Platelets and Hemostasis

Platelet Essentials - Tiny Clotting Titans

• Origin: Megakaryocytes (bone marrow). Lifespan: 7-10 days.
• Normal Count: 1.5 - 4.5 lakh/µL (or 150,000 - 450,000/µL).
• Structure: Anucleated, discoid. Granules:
  • Alpha (α): Fibrinogen, vWF, PF4.
  • Dense (δ): ADP, ATP, $Ca^{2+}$, Serotonin.
• Key Receptors: GpIb (binds vWF for adhesion), GpIIb/IIIa (binds fibrinogen for aggregation).
• Functions: Primary hemostasis (platelet plug), secondary hemostasis (phospholipid surface), clot retraction.

⭐ Bernard-Soulier syndrome is due to GpIb deficiency, causing impaired platelet adhesion. Glanzmann thrombasthenia is due to GpIIb/IIIa deficiency, causing impaired aggregation.

Primary Hemostasis - The Initial Plug

• Vascular Spasm: Immediate but transient vasoconstriction post-injury.
• Platelet Adhesion:
  • Endothelial damage exposes subendothelial collagen & von Willebrand Factor (vWF).
  • Platelets adhere: GP Ia/IIa to collagen; GP Ib-IX-V to vWF.

  ⭐ vWF is critical, acting as a molecular bridge between platelet GP Ib and exposed subendothelial collagen.

• Platelet Activation & Degranulation:
  • Adhesion triggers shape change (disc to spiny sphere) and degranulation.
  • Release of mediators:
    • ADP (potent activator & aggregator)
    • Thromboxane A₂ (TXA₂) (vasoconstrictor, promotes aggregation)
    • Serotonin (5-HT) (vasoconstrictor)
• Platelet Aggregation:
  • Activated platelets express GP IIb/IIIa receptors.
  • Fibrinogen binds to GP IIb/IIIa, linking adjacent platelets.
  • Forms the initial, unstable primary hemostatic plug.
  • 📌 Mnemonic: GP 1b for vWF Binding; GP 2b/3a for 2 (two) platelets to Aggregate.

Coagulation Cascade - The Fibrin Factory

• Enzymatic cascade amplifying initial hemostatic plug. Goal: Fibrin mesh formation.
• Pathways:
  • Intrinsic Pathway: Activated by subendothelial collagen. Factors XII, XI, IX, VIII. Lab: aPTT.
  • Extrinsic Pathway: Tissue Factor (TF/Factor III) release from injury. Factor VII. Lab: PT.
  • Common Pathway: Factor X activation (Xa). Xa + Va convert Prothrombin (Factor II) to Thrombin (IIa). Thrombin converts Fibrinogen (Factor I) to Fibrin. Factor XIIIa stabilizes clot.
• 📌 Vit K-dependent factors: II, VII, IX, X, Proteins C & S.

⭐ Calcium (Factor IV) is essential for multiple steps, acting as a cofactor for many coagulation factors.

Fibrinolysis & Regulation - Clot Control Crew

• Fibrinolysis: Enzymatic clot breakdown by Plasmin.
  • Plasminogen $\xrightarrow{\text{tPA, uPA}}$ Plasmin.
  • Plasmin degrades fibrin $\rightarrow$ FDPs; D-dimer (specific thrombosis marker).
• Key Regulators (Anticoagulants/Inhibitors):
  • Antithrombin III: Major inhibitor of thrombin (IIa), Xa. Heparin augments.
  • Protein C & S (Vit. K-dep): Inactivate factors Va, VIIIa. Thrombin-thrombomodulin activates.
  • TFPI (Tissue Factor Pathway Inhibitor): Inhibits Xa & TF-VIIa.
  • PAI-1: Plasminogen Activator Inhibitor-1; inhibits tPA, uPA.
  • $\alpha_2$-Antiplasmin: Directly inhibits circulating plasmin.

⭐ D-dimer elevation is crucial in diagnosing DVT, PE, DIC, reflecting ongoing fibrinolysis.

Hemostasis Disorders & Drugs - Clotting Chaos & Cures

• Platelet Disorders:
  • Thrombocytopenia: ITP, TTP (ADAMTS13↓), HUS, DIC (consumptive).
  • Qualitative: Glanzmann (GpIIb/IIIa↓), Bernard-Soulier (GpIb↓ - 📌 Big Suckers).
• Coagulation Disorders:
  • Hemophilia A (FVIII↓), B (FIX↓): X-linked, PTT↑.
  • vWD: Most common inherited; BT↑, PTT variable.
  • Vit K def: FII, VII, IX, X, C, S↓; PT↑.
• Drugs:
  • Antiplatelets: Aspirin (COX inh), Clopidogrel (ADP-R inh).
  • Anticoagulants: Heparin (PTT), Warfarin (PT/INR), DOACs (e.g., Rivaroxaban).
  • Thrombolytics: Alteplase (tPA).

⭐ Fresh Frozen Plasma (FFP) reverses warfarin rapidly; contains all clotting factors.

High‑Yield Points - ⚡ Biggest Takeaways

• Platelet adhesion: GpIb binds vWF. Aggregation: GpIIb/IIIa binds fibrinogen.
• Bernard-Soulier syndrome: GpIb defect (↓ adhesion). Glanzmann thrombasthenia: GpIIb/IIIa defect (↓ aggregation).
• Aspirin inhibits COX-1 (↓ TXA2). Clopidogrel blocks ADP (P2Y12) receptors.
• Vitamin K is vital for factors II, VII, IX, X, Protein C & S.
• Hemophilia A: Factor VIII deficiency. Hemophilia B: Factor IX deficiency. vWD: vWF defect.
• PT assesses extrinsic/common pathways; aPTT assesses intrinsic/common pathways.