Muhammad Asif Zeb LecturerHematology IPMS-KMU

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.

Or

Maintenance of normal blood flow within the circulatory system

Prime function of hemostasis

To maintain blood in fluid state.

To arrest bleeding followed by trauma

To remove platelet plug when healing is

complete.

Type of hemostasis

Primary hemostasis

Formation of PLT plug after vessel injury

Secondary hemostasis

Due to coagulation protein

Events in Hemostasis

Vascular Constriction

- local myogenic spasm

- local autacoid factors from traumatized tissues and platelets

- nervous reflex

Structure of Platelet

STAGES OF

PRIMARY HEMOSTASIS

Platelet Adhesion

Platelet Activation and release reaction

Platelet Aggregation

platelet adhesion

Platelet activation : platelet release action

Platelet aggregation

Clot formation

Secondary hemostasis

“Cascade of reactions” by Macfarlane, R.G.,1967

It states that ‘inactive’ enzymes are activated, and the ‘activated’ enzymes in turn activates other inactive enzymes until final step is reached.

Clotting or coagulation Factors

Factor I Fibrinogen

Factor II Prothrombin

Factor III Thromboplastin

Factor IV Calcium

Factor V Labile factor, or proaccelerin

Factor VI Non – existent

Factor VII Stable factor or proconvertin

Factor VIII Antihaemophilic factor / globulin A

Factor IX Christmas factor or Antihaemophilic factor B

Factor X Stuart – Prower factor

Factor XI Plasma thromboplastin antecedent or

Antihaemophilic factor C

Factor XII Hageman factor or Contact factor

Factor XIII Fibrin stabilizing factor or Laki – Lorand factor

Secondary hemostasis

Blood coagulation

The 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.

Mechanism

Clotting pathway is activated by:

Intrinsic pathway

Extrinsic pathway

Common pathway

Intrinsic pathway

The 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 pathway

It 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

Fibrinolytic system

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.

Activators of plasminogen:

◦ TPA

◦ Thrombin

◦ XIIa

◦ Kallikerin

◦ HMWK

Inhibitors of plasmin :

Alpha 1 anti trypsin

Alpha 2 anti plasmin

Alpha 2 macroglobulin