BLOOD COAGULATION

 

                           5. BLOOD  COAGULATION/ CLOTTING

   Introduction:

                     When blood comes out of the blood vessel, it looses its fluidity & becomes a

                     semisolid jelly. This process  called clotting.

   Definition:

         Defined as the sequence of events leading to the formation of fibrin from fibrinogen

   Coagulation factors: 

            -  these are substances required for coagulation

            -  all these substances are present in plasma in an inactive form

            -  these are activated and take part in coagulation when the blood vessel wall is

               injured

 I       -      Fibrinogen

ii       -      Prothrombin

iii      -      Tissue thromboplastin

iv      -      Calcium

v       -      Proaccelerin,labile factor

vi      -      Accelerin

vii     -      Proconvertin,stable factor

viii    -      Anti hemophilic factor A

ix       -     Plasma Thromboplastic Component(PTC),  christmas factor, antihemophilic

               factor b

x        -      Stuart-prower factor

xi     -    Plasma Thromboplastin antecedent(PTA),

 

xii    -    Hageman factor,

xiii   -    Fibrin stabilizing factor

HMW- K - High Molecular Weight Kininogen,                                

pre-k   - Prekallikrein

ka      -   kallikrein

PL     -    Platelet Phospholipid

Mechanism of coagulation:

3 main steps are involved

1. Damage to the blood vessel wall/ blood -à Formation of Prothrombin Activator

    Complex

2. Prothrombin Activator Complex activates prothrombin to thrombin                                             

3. Thrombin converts fibrinogen to fibrin

Pathways of coagulation:

       1. Extrinsic pathway

       2. Intrinsic pathway

 

 Extrinsic mechanism of clotting: 

Triggered by tissue injury

 Injured tissues release factor III (tissue factor)

                                        

                                     

                            VII                                      VIIa

                                              PF3

                                              Ca₂ + &  III                                             

                                                        

               

                                                                 X                     Xa  

                                                                                          Va          Prothrombin

                                                                                          Ca₂+       Activator

                                                                     tissue phospholipids     Complex

 

Intrinsic mechanism of clotting:

Contact of blood with collagen, HMW kininogen, kallikrein

                       ↓

Release of platelet phospholipids

                 &

activation of  factor  XII         XIIa

                                                 ↓

                                XI                     XI a

                                                          ↓       

                                            IX                  IXa

                                                                    PL

                                                                    Ca₂+

                                                                   VIIIa 

                                                                                                                                                                                                                                  

                                                               X                            Xa

                                                                                              Va           Prothrombin

                                                                                              Ca₂+       Activator

                                                                    Platelet phospholipids       Complex