Hemostasis is the process of forming clots in walls of damaged blood vessels and preventing blood loss while maintaining blood in a fluid state within the vascular system. 

Physiological balance exists between:

• Factors promoting coagulation i.e. Procoagulants. 
• Factors inhibiting coagulation i.e. Anticoagulants. 

The three important steps in hemostasis are: 

• Vasoconstriction - Constriction of blood vessel which is a local phenomenon. 
• Platelet plug formation – Here platelets undergo adhesion, activation and aggregation. Platelets aggregate and form a temporary loose plug to prevent blood loss. 
• Coagulation of blood -  Process of conversion of fluid blood into semisolid jelly like mass is known as coagulation or clotting of blood. 

There is intrinsic and extrinsic pathway. Both leads to formation of prothrombin activator. Then, prothrombin gets converted into thrombin. Thrombin activates fibrinogrn to form fibrin and further blood clot is formed. 

Coagulation of blood: Clotting factors 

I- Fibrinogen, 

II- Prothrombin, 

III- Tissue thromboplastin, 

IV- Calcium, 

V- Labile factor, Proaccelerin, 

VII- Stable factor Proconvertin 

VIII- Anti haemophilic factor, AHF-A 

IX- Christmas factor, AHF-B 

X- Stuart prower factor, 

XI- Plasma Thromboplastin Antecedent, AHF-C 

XII- Hagemans factor, Glass factor 

XIII- Fibrin stabilizing F, Laki-Lorand F 

Steps of coagulation includes 

• Prothrombin activator complex formation
• Conversion of prothrombin to thrombin
• Conversion of fibrinogen to fibrin & clot formation

Special attributes of Coagulation Process

• Enzyme substrate reaction
• System acts as a bio amplifier system
• Cascade reaction

There are positive and negative feedback mechanisms. Calcium is essential in almost all steps.

Functions of Platelets

• Role in Vasoconstriction
• Primary Hemostasis-by temporary Platelet Plug
• Secondary hemostasis-Help in clot formation
• Clot retraction: Due to actin, & Myosin
• Repair of capillary endothelium: PDGF- Multiplication of endothelial cells & fibroblasts

Commonly Performed Lab Tests 

• Bleeding time
• Clotting Time
• Prothrombin time 
• Activated partial thromboplastin time (aPTT or APTT)
• Assay of individual clotting factors

Some must known facts

• Bleeding time is prolonged in – Thrombocytopenia
• Classic hemophilia is due to - Factor 8 deficiency
• Prothrombin time is the test for - Extrinsic pathway 
• Anticoagulant potency of Heparin increases by combining with – antithrombin
• Fibrin stabilizing factor is mainly released from – Platelet.  

 Article by Dr. Siri P. B.