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Muhammad Asif Zeb LecturerHematology IPMS-KMU
DEFINITION
The term haemostasis means prevention of blood loss.
Haemostasis is the arrest of blood flow and control of hemorrhage from an injured blood vessels.
or
It is the process by which bleeding at any site is arrested by formation of hemostatic plug.
Or
Maintenance of normal blood flow within the circulatory system
Prime function of hemostasis
To maintain blood in fluid state.
To arrest bleeding followed by trauma
To remove platelet plug when healing is
complete.
Type of hemostasis
Primary hemostasis
Formation of PLT plug after vessel injury
Secondary hemostasis
Due to coagulation protein
Events in Hemostasis
Vascular Constriction
- local myogenic spasm
- local autacoid factors from traumatized tissues and platelets
- nervous reflex
Structure of Platelet
STAGES OF
PRIMARY HEMOSTASIS
Platelet Adhesion
Platelet Activation and release reaction
Platelet Aggregation
platelet adhesion
Platelet activation : platelet release action
Platelet aggregation
Clot formation
Secondary hemostasis
“Cascade of reactions” by Macfarlane, R.G.,1967
It states that ‘inactive’ enzymes are activated, and the ‘activated’ enzymes in turn activates other inactive enzymes until final step is reached.
Clotting or coagulation Factors
Factor I Fibrinogen
Factor II Prothrombin
Factor III Thromboplastin
Factor IV Calcium
Factor V Labile factor, or proaccelerin
Factor VI Non – existent
Factor VII Stable factor or proconvertin
Factor VIII Antihaemophilic factor / globulin A
Factor IX Christmas factor or Antihaemophilic factor B
Factor X Stuart – Prower factor
Factor XI Plasma thromboplastin antecedent or
Antihaemophilic factor C
Factor XII Hageman factor or Contact factor
Factor XIII Fibrin stabilizing factor or Laki – Lorand factor
Secondary hemostasis
Blood coagulation
The clotting mechanism involves a cascade of reactions in which clotting factors are activated.
Most of them are plasma proteins synthesized by the liver (vitamin K is needed for the synthesis of factor II, VII, IX and X).
They are always present in the plasma in an inactive form.
When activated they act as proteolytic enzymes which activate other inactive enzymes.
Several of these steps require Ca++ and platelet phospholipid.
Mechanism
Clotting pathway is activated by:
Intrinsic pathway
Extrinsic pathway
Common pathway
Intrinsic pathway
The initial reaction is the conversion of
inactive factor XII to active factor XIIa.
Factor XII is activated in vitro by exposing
blood to foreign surface (glass test tube).
Activation in vivo occurs when blood is
exposed to collagen fibers underlying the
endothelium in the blood vessels.
Extrinsic pathway
Requires contact with tissue factors external to blood.
This occurs when there is trauma to the vascular wall and surrounding tissues.
The extrinsic system is triggered by the release of tissue factor (thromboplastin from damaged tissue), that activates factor VII.
The tissue thromboplastin and factor VII activate factor X.
Common pathway
It involves the activation of factor X to Xa
via intrinsic and extrinsic pathway.
Xa further activates prothrombin to active
enzyme thrombin.
Thrombin act on fibrinogen to fibrin.
Factor XIII helps in the formation of
stabilized fibrin.
Clot formation
Fibrinolytic system
Fibrinolysis
The process of removing unwanted
insoluble deposit formed as a result of
coagulation is called fibrinolysis.
It is a physiological process in which fibrin
clot is broken down by enzyme into
soluble fragment.
Activators of plasminogen:
◦ TPA
◦ Thrombin
◦ XIIa
◦ Kallikerin
◦ HMWK
Inhibitors of plasmin :
Alpha 1 anti trypsin
Alpha 2 anti plasmin
Alpha 2 macroglobulin