Hemostasis Lecture Notes (Module 2, Topic 2.5)

Learning Outcomes

• Describe the processes involved in hemostasis.
• Describe fibrinolysis.

Introduction

• Blood is life-giving but needs mechanisms to prevent excessive loss (hemostasis).
• Blood needs to remain fluid to circulate properly.
• Hemostasis is the body's response to injury in blood vessels.

Importance of Hemostasis

• Improper clotting (e.g., hemophilia) can threaten life.
• Inappropriate clotting can lead to disseminated clotting, cardiovascular issues.

Complexity of Blood Clotting

• Involves control and amplification.
• Uses many steps and substances (e.g., proteins, calcium).
• Proteins involved are mostly produced in the liver.

Three Vital Steps in Hemostasis

1. Vascular Spasm (Vasoconstriction):

   • Stimulated by injury, endothelial cell agents, and pain receptor reflexes.
   • Reduces blood flow, effective in small arteries and arterioles.
   • Can reduce blood loss significantly for 20-30 minutes.

2. Platelet Plug Formation:

   • Platelets adhere to collagen exposed by damaged endothelium.
   • Von Willebrand factor (VWF) enhances platelet-collagen binding.
   • Activated platelets release chemical messengers (serotonin, thromboxane) to enhance spasm and aggregation.
   • Formation of platelet plug by increasing platelet stickiness (positive feedback loop).

3. Coagulation:

   • Conversion of plasma fibrinogen to fibrin, forming a mesh that traps cells and solidifies clot (gel formation).
   • Involves intrinsic and extrinsic pathways, leading to prothrombin activator production.
   • Final clot undergoes retraction and fibrinolysis (clot resolution).

Pathways in Clot Formation

• Intrinsic Pathway:
  • Involves factors within blood, activated by platelets and collagen.
  • Slower due to intermediate steps.
• Extrinsic Pathway:
  • Involves tissue factor (TF) external to blood.
  • Faster, bypasses many steps.
• Both pathways produce prothrombin activator, leading to clot formation.

Clot Retraction and Fibrinolysis

• Clot retraction: Platelets contract, pulling vessel walls together, squeezing fluid out.
• Fibrinolysis: Plasminogen in clot converts to plasmin (fibrin-digesting enzyme) to dissolve clot after healing.

Bleeding Disorders

• Thrombocytopenia: Low platelets causing hemorrhages and skin petechiae.
• Vitamin K Deficiency: Affects liver-produced procoagulants, reduces clotting.
• Hemophilia: Genetic disorder, affects clotting pathway, leads to severe bleeding.
• Thromboembolic Disorders: Unwanted clotting (e.g., thrombosis and embolism).

Anticoagulant Drugs

• Aspirin: Inhibits platelet aggregation, used in cardiovascular disease prevention.
• Heparin: Natural anticoagulant, inhibits thrombin formation, used for DVT and pulmonary embolism.
• Warfarin: Interferes with vitamin K action, used to prevent stroke and thrombosis.

Summary

• Hemostasis involves a sequence of reactions to stop bleeding: vascular spasm, platelet plug formation, and coagulation.
• Following healing, clots contract and dissolve.
• Balance between procoagulants and anticoagulants is crucial.

Review Questions

• Engage with workbook to test understanding of topic 2.5.