QS Study


A spontaneous arrest of bleeding or prevention of blood loss by physiological phenomenon is called Hemostasis. It is the natural process by which blood flow slows and clot forms to prevent blood loss during an injury, with hemo- meaning blood, and stasis meaning stopping.

Events of Hemostasis

When a blood vessel is severed or ruptured, hemostasis is achieved by several different events, including –

(i) Vascular spasm (vasoconstriction)

(ii) Formation of a platelet plug

(iii) Formation of a blood clot.

The eventual growth of Fibrous tissue into the blood clot to close the hole in the vessel permanently.

Fig: Events of Hemostasis

(i) Vascular spasm: Just after a blood vessel is cut, the wall of vessel contract and thereby reduces the flow of blood outside. The vasoconstriction that occurs during hemostasis is a brief reflexive contraction that causes a decrease in blood flow to the area. The contraction results from both –

  • Nervous reflexes initiated by pain impulses and Local myogenic contraction of the blood vessel.
  • Local humoral factors (like serotonin) released by damaged tissue and platelet.

(ii) Formation of platelet plug: At the site of vessel injury, platelets stick together to create a plug, which is the beginning of blood clot formation. When platelet comes in contact with a damaged vascular wall, they immediately change their character and begin to swell. They become sticky and adhere to the collagen fibers of the ruptured vessel and secretes a large quantity of ADP and thromboxan A2. This thromboxan A2 and ADP activate the other platelets and accumulates them to form a loose platelet plug

(iii) Blood coagulation – It is the process by which a blood clot forms to reduce blood loss after damage to a blood vessel. Stages of blood coagulation are –

  • Formation of prothrombin activator by the extrinsic or intrinsic pathway.
  • Conversion of prothrombin into thrombin.
  • Conversion of fibrinogen into fibrin by the action of thrombin.

The growth of fibrous tissue in the blood clot: When a blood clot is formed, it is invaded by fibroblasts which form connective tissue all over the clot or it becomes dissolute.