The important factors that prevent the process of clotting within the vascular system are:
Smooth endothelium, which prevents contact activation of intrinsic system
Glycocalyx layer adsorbed to the inner surface of the endothelium that repels clotting factors and platelets
Protein bound with endothelium which binds thrombin and removes its effect; this protein also activates a plasma protein called protein C which inactivates the activated factor V and VIII. Damage to endothelium causes loss of smoothness and the glycocalyx layer and thus initiates coagulation.
Anticoagulants in blood also prevent clotting within the vessels. Important anticoagulants are
Fibrin threads formed during clotting
A globulin known asantithrombin III, heparin and alpha2 macroglobulin .
The thrombin formed during clotting becomes adsorbed to fibrin and thus prevents further spreading of the blood clotting. Thrombin not adsorbed to fibrin combines with antithrombin and gets inactivated. Heparin increases the effectiveness of antithrombin in removing thrombin. The heparin and antithrombin also inactivates the activated factors XII, XI, and X. The alpha2 macroglobulin acts similar to antithrombin.
Clotting time‚ or coagulation time is the time required for the blood to clot after it is drawn out from the blood vessels. When blood is exposed to air, it clots within a short time. Clotting time is less than 5 min. in most of the domestic animals except cow and horse. Average clotting time (in minutes):
Horse : 11.5,
Sheep : 2.5,
Cow : 6.5,
Pig : 3.5,
Dog : 2.5,
Human : 5.0.
The bleeding time is the time required for bleeding to stop after puncture of the skin. The blood is blotted away every 30 seconds or the wound is immersed in physiological salt solutions: (ranges from 3- 6 min)
The prothrombin time is a measure of the clotting time of plasma to which an excess of thromboplastin has been added so that this coagulation factor will not be a limiting one. Since oxalated plasma is used, CaCl2 is also added to the plasma.
Longer the clotting time, the smaller the prothrombin concentration. Prothrombin time in dogs: 9-12 sec. in sheep 13-25 sec.
Causes of excessive bleeding
Vitamin K deficiency: This vitamin is required for hepatic synthesis of prothrombin factors V, VII, IX and X and deficiency leads to reduced synthesis of these coagulation factors resulting in excessive bleeding.
Hepatitis or cirrhosis of liver: Results in less synthesis of coagulation factors derived from liver.
Thrombocytopenia: Reduction in platelet count, which may occur in irradiation of bone marrow, drug sensitivity and some infections.
Hereditary hemorrhagic disorders
Hemophilia A due to deficiency of factor VIII. It is congenital, seen in dogs and also reported in cat and horse.
Hemophilia B: caused by reduction in factor IX. Seen in dogs and it is inherited.
Von Willebrand’s disease: Inherited diseases resembles hemophilia A caused by reduction in platelet function and factor VIII. Deficiency of factor IX is reported in cattle.
