II. Definitions

  1. Platelet
    1. Irregular, small ( 3um) disc shaped fragments, pieces of Megakaryocytes in the Bone Marrow
    2. Key component of Blood Clotting

III. Physiology

  1. Platelets are generated in the Bone Marrow at a rate of 10% of total Platelet Count per day
  2. Aggregate at damaged blood vessels to form clot
    1. Cross-bridged to the vessel wall via the Glycoprotein, Von Willebrand Factor
  3. Contain actin and myosin (found in Muscle fibers)
    1. Allow Platelets to contract and close a vessel wall opening
  4. Promote Clot Formation
    1. Attract additional Platelets by Secreting chemoattractants (ADP, Thromboxane A2)
    2. Produce Fibrin stabilizing factor (binds Fibrin into mesh)
    3. Platelet factor 3 (PF3) and phospholipids, contained in the Platelet membrane, promote Clot Formation
  5. Release vasoactive mediators (Arachidonic Acid metabolites, biogenic amines) - especially Vasoconstrictors
    1. Thromboxane A2
      1. Thromboxane A2 activates Platelets, increasing their "stickiness" in Clot Formation
      2. Thromboxane A2 production is dependent on the enzyme cyclooxygenase
    2. PGF2 alpha
    3. Serotonin (5-HT)
  6. Other functionality
    1. Phagocytosis of Antigen-Antibody complexes
  7. Inhibitors of Platelet aggregation and stickiness
    1. Aspirin
      1. Inhibits cyclooxygenase, reducing Thromboxane A2 levels and Clot Formation
    2. Platelet ADP Receptor Antagonist (e.g. Clopidogrel, Prasugrel, Ticagrelor)
      1. Inhibit ADP binding to Platelets and Platelet aggregation
    3. Glycoprotein IIB/IIIA Inhibitor (e.g. Abciximab, Tirofiban, Eptifibatide)
      1. Inhibit Fibrinogen and Von Willebrand Factor binding to activated platelet Glycoprotein IIB/IIIA receptors
      2. Inhibits Platelet aggregation, by inhibiting Platelet binding to one another
    4. Dipyridamole
      1. Platelet Phosphodiesterase Inhibitor (PDE3 inhibitor)
      2. PDE3 would normally breakdown cAMP (Cyclic Adenosine monophosphate)
      3. Excess CAMP buildup inhibits Platelet aggregation (and also results in vasodilation)
  8. Measures of Platelet function
    1. Platelet Function Closure Time
    2. Bleeding Time

IV. Labs: Platelet Count Normal Range

  1. Newborns
    1. Preterm: 100,000 to 300,000 per mm3
    2. Term: 140,000 to 300,000 per mm3
  2. Infant: 200,000 to 475,000 per mm3
  3. Child: 150,000 to 450,000 per mm3
  4. Adults: 150,000 to 400,000/mm3

V. Interpretation: Increased Platelet Count

VI. Interpretation: Decreased Platelet Count

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