von Willebrand’s Factor Vascular Endothelial Growth Factor

α2-Antiplasmin and α2-Macroglobulin Transforming Growth Factor β

Plasminogen Activator Inhibitor Epidermal Growth Factor

Serotonin

Vascular injury

Exposure of ECM

Adhesion and shape change

Secretion of granules

Aggregation

HEMOSTASIS

Platelets

1 3 2

3 responses of platelets to vascular injury

HEMOSTASIS

Platelets

1. Adhesion and Shape change

vWF

HEMOSTASIS

Platelets

1. Adhesion and Shape change

vWF

HEMOSTASIS

Platelets

2. Secretion of granules

Calcium ADP

vWF

TxA2

HEMOSTASIS

Platelets

Calcium ADP TxA2

2. Secretion of granules

vWF

Thromboxane A2 induces vasoconstriction and platelet aggregation

• ADP mediates platelet aggregation

• Calcium is required for coagulation cascade

HEMOSTASIS

Platelets

3. Aggregation

Calcium ADP

vWF

TxA2

HEMOSTASIS

Platelets

3. Aggregation

Calcium ADP

vWF

TxA2

TxA2 + ADP → Enlarging platelet aggregate → Primary hemostatic plug

HEMOSTASIS

Platelets Summary

1 3

2

HEMOSTASIS

Platelet Disorders

• Decreased circulating platelets

• Normal <200 x 109 Thrombocytopenia

Diagnosis

History of bleeding

Low platelet counts

Mechanisms

Deficient formation of platelets

Excessive utilization of platelets

Premature destruction of platelets

Estrogen toxicity

Consumptive coagulopathy

Antibodies to platelets

HEMOSTASIS

Platelet Disorders

•Defective platelet function Thrombocytopathy

Diagnosis

History of bleeding

Normal platelet counts

Abnormal platelet function tests

Mechanisms

Defect in Adhesion

Defect in Aggregation

Defect in release of granules

von Willebrand’s disease

HEMOSTASIS

Platelet Disorders

http://vet.uga.edu/ivcvm/courses/VPAT5200/01_circulation/hemorrhage/images/Quiz4.jpg

HEMOSTASIS

Coagulation Cascade

Enzyme

Substrate

Activated coagulation

factor

• An enzymatic cascade • A reaction pathway that occurs on a platelet phospholipid complex and is held

together by calcium ions

Thrombin

Coverts fibrinogen to fibrin

HEMOSTASIS

Coagulation Cascade

Thrombin • Bound to the platelet surface • Converts soluble fibrinogen to fibrin → anchors/ stabilizes the hemostatic plug

HEMOSTASIS

Coagulation Cascade

• TF from injured vessel wall interacts with factor 7

• All factors present in plasma

• Exposure of collagen for activation

Common Pathway

Intrinsic Pathway

Extrinsic Pathway

HEMOSTASIS

Coagulation Cascade

Thrombin

Fibrinogen Fibrin

HEMOSTASIS

Coagulation Cascade

HEMOSTASIS

Coagulation Cascade

Common

CLOT

12

10

7

11

9

5

2

1

Intrinsic Extrinsic

=Thrombin

=Fibrinogen

8

13

TF

HEMOSTASIS

Coagulation Cascade

End result of the cascade:

•Contraction of fibrin-platelet clot

•Reduces size of clot (restores flow)

•Draws damaged vessels edges closer (for healing)

CLOT

12

10

7

11

9

5

2

1

Intrinsic Extrinsic

=Thrombin

=Fibrinogen

8

13

TF

Common

• In general, large hemorrhages or hematomas suggest a coagulation disorder

• Coagulation disorders can by inherited or acquired

• Chronic bleeding from a mucosal surface is more indicative of a platelet deficiency or abnormality

HEMOSTASIS

Coagulation Disorders

Peri-renal hematoma

(See Box 2-5 Zachary and McGavin – Pathologic Basis of Veterinary Disease)

HEMOSTASIS

For your information only Coagulation Disorders - Inherited

HEMOSTASIS

Coagulation Disorders - Acquired

• Vitamin K deficiency

• Affects factors 10, 9, 7 and 2 and proteins C and S

• Liver failure

• Site of synthesis of coagulation factors

Decreased Production of Coagulation Factors

• Disseminated intravascular coagulation

• Sepsis

• Severe trauma or deep burns

• Snake venoms and plant toxins

Increased Use of Coagulation Factors

• Fibrinolytic cascade limits the size and dissolves the thrombus • Primarily by the generation of plasmin (from plasminogen)

• Plasminogen activated by tPA and intrinsic coagulation (factor 12a-kallikrein) • FDPs have anticoagulant activity & used as measure of thrombotic states

HEMOSTASIS

Fibrinolytic System – Anticoagulation

Plasminogen

Plasmin

tPA F12-kallikrein

Fibrin FDPs

• Fibrinolytic cascade limits the size and dissolves the thrombus • Primarily by the generation of plasmin (from plasminogen)

• Plasminogen activated by tPA and intrinsic coagulation (factor 12a-kallikrein) • FDPs have anticoagulant activity & used as measure of thrombotic states

HEMOSTASIS

Fibrinolytic System – Anticoagulation

Questions?