STAGES OF HEMOSTASIS

Hemostasis is defined as arrest or stoppage of bleeding.

STAGES OF HEMOSTASIS

When a blood vessel is injured, the injury initiates a series of reactions, resulting in hemostasis. It occurs in three stages :

1. Vasoconstriction

2. Platelet plug formation

3. Coagulation of blood.

VASOCONSTRICTION

Immediately after injury, the blood vessel constricts and decreases the loss of blood from damaged

portion. Usually, arterioles and small arteries constrict. Vasoconstriction is purely a local phenomenon. When the blood vessels are cut, the endothelium is damaged and the collagen is exposed. Platelets adhere to this collagen and get activated. The activated platelets secrete serotonin and other vasoconstrictor substances which cause constriction of the blood vessels. Adherence of

VASOCONSTRICTION PLATELET PLUG FORMATION COAGULATION OF BLOOD

platelets to the collagen is accelerated by von Willebrand factor. This factor acts as a bridge between a specific glycoprotein present on the surface of platelet and collagen fibrils.

PLATELET PLUG FORMATION

Platelets get adhered to the collagen of ruptured blood vessel and secrete adenosine diphosphate (ADP) and thromboxane A2. These two substances attract more and more platelets and activate them. All these platelets aggregate together and form a loose temporary platelet plug or temporary hemostatic plug, which closes the ruptured vessel and prevents further blood loss. Platelet aggregation is accelerated by plateletactivating factor (PAF).

COAGULATION OF BLOOD

During this process, the fibrinogen is converted into fibrin. Fibrin threads get attached to the loose platelet

plug, which blocks the ruptured part of blood vessels and prevents further blood loss completely. Mechanism of blood coagulation is explained in the next chapter. blood loses its fluidity and becomes a jelly-like mass few minutes after it is shed out or collected in a container.

FACTORS INVOLVED IN BLOOD CLOTTING

Coagulation of blood occurs through a series of reactions due to the activation of a group of substances.

Substances necessary for clotting are called clotting factors.

Thirteen clotting factors are identified:

Factor I Fibrinogen

Factor II Prothrombin

Factor III Thromboplastin (Tissue factor)

Factor IV Calcium

Factor V Labile factor (Proaccelerin or accelerator globulin)

Factor VI Presence has not been proved

Factor VII Stable factor

Factor VIII Antihemophilic factor (Antihemophilic globulin)

Factor IX Christmas factor

Factor X Stuart-Prower factor

Factor XI Plasma thromboplastin antecedent

Factor XII Hageman factor (Contact factor)

Factor XIII Fibrin-stabilizing factor (Fibrinase).

Clotting factors were named after the scientists who discovered them or as per the activity, except factor

IX. Factor IX or Christmas factor was named after the patient in whom it was discovered.

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