08 Respiration and Circulation - part 11 - Thrombocytes / Platelets

 

08 Respiration and Circulation - part 11 - Thrombocytes / Platelets


Thrombocytes / Platelets:
  • Are cellular fragments formed from the large cells called megakaryocytes. 
  • Produced in bone marrow. 
  • Very small, oval shaped cell fragments without nucleus. 
  • Normal count of thrombocytes in human blood is about 2.5 – 4.5 lakh / mm3 of blood. 
  • If number of thrombocytes decreases than normal, condition is called as thrombocytopenia. 
  • This condition causes internal bleeding (haemorrhage). 
  • Platelets secrete platelet factors which are essential in blood clotting. 
  • Also seal the ruptured blood vessels by formation of platelet plug/ thrombus. 
  • Secrete serotonin a local vasoconstrictor.
Blood Clotting/ Coagulation of blood :
  • Process of converting the liquid blood into a solid form.
  • Initiated by contact of blood with any foreign surface (intrinsic process) or with damaged tissue (extrinsic process). 
  • Intrinsic and extrinsic processes involve interaction of various substances called clotting factors by a step wise or cascade mechanism. 
  • There are in all twelve clotting factors numbered as I to XIII (factor VI is not in active use). 
  • Interaction of these factors in a cascade manner leads to formation of the enzyme thrombin. 
  • Thromboplastin, helps in the formation of enzyme prothrombinase. 
  • This enzyme inactivates heparin and it also converts inactive prothrombin into its active thrombin.
  • Thrombin converts soluble blood protein fibrinogen into insoluble fibrin. 
  • Fibrin forms a mesh in which platelets and other blood cells are trapped to form the clot. 
  • Blood clotting occurs as shown in the following flowchart.

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