In: Anatomy and Physiology
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.
Primary hemostasis involves the process till platelet plug formation. Coagulation of blood falls under secondary hemostasis.
*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 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 throm boxane 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 platelet activating factor (PAF).