Lecture 5

Hemostasis and Platelets

Hemostasis: Prevention of Blood Loss

  • Hemostasis is the process of preventing blood loss.
  • Platelets play a crucial role in hemostasis.
  • Platelets are found in the buffy coat, mixed with white blood cells.
Why Hemostasis is Necessary
  • Pro-hemostatic factors: Prevent blood loss.
  • Anti-hemostatic factors: Maintain blood fluidity.
  • The body aims to maintain a balance for smooth blood circulation without unnecessary clot formation.
Key Steps of Hemostasis
  • Step 1: Vasoconstriction (Vascular Spasm)
    • A ruptured blood vessel contracts to minimize blood loss.
    • Triggers:
      • Pain receptor stimulation activates nerve endings.
      • Injury to blood vessel smooth muscle.
      • Local injury to platelets releases serotonin.
    • Serotonin acts as a vasoconstrictor.
  • Step 2: Primary Hemostasis (Platelet Plug Formation)
    • Platelets aggregate to form a platelet plug or white thrombus (colorless cells).
  • Step 3: Secondary Hemostasis (Blood Clotting/Coagulation)
    • If bleeding continues, blood clotting enzymes are activated.
    • A stronger, gel-like clot (red thrombus) is formed.
Platelet Origin
  • Platelets, also known as thrombocytes, originate from pluripotent stem cells in the bone marrow.
  • Pluripotent stem cells convert into megakaryocytes.
  • Platelets are pinched off from the cytoplasm of megakaryocytes.

Platelet Structure and Function

Structure of Platelets
  • Platelets contain:
    • Organelles
    • Alpha granules and dense granules
    • Glycogen for energy
    • Contractile proteins (actin and myosin)
    • Surface glycoproteins (receptors)
    • Canaliculi
  • Platelets do not contain a nucleus.
Contents of Platelet Granules
  • Alpha granules:
    • Large molecules, including:
      • von Willebrand factor (adhesive protein)
      • Growth factors
      • Blood clotting factors
      • Cytokines
  • Dense granules:
    • Small molecules, including:
      • ADP and ATP
      • 5-hydroxytryptamine (serotonin)
      • Calcium

Platelet Plug Formation (Primary Hemostasis)

Step 1: Adhesion
  • Platelets adhere to a surface.
  • Normally, they do not stick to the smooth endothelium of blood vessels.
  • Injury disrupts the endothelial layer, exposing collagen.
  • Platelets adhere to collagen using von Willebrand factor.
  • von Willebrand factor is secreted by platelets and endothelial cells.
  • von Willebrand factor changes conformation and binds to platelets, forming a bridge between the damaged vessel wall and platelets.
Step 2: Activation of Platelets
  • Binding to collagen triggers the release of chemicals (ADP and serotonin) from storage granules.
  • ADP and serotonin act locally to change:
    • Metabolism
    • Shape
    • Receptor expression
Step 3: Aggregation of Platelets
  • New platelets adhere to old ones via a positive feedback effect, rapidly forming a platelet plug.
The Platelet Plug
  • von Willebrand factor (secreted by platelets and endothelial cells) binds to exposed collagen in the damaged blood vessel wall.
  • von Willebrand factor changes conformation and binds to platelets, bridging the vessel wall and platelets.
  • Activated platelets express a fibrinogen receptor.
  • The receptor binds to fibrinogen (a plasma protein) and other platelets, forming a lattice-like structure.
  • Activated platelets secrete thromboxane A2 and ADP, which attract more platelets to the cut site, continuing aggregation.
  • The plug contracts using actin and myosin to tighten and seal the cut site.
Roles of Activated Platelets
  • Adhesion triggers activation.
  • Activated platelets secrete:
    • Serotonin (5HT) and ADP:
      • Serotonin: Vasoconstrictor.
      • ADP: Promotes further platelet aggregation.
    • Thromboxane A2:
      • Promotes further platelet aggregation.
      • Causes vasoconstriction of vascular smooth muscle, reducing blood flow to the cut site.
    • Phospholipids exposed on the surface aid in converting prothrombin to thrombin, which promotes further platelet aggregation.

Factors Affecting Platelet Plug Formation

Limiting Plug Expansion
  • Why doesn't the plug expand along undamaged endothelium?
    • Adjacent undamaged endothelial cells synthesize and release prostacyclin and nitric oxide.
      • Prostacyclin (prostaglandin I2 or PGI2) inhibits platelet aggregation.
      • Nitric oxide (NO) inhibits platelet adhesion, activation, and aggregation.
Effects of Arachidonic Acid Metabolites
  • Inflammation occurs rapidly after injury.
  • Chemicals released from the blood vessel wall initiate platelet plug formation.
  • Membrane damage initiates the production of arachidonic acid from membrane phospholipids.
  • Lipoxygenase pathway:
    • Arachidonic acid is converted to leukotrienes by lipoxygenase.
    • Leukotrienes initiate inflammatory responses (swelling).
  • Cyclooxygenase pathway:
    • Arachidonic acid is converted to a prostaglandin by cyclooxygenase (COX).
    • Platelets and endothelial cells have different isoforms of COX, resulting in varying hemostatic responses.
    • Prostaglandins play a role in hemostatic effects that prevent blood loss.
    • Generally, the acute inflammatory response precedes hemostatic effects.
Effect of Aspirin on Hemostasis
  • Aspirin is prescribed to prevent clot formation in patients at risk of heart attacks.
  • Cyclooxygenase pathway:
    • Arachidonic acid is produced from membrane phospholipids due to membrane rupture and injury.
    • Two isoforms of cyclooxygenase enzyme: COX 1 and COX 2.
    • Healthy endothelial cells:
      • Subject to COX 2 enzyme activity.
      • COX 2 activity results in the synthesis of prostacyclin, which has an anti-hemostatic effect.
      • Prostacyclin keeps the plug from expanding to adjacent undamaged endothelial cells.
    • Platelets:
      • Subject to COX 1 enzyme activity.
      • COX 1 produces thromboxane A2, which has a pro-hemostatic effect (favors platelet plug formation).
  • Aspirin inhibits both COX-1 and COX-2.
    • Inhibition of COX-1 in platelets completely blocks thromboxane A2 production.
    • Platelets lack nuclei and cannot synthesize new COX 1 enzyme, so the effect is prolonged.
    • Healthy endothelial cells can eventually synthesize new COX-2 enzymes despite aspirin.
    • These COX-2 enzymes convert arachidonic acid to prostacyclin.
  • Aspirin can selectively block the COX 1 pathway (pro-hemostatic).
  • COX 2 pathways can overcome aspirin's inhibition to make prostacyclin, maintaining vasodilation and decreasing platelet aggregation.
  • Low-dose aspirin is used in patients at increased risk of heart attacks to prevent clot formation.