HI15 - Hemostasis

Biomedical Sciences IV

What is hemostasis?

Hemostasis is the process of arresting bleeding, involving blood clot formation followed by clot dissolution once the injury is repaired.

What are the 4 coordinated events of hemostasis?

1. Vascular constriction

2. Platelet activation, adhesion, and aggregation → temporary platelet plug

3. Coagulation → fibrin clot (thrombus) formation

4. Dissolution of the clot after tissue repair

What substances secreted by endothelial cells inhibit clot formation?

• Prostacyclin (PGI₂): inhibits platelet function

• Nitric oxide (NO): vasodilator, inhibits platelet activation/aggregation

What endothelial surface molecules inhibit coagulation?

• Heparin sulfate (HS): activates antithrombin III

• Thrombomodulin (TM): shifts thrombin activity from procoagulant to anticoagulant

• Tissue factor pathway inhibitor (TFPI): inhibits TF–VIIa/Xa complex

What is the purpose of vascular constriction in hemostasis?

To immediately reduce blood flow to the injured area.

What mediates vascular constriction?

Endothelin, a potent endothelium-derived vasoconstrictor.

Is vascular constriction permanent?

No, it is transient—bleeding resumes if a clot does not form.

Where are platelets derived from?

Megakaryocytes in bone marrow through endomitosis (nuclear division without cytoplasmic division).

Approximately how many platelets are released from one megakaryocyte?

Around 4,000.

What stimulates platelet production?

Thrombopoietin.

What is the normal platelet count and lifespan?

150,000–400,000/μL; lifespan ≈ 10 days.

What granules are found in platelets?

• Dense granules: Ca²⁺, ATP/ADP, serotonin

• Alpha granules: fibrinogen, von Willebrand factor (vWF), factor V

• Lysosomal granules: hydrolytic enzymes

What are key platelet membrane glycoprotein receptors and their functions?

• GPIb: binds vWF on exposed collagen

• GPIa/IIa: binds directly to collagen

• GPIIb/IIIa: binds free fibrinogen and vWF to promote aggregation

What is von Willebrand factor and where is it produced?

A large glycoprotein made by platelets (alpha-granules) and endothelial cells.

What are the binding sites on vWF?

Collagen, GPIb, GPIIb/IIIa, and Factor VIII.

What is the function of vWF?

Acts as a bridge between platelet receptors (GPIb) and exposed collagen fibers; stabilizes Factor VIII.

What disease results from inherited vWF deficiency?

von Willebrand Disease.

What triggers platelet activation?

• Exposed collagen

• ADP from platelet degranulation

• TXA₂ (thromboxane A₂)

• Thrombin

What occurs during platelet adhesion?

• GPIb–vWF binds to exposed collagen

• GPIa/IIa binds directly to collagen

• Exposure of GPIIb/IIIa allows binding to vWF and fibrinogen

What substances are released during platelet degranulation (secretion)?

ADP, vWF, serotonin, Ca²⁺

What morphological change do activated platelets undergo?

Shape change from discoidal to irregular with long pseudopods → increased surface area for binding.

What is platelet aggregation?

Binding of platelets to each other via GPIIb/IIIa–fibrinogen bridges.

Outline platelet plug formation (3–5 min):

1. Platelet adhesion to collagen (directly or via vWF)

2. Shape change and release of procoagulants (ADP, TXA₂, fibrinogen)

3. Recruitment and aggregation of more platelets → primary hemostatic plug

What is the coagulation process?

A cascade of enzymatic reactions producing an insoluble fibrin clot.

What are the major components of the coagulation cascade?

Glycoproteins (mostly liver-derived) that circulate as inactive zymogens and are activated by proteolysis.

What facilitates coagulation complex assembly?

• Phosphatidylserine (PS) on activated platelet membranes

• Ca²⁺ binding to γ-carboxylated glutamate residues on Factors II, VII, IX, and X

What vitamin is required for γ-carboxylation of coagulation factors?

Vitamin K.

What are the two natural forms of vitamin K?

• K₁ (phyloquinone): from green vegetables

• K₂ (menaquinone): produced by gut bacteria

How does warfarin act as an anticoagulant?

It inhibits vitamin K epoxide reductase (VKOR), blocking regeneration of active vitamin K and thus preventing γ-carboxylation of clotting factors.

What happens in the Initiation (extrinsic) pathway?

• Tissue Factor (TF) binds FVII → TF–FVIIa complex

• Activates FX → FXa

• Starts coagulation, but generates limited Xa.

What occurs in the Amplification (intrinsic) pathway?

• Thrombin activates FV, FVII, FVIII, FXI, FXII

• FIXa + FVIIIa (bound to vWF) → activates FX

• Produces a large burst of thrombin generation (≈4000× amplification).

What occurs in the Propagation (common) pathway?

• FXa converts prothrombin → thrombin

• Thrombin converts fibrinogen → fibrin

• Fibrin monomers polymerize into a mesh, stabilized by FXIIIa cross-links.

List the 3 main steps of clot formation:

1. Release of clotting factors from damaged cells and platelets → platelet plug

2. Cascade reactions → formation of thrombin

3. Conversion of fibrinogen → fibrin → clot trapping blood cells

What two mechanisms terminate clot propagation?

1. Antithrombin (AT III): inhibits thrombin, VIIa, IXa, Xa, XIa; activated by heparin

2. Protein C pathway:

• Thrombomodulin alters thrombin activity

• Activated thrombin → activates Protein C

• Protein C + Protein S → inactivate Va and VIIIa

What is fibrinolysis?

The process of clot dissolution after tissue repair.

What enzymes activate plasminogen to plasmin?

• t-PA (tissue plasminogen activator) – from endothelial cells

• u-PA (urokinase) – from various cells

What is the function of plasmin?

Degrades fibrin and other clotting factors, producing fibrin fragments (D-dimers).

What inhibits fibrinolysis?

• PAI-1 (plasminogen activator inhibitor-1): blocks t-PA

• α₂-antiplasmin (α₂AP): rapidly inactivates free plasmin