Hemostasis: Platelets and Clotting Mechanisms

• Hemostasis maintains a fluid, clot-free state in normal vessels, while ensuring rapid, localized clot formation during vascular injury.
• The lecture focuses on the roles of platelets, endothelium, and von Willebrand factor in clotting, with coagulation factors discussed separately.

Platelets: Key Players in Hemostasis

• Platelets are cytoplasmic fragments from megakaryocytes in bone marrow, essential for hemostasis.
• Megakaryocytes remain in the bone marrow, releasing small pieces of cytoplasm into circulation as platelets.
• Platelets are anucleated and contain granules storing key molecules for clotting, appearing purplish-pink on stained blood smears.

Platelet Granules and Their Contents

• Platelets contain specialized storage compartments called granules, contain various substances essential for their activation and function during clot formation.
• Dense Granules: Contain ADP and calcium, crucial for platelet activation and aggregation.
  • ADP promotes platelet aggregation by binding to P2Y12 receptors, amplifying the response to vascular injury.
  • Calcium is essential for platelet shape change, aggregation, and coagulation cascade activation.
• Alpha Granules: Contain clotting factors, modulatory factors, and components for wound healing and inflammation.
  • Clotting factors include fibrinogen and von Willebrand factor (vWF).
  • Modulatory factors include platelet-derived growth factor (PDGF) for wound healing.
  • Fibrinogen is converted to fibrin by thrombin, forming a mesh that stabilizes the platelet plug.
  • vWF helps platelets adhere to exposed collagen and acts as a carrier for factor VIII.
• Granule release amplifies the hemostatic response, ensuring clot growth and strength, contributing to the structural integrity by providing clotting factors and growth factors stimulating regeneration of blood vessels and tissue repair.

Platelet Surface Receptors

• Glycoprotein receptors on platelets facilitate interaction with clotting factors, playing a major role in adhesion, activation, and aggregation, including GPIIB/IIIa and GPIb
• vWF binds to GPIb on platelets, allowing adhesion to damaged vessel walls which is essential for initial platelet capture at the injury site.
• Activated GPIIb/IIIa receptors bind fibrinogen, connecting adjacent platelets and forming a platelet plug.

Role of Endothelial Cells in Hemostasis

• Endothelial cells lining blood vessels regulate hemostasis by preventing unnecessary clotting.
• Key anticoagulant functions:
  • Secrete Tissue Plasminogen Activator (tPA) to activate fibrinolysis.
  • Produce Vasodilators (Prostacyclin & Nitric Oxide) to keep blood vessels open.
  • Secrete Adenosine Diphosphatase (ADPase) to prevent platelet aggregation.
  • Express Thrombomodulin to activate Protein C, inactivating clotting factors.
  • Express Heparin-like Molecules to bind to Antithrombin III, inhibiting thrombin.
  • Produce Tissue Factor Pathway Inhibitor (TFPI) to prevent activation of the extrinsic coagulation pathway.

Hemostasis Process: Response to Injury

• Vascular Spasm (Neurogenic Reflex & Vasoconstriction) occurs upon injury to minimize blood loss which is caused by neural reflexes and vasoconstrictive factors.

Primary Hemostasis: Formation of the Platelet Plug

• Platelet Adhesion:
  • Platelets bind to von Willebrand factor (vWF) via Glycoprotein Ib (GpIb) receptor connecting platelets to exposed subendothelial collagen at the injury site.
• Platelet Activation:
  • Platelets change shape, increasing their surface area, and release ADP and thromboxane A₂ to recruit more platelets.
• Platelet Aggregation:
  • Platelets bind to each other using Glycoprotein IIb/IIIa (GpIIb/IIIa) receptors with Fibrinogen acting as a bridge between platelets.
• GpIIb/IIIa receptors undergo a conformational change when platelets are activated allowing them to bind fibrinogen facilitating platelet aggregation.
• Phospholipids move from the inner side (cytoplasmic side) of the platelet membrane to the outer surface of the platelet surface, facilitating coagulation factor binding.
• Mnemonic: "One binds first" → GpIb binds to vWF first (Platelet Adhesion).
• Mnemonic: "Two-three" (GpIIb/IIIa) needed for aggregation → Helps platelets stick together via fibrinogen.

Secondary Hemostasis: Formation of a Stable Clot

• Tissue Factor is exposed, activating the coagulation cascade.
• Thrombin is generated, converting fibrinogen to fibrin, forming a stable clot.
• Tissue Factor binds to Factor VII, forming a complex that activates Factor VIIa, beginning the coagulation cascade.
• Factor Xa activates Factor V to form the prothrombinase complex, converting prothrombin into thrombin.

Clot Stabilization and Resolution

• Fibrin mesh strengthens the clot, intertwining and binding platelets together trapping blood cells.
• Fibrinolysis (clot breakdown) occurs to restore normal blood flow.
• Plasminogen is activated by tPA to form plasmin breaking down fibrin into fibrin degradation products (FDPs), which dissolve the clot.