Lecture 9: Vascular Disorders and Thrombosis: Hemostasis and Fibrinolysis

Define hemostasis and its primary function.

Hemostasis is the arrest of bleeding. It is the body's response to vascular damage, sealing injured vessels to prevent blood loss.

Why must hemostasis be closely regulated?

Poor regulation can lead to hypercoagulable or hypocoagulable states, resulting in bleeding or thromboses.

What are the three primary "players" in hemostasis?

Endothelial cells, platelets, and coagulation factors.

Describe the role of normal (intact) endothelium in hemostasis.

Provides a smooth, non-thrombogenic surface for laminar blood flow, releases vasodilators and coagulation inhibitors to maintain circulation.

How does damaged endothelium contribute to hemostasis?

Becomes procoagulant—promotes vasoconstriction, platelet adhesion and aggregation, and activation of coagulation factors.

What are platelets, and how are they formed?

Platelets are anucleate cell fragments (nucleated in some exotic species) derived from megakaryocytes in the bone marrow via shearing.

What are coagulation factors, and where are they produced?

Plasma proteins, mostly produced by the liver, that undergo enzymatic transformations to become active in clot formation.

List the four goals of hemostasis.

1. Vasoconstriction of the damaged vessel.

2. Formation of a temporary platelet plug.

3. Activation of the coagulation cascade to form a stable fibrin clot.

4. Fibrinolysis and vessel repair after healing.

What is the goal of primary hemostasis?

To constrict the damaged vessel and form a temporary platelet plug to immediately stop or reduce bleeding.

What initiates vasoconstriction in primary hemostasis?

Neurogenic stimuli and mediators released by endothelial cells and platelets.

What factors determine the degree of vasoconstriction during primary hemostasis?

Vessel size, amount of smooth muscle, and endothelial integrity.

Name the key players in primary hemostasis.

- Exposed subendothelial collagen

- Endothelial cells

- von Willebrand Factor (VwF)

= Platelets (ADP, thromboxane A₂)

- Fibrinogen (liver)

How do platelets initially adhere to the site of injury?

Platelets bind directly to exposed subendothelial collagen (negatively charged) with the help of VwF, which forms a bridge between collagen and platelet GPIb receptors.

What happens if platelet aggregation does not proceed beyond initial adhesion?

Platelets may disaggregate without further stabilization.

How is platelet aggregation propagated after adhesion?

Release of thromboxane A₂ and tissue factor promotes further platelet recruitment and activation.

What is the role of ADP in primary hemostasis?

ADP released from platelet granules triggers fibrinogen binding to GPIIb-IIIa receptors, forming fibrinogen bridges between platelets to create loose aggregates.

How does fibrinogen strengthen the platelet plug in primary hemostasis?

Fibrinogen crosslinks platelets via GPIIb-IIIa, forming a denser platelet plug with small amounts of fibrin.

Place the four stages of hemostasis in order.

1. Primary hemostasis - Vasoconstriction and platelet plug formation.

2. Secondary hemostasis - Coagulation cascade forming fibrin mesh.

3. Fibrinolysis - Controlled removal of clot to allow healing.

4. Tissue repair - Permanent repair via fibrosis or re-endothelialization.

What is the ultimate goal of secondary hemostasis?

To enzymatically convert fibrinogen (Factor I) into fibrin, creating a stable, polymerized meshwork that interlocks platelets.

Why is fibrinolysis important even during hemostasis?

Prevents excessive clot formation and begins the process of removing the platelet plug once vascular integrity is restored.

How does tissue repair conclude the hemostatic process?

By permanently repairing the damaged site, which may involve fibrosis or simple re-endothelialization depending on injury severity.

What is the goal of secondary hemostasis?

To form a stable fibrin plug that reinforces the platelet plug and ensures lasting hemostasis.

Why is fibrin necessary in most cases of vascular injury?

A platelet plug alone is insufficient for long-term vessel sealing; fibrin provides mechanical strength and stability.

What are the three main segments of the classical coagulation cascade?

Intrinsic pathway, extrinsic pathway, and common pathway.

What initiates the extrinsic pathway?

Exposure of tissue factor (Factor III) from perivascular cells (fibroblasts) or microparticles from activated endothelium, platelets, monocytes, and apoptotic cells.

How is Factor VII activated in the extrinsic pathway?

Tissue factor binds calcium to form a tissue factor-VII complex, converting VII → VIIa.

What is the next step after VIIa formation in the extrinsic pathway?

VIIa, with calcium, activates Factor X to Xa, initiating the common pathway.

Why can't the extrinsic pathway alone produce enough thrombin for fibrin formation?

It generates limited thrombin; the intrinsic pathway is needed for sufficient amplification.

What initiates the intrinsic pathway of hemostasis?

Activation of Factor XII (Hageman factor) by contact with negatively charged surfaces, leading to sequential activation of Factors XI, IX, and VIII.

What is the basic order of factor activation in the classical coagulation pathway?

Intrinsic: XII → XI → IX (with VIII) → X

Extrinsic: III + VII → VIIa → X

Common: X (with V) → II (prothrombin) → IIa (thrombin) → I (fibrinogen) → fibrin → XIII cross-linking

What is the main enzymatic action of thrombin in hemostasis?

Cleaves fibrinopeptides from fibrinogen to form fibrin monomers that polymerize into insoluble fibrin strands.

Which factor catalyzes covalent cross-linking of fibrin?

Factor XIIIa, which strengthens the clot along with platelet contraction.

Which coagulation pathway results in greater thrombin production?

The intrinsic pathway, due to its role in amplifying and propagating coagulation factor activation.

How does thrombin "grease the wheel" in coagulation?

By promoting further activation of multiple coagulation factors, increasing the efficiency and speed of fibrin generation.

Which coagulation factors are Vitamin K-dependent?

Factors II, VII, IX, and X.

How is the intrinsic pathway initiated?

Binding of high molecular weight kininogen (HMWK), Factor XII, and prekallikrein to the endothelial surface, leading to activation of Factor XII → XIIa.

After Factor XIIa activation, what is the sequence of events in the intrinsic pathway?

XIIa activates XI → XIa, which activates IX (with VIIIa) → IXa-VIIIa complex activates Factor X.

Why is the intrinsic pathway more potent than the extrinsic pathway alone?

It produces much larger quantities of thrombin by amplifying coagulation factor activation beyond what tissue factor-mediated activation can achieve alone.

What is the purpose of tissue factor (Factor III) in hemostasis?

To initiate the extrinsic coagulation pathway by forming a calcium-dependent complex with Factor VII, leading to activation of Factor X.

What stimuli can cause endothelial cells to produce tissue factor?

Endotoxin, TNF, IL-1, TGF-β, and thrombin.

What is the process of clot dissolution called?

Thrombolysis.

What is the most important part of thrombolysis?

Fibrinolysis - the enzymatic breakdown of fibrin.

Which plasma protein is cleaved to initiate fibrinolysis?

Plasminogen.

What enzyme converts plasminogen to plasmin after vessel injury?

Tissue plasminogen activator (tPA).

Besides tPA, what other activators can cleave plasminogen to plasmin?

Extracellular matrix and fluid activators, including proteases like Factor XIIa.

What is the function of plasmin in fibrinolysis?

To degrade fibrin into fibrinogen degradation products (FDPs).

What are FDPs?

Various-sized fragments of fibrin and fibrinogen produced during fibrinolysis.

How do FDPs impair hemostasis?

They inhibit thrombin, interfere with fibrin polymerization, and coat platelet membranes to reduce coagulation efficiency.

What are the two primary inhibitors of fibrinolysis?

Plasminogen activation inhibitor-2 (PAI-2) and alpha-2 antiplasmin.

What is the function of PAI-2?

Inhibits conversion of plasminogen to plasmin, reducing fibrinolysis.

What is the function of alpha-2 antiplasmin?

Directly binds and inactivates plasmin to prevent fibrin breakdown.

Name three major coagulation inhibitor systems that indirectly regulate fibrinolysis.

- Protein C-Protein S-Thrombomodulin system.

- Endothelial heparan sulfate.

- Antithrombin III and Tissue Factor Pathway Inhibitor (TFPI)

How does the Protein C-Protein S-Thrombomodulin system work?

Vitamin K-dependent proteins C and S degrade Factors Va and VIIIa, reducing thrombin generation.

What is the role of TFPI (Tissue Factor Pathway Inhibitor)?

Inhibits the extrinsic pathway by binding Factor Xa and suppressing thrombin formation.

Where is TFPI produced?

Endothelial cells and smooth muscle cells.

When does TFPI substantially inhibit the extrinsic coagulation pathway?

Only after Factor Xa levels rise.

What is the most potent endogenous anticoagulant?

Antithrombin III (ATIII).

Where is Antithrombin III produced?

Hepatocytes and endothelial cells.

Which coagulation factors does ATIII degrade?

All coagulation factors except Factor VIIa.

What additional functions does ATIII have besides degrading clotting factors?

Neutralizes thrombin and Xa, inhibits fibrinolysis, kinin formation, and complement activation.

What are the two possible outcomes when coagulation is dysregulated?

Too much coagulation: Excessive thrombosis.

Too little coagulation: Hemorrhage

What is disseminated intravascular coagulation (DIC)?

A pathological state with abnormal and excessive thrombin and fibrin generation, increased platelet aggregation, and coagulation factor consumption, leading to systemic thromboses and hemorrhage.

List major causes of DIC.

- Obstetric events (placental abruption, abortion, retained dead fetus/products, amniotic fluid embolism).

- Infection, especially Gram-negative sepsis.

- Cancer.

- Shock (ischemic injury → tissue factor release)

- Severe tissue damage (head trauma, burns, frostbite).

- Snake envenomation.

- Intravascular hemolysis

What pathophysiologic mechanisms lead to DIC in most cases?

Increased tissue factor production, direct endothelial injury, or both.

How do petechiations differ from ecchymoses grossly?

Petechiations: Small pinpoint hemorrhages.

Ecchymoses: Larger, "paintbrush-like" hemorrhages

What do petechiations and ecchymoses generally indicate?

Coagulation abnormalities, thrombocytopenia, or vascular fragility.

What is thrombin's primary role in coagulation?

Acts as a procoagulant by converting fibrinogen to fibrin.

How can excessive thrombin act as an anticoagulant?

By destroying Factors V and VIII.

How does a prothrombotic state influence inflammation?

A prothrombotic environment promotes a proinflammatory environment.