pathology

Mid Semester Exams

• Discussion on the difficulty level of the mid-semester exams
  • Responses: A bit difficult; manageable
• Results Announcement
  • Results will be posted next week
  • Class reps to communicate results to the rest of the class

Topic: Hemostasis and Thrombosis

Hemostasis

• Definition: The process of blood clot formation at the site of vascular injury.
• Key components needed for blood clot formation:
  • Platelets
  • Clotting factors
  • Exposure of the epithelium
• Purpose: Limits the extent of bleeding.

Sequence of Events Leading to Hemostasis

1. Arteriolar Vessel Constriction

   • Type: Transient
   • Mediated by:
     • Neurogenic mechanisms
     • Endothelin (a potent vasoconstrictor)
   • Purpose: Reduces blood flow to the site of bleeding; bleeding resumes if not followed by subsequent processes.

2. Primary Hemostasis

   • Formation of a platelet plug.
   • Triggered by vascular endothelial damage leading to:
     • Exposure of:
     • Von Willebrand's Factor
     • Collagen
   • Mechanism:
     • Platelet circulation binds to von Willebrand's factor, undergoes activation (shape change to more spiky).
     • Release of secretory granules that contain:
     • Thromboxane A2
     • Adenosine diphosphate (ADP)
     • Role of Thromboxane A2 and ADP:
     • Potent in enhancing platelet aggregation.
     • Act as additional vasoconstrictors.

3. Secondary Hemostasis

   • Formation of a fibrin strand mesh.
   • Triggered by:
     • Exposure of tissue factor (TF) due to endothelial damage.
   • Mechanism:
     • TF activates Factor VII in the extrinsic pathway of the coagulation cascade.
     • Leads to a series of reactions culminating in the generation of thrombin.
   • Functions of thrombin:
     • Converts fibrinogen to insoluble fibrin, creating a fibin meshwork.
     • Activates more platelets, leading to increased platelet aggregation.

4. Clot Stabilization and Resorption

   • Process:
     • The polymerized fibrin and platelet aggregates contract and form a solid, stable clot that prevents further bleeding.
     • Polymerization of fibrin leads to the formation of stable fibrin mesh within the clot.

Counterregulatory Mechanism: Fibrinolysis

• Tissue Plasminogen Activator (tPA): Converts plasminogen to plasmin, which acts on fibrin to break it down.
• Clinical significance: Used in thrombolysis for myocardial infarction, pulmonary embolism, and thrombotic strokes.

Thrombosis

• Definition: A pathological state where there is inappropriate activation of the coagulation cascade, resulting in blood clots that can obstruct blood vessels.
• Thrombus: Abnormal blood clot that can partially or completely occlude a blood vessel.

Virchow's Triad

• Definition: Three conditions that contribute to thrombus formation:

  1. Endothelial Injury
  • Examples of conditions that can cause endothelial injury:
    • Myocardial infarction
    • Ulcerated plaques
    • Inflammatory vascular injury (vasculitis)
  1. Abnormal Blood Flow
  • Stasis vs. Turbulence:
    • Stasis: Slow blood flow (e.g., prolonged immobility).
    • Turbulence: Rapid flow leading to injury.
  • Conditions contributing to stasis:
    • Prolonged immobilization
    • Heart failure
    • Sickle cell disease
    • Polycythemia vera
  1. Hypercoagulable State
  • Conditions that increase thrombus formation:
    • Pregnancy
    • Advanced cancers
    • Nephrotic syndrome
    • Trauma and burns

Thrombus Behavior

1. Propagation

   • Growth of the thrombus due to additional platelet aggregation and fibrin deposition.

2. Embolization

   • Dislodgement of part or all of the thrombus to a distant site within the vasculature.

3. Dissolution

   • Breakdown of the clot through fibrinolysis.

4. Organization

   • The transformation of the clot into fibrous tissue, integrating into the vascular wall.

Types of Embolism

1. Pulmonary Embolism (PE)

   • Major cause of mortality; occurs primarily from deep vein thrombosis.
   • Symptoms include dyspnea, cyanosis, and potential cardiovascular collapse.
   • Risk factors include previous DVT.

2. Fat Embolism

   • Strongly associated with long bone fractures (90% incidence).

3. Air Embolism

   • Gas bubbles that obstruct blood flow; causes ischemia and can result from injuries or medical procedures.
   • Notable conditions:
     • Decompression sickness in divers.
     • Can lead to "the bends" or chokes.

4. Amniotic Fluid Embolism

   • Accounts for 10% of maternal deaths in the U.S.
   • Presents with sudden dyspnea, cyanosis; neurologic complications possible.

Conclusion

• Questions posed in class to encourage participation and reinforce understanding of hemostasis and thrombosis.
• Review of definitions and mechanisms essential for understanding the physiological processes and pathologies associated with hemostasis and thrombosis.