By Mr. Govardhan Joshi

HAEMOSTASIS

OBJECTIVES:Define haemostasis.Describe the main mechanisms

that prevent blood loss after an injury.

Describe role of platelets in haemostasis.

Outline the mechanism of platelet plug formation.

Describe the mechanisms of blood coagulation.

DEFINITION The term haemostasis means

prevention of blood loss. Haemostasis is the arrest of

blood flow and control of hemorrhage from an injured blood vessels.

or It is the process by which

bleeding at any site is arrested by formation of hemostatic plug.

Prime function of hemostasisTo maintain blood in fluid state.To arrest bleeding followed by

traumaTo remove platelet plug when

healing is complete.

MECHANISM

Haemostasis involves 4 main steps: Vascular spasm Formation of platelet plug Blood coagulation Fibrinolysis

Vascular spasmReduces flow of blood from

injured vessel. Cause:

Sympathetic reflex Release of vasoconstrictors from platelets

that adhere to the walls of damaged vessels.

PLATELET PLUG FORMATION

Platelets adhere to damaged endothelium to form platelet plug (primary hemostasis) that temporarily seals the break in the vessel wall.Mechanism:

Platelet adherence Platelet activation Platelet aggregation

Platelet adhesion

When a blood vessel wall is injured, platelets adhere to the exposed collagen and von Willebrand factor in the wall via platelet receptors.

Platelet activation

Activated platelets release the contents of their granules including ADP and secrete TXA2 → activates nearby platelets to produce further accumulation of more platelets and forming a platelet plug(platelet aggregation)

Blood coagulationThe clotting mechanism involves a cascade of

reactions in which clotting factors are activated.

Most of them are plasma proteins synthesized by the liver (vitamin K is needed for the synthesis of factor II, VII, IX and X).

They are always present in the plasma in an inactive form.

When activated they act as proteolytic enzymes which activate other inactive enzymes.

Several of these steps require Ca++ and platelet phospholipid.

Clotting factors

Mechanism Clotting pathway is activated by:

Intrinsic pathway Extrinsic pathway Common pathway

Intrinsic pathwayThe initial reaction is the

conversion of inactive factor XII to active factor XIIa.

Factor XII is activated in vitro by exposing blood to foreign surface (glass test tube).

Activation in vivo occurs when blood is exposed to collagen fibers underlying the endothelium in the blood vessels.

Extrinsic pathway

Requires contact with tissue factors external to blood.

This occurs when there is trauma to the vascular wall and surrounding tissues.

The extrinsic system is triggered by the release of tissue factor (thromboplastin from damaged tissue), that activates factor VII.

The tissue thromboplastin and factor VII activate factor X.

Common pathwayIt involves the activation of factor

X to Xa via intrinsic and extrinsic pathway.

Xa further activates prothrombin to active enzyme thrombin.

Thrombin act on fibrinogen to fibrin.

Factor XIII helps in the formation of stabilized fibrin.

Clot formation

Fibrinolysis The process of removing

unwanted insoluble deposit formed as a result of coagulation is called fibrinolysis.

It is a physiological process in which fibrin clot is broken down by enzyme into soluble fragment.