Outline the physiological production of blood and its constituents

Describe the process and regulation of haemostasis, coagulation and fibrinolysis

Platelets are small cell fragments which are vital in haemostasis via forming a platelet plug. They:

  • Have a lifespan of 7-10 days
  • Are removed by the reticuloendothelial system in the spleen and liver


Platelets are:

  • Anuclear circulating cell bodies, which bud from megakaryocytes
    As the megakaryocyte cell volume increases, the cell membrane invaginates and small platelets bud off.
    • The time from stem cell to platelet is ~10 days, and is stimulated by thrombopoietin
  • New platelets are held in the spleen for 36 hours until they mature


  • α-granules
    Contain fibronectin, fibrinogen, vWF, PDGF, and Thrombospondin, platelet factor 4.
  • δ-granules
    Contain 5-HT, ATP, ADP, and Ca2+.
  • Contractile proteins
    Facilitate platelet deformation when activated.


  • Platelets are activated by:
    • Collagen
      Exposed by damaged endothelium.
    • Adrenaline
    • ADP
    • Thrombin
  • Activation results in several events:
    • Exocytosis of granules
    • Activation of membrane phospholipase A2 to form thromboxane A2
    • Deformation from a disc to a sphere with long projections
    • Promotion of the coagulation cascade
    • Change in glycoprotein (GP) expression by the action of ADP:
      ADP antagonists (e.g. clopidogrel) prevent expression of the GPIIb/IIIa complex.
      • GP Ib/IIb/IIIa facilitate platelet attachment to vWF
        vWF also binds to sub-endothelial connective tissue.
      • GP IIb/IIIa are also receptors for fibrinogen, which encourages platelet aggregation


  1. Kam P, Power I. Principles of Physiology for the Anaesthetist. 3rd Ed. Hodder Education. 2012.
  2. Krafts K. Clot or Bleed: A Painless Guide for People Who Hate Coag. Pathology Student.
Last updated 2019-07-18

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