Blood coagulation

- Dr. E. R. Venkata Subba Reddy

Coagulation

Blood coagulation is part of an important host defense mechanism termed hemostasis.

Hemostasis : arrest or stoppage of bleeding from a blood vessel.

Injury to a blood vessel initiates a series of reactions resulting in hemostasis.

Three stages : 1. Vasoconstriction 2. Formation of platelet plug 3. Coagulation of blood

Hemostasis

Injury to blood vessel ↓

Endothelial damage and collagen exposure VonWillebrand factor → I ↓

Activation of platelets FIBRINOGEN ↓ ↓ I

Serotonin ADP + I I TX A2 I I ↓ I

I Aggregation of I I more platelets I

I ↓ ↓

Vasoconstriction platelet plug FIBRIN (clot) (STAGE 1) (STAGE 2) (STAGE 3)

Coagulation / clotting

Definition : The process in which blood looses its fluidity and becomes a jelly like mass few minutes after its shed out or collected in a container.

The so formed mass is called clot The clot is a mesh of thin fibrils

entangling the blood cells .These fibrils consist of fibrin. The fibrin is formed from fibrinogen.

Sequence of clotting mechanism

ENZYME CASCADE THEORY - Factors in inactive proenzyme forms - Series of proenzyme-enzyme conversions ( Zymogen activation reactions ) - At each stage a precursor protein

(zymogen) is converted to an active protease by cleavage of one or more peptide bonds

- Helped by cofactors, calcium

CLOTTING MECHANISM

3 STAGES

1. Formation of Prothrombin Activator 2. Conversion of Prothrombin to

Thrombin 3. Conversion of Fibrinogen to Fibrin

1. Formation of Prothrombin Activator

Two ways 1. Contact activation pathway

(formerly known as the intrinsic pathway)

2. Tissue factor pathway (extrinsic pathway)

INTRINSIC PATHWAY

Endothelial damage + collagen exposure

I Kallikrein & I ↓ HMW kinogen I XII ------XIIa ↓ ↓ HMW kinogen Platelets XI ------XIa I ↓ Calcium I IX ------------ IXa ↓ ↓ VIII & Ca Phospholipid X------------------ Xa I I I ↓ ↓

PROTHROMBIN ACTIVATOR ( PTA ) (Xa + V + Ca +Phospholipid)

EXTRINSIC PATHWAY

Tissue trauma + Tissue thromboplastin ↓ Glycoprotein Phospholipid I

VII --------------------------------- VIIa I Ca ↓ Phospholipid

X ----------------------------------- Xa I Ca I Factor V I Thrombin ↓

Prothrombin activator (Xa + V + Ca +phospholipid)

STAGE 2 & 3 PROTHROMBIN ACTIVATOR

I ↓

PROTHROMBIN-----------------→THROMBIN

I

↓ FIBRINOGEN a ←------------------------- FIBRINOGEN I Polymerization I ↓ XIII & Ca

LOOSE STRANDS OF FIBRIN-------------------------------→FIBRIN

TIGHT BLOOD CLOT

Factors involved in Blood coagulation

The substances necessary for clotting are called clotting factors

Thirteen identified

I – Fibrinogen II – Prothrombin III – Thromboplastin IV – Calcium V – Labile factor ( proaccelerin ) VI – presence not proved VII – Stable factor

Clotting Factors

VIII – Anti hemophilic factorIX – Christmas factorX – Stuart-Prower factorXI – Plasma Thromboplastin antecedentXII – Hageman factorXIII – Fibrin stabilizing factor (Fibrinase)

Cofactors

Calcium Phospholipid Vitamin K is an essential factor to a

hepatic gamma-glutamyl carboxylase that adds a carboxyl group to glutamic acid residues on factors II, VII, IX and X, as well as protein S, Protein C and protein Z.

Regulators Five mechanisms keep platelet activation and the

coagulation cascade in check.

Protein C is a major physiological anticoagulant. It is a vitamin K-dependent serine protease .

Activated by thrombin with the help of thrombomodulin.

The activated form, along with protein S and a phospholipid as cofactors, degrades Va and VIIIa.

Antithrombin is a serine protease inhibitor Tissue pathway factor inhibitor(TFPI) Plasmin cleaves fibrin into fibrin degradation

products that inhibit excessive fibrin formation. Prostacyclin (PGI2) through cAMP inhibits platelet

activation

BLOOD CLOT

Clot consists of entrapped platelets, RBCs, WBCs within the fibrin meshwork.

Clot Retraction : after 30 to 45 min. -Clot contracts. Serum oozes out. -Actin, myosin and Thrombosthenin. FIBRINOLYSIS : lysis in blood vessel.

Requires plasmin (fibrinolysin)

Testing of coagulation

Commonly used:

aPTT ( activated partial Thromboplastin time) - The contact factor pathway

PT (also used to determine INR)- The tissue factor pathway

Fibrinogen testing (often by the Clauss method) Platelet count Platelet function testing (often by PFA-100).

Testing

Others: Thrombin clotting time, Bleeding time, Mixing test (whether an abnormality corrects if the patient's plasma is mixed with normal plasma), Coagulation factor assays, Antiphospholipid antibodies, D-dimer, Genetic tests (factor V leiden), Miscellaneous platelet function tests, Thromboelastography (TEG or ROTEM), Euglobulin lysis test(ELT), .

Role in disease

Platelet disorders:-Inborn or Acquired-congenital: > Glanzmann’s thrombasthenia - It is characterized

by a defect in GPIIb/IIIa fibrinogen receptor complex,

> Bernard-sourlier syndrome (abnormal glycoprotein Ib-IX-V complex),

> Gray platelet syndrome(deficient α-granules), > Delta storage pool deficiency(deficient dense

granules). -Acquired: MDS, ITP, TTP, HUS, PNH, DIC, HIT.

HAEMOPHILIA: 3 main forms- Hemophilia A : VIII deficiency- Hemophilia B : IX- Hemophilia C : XI Von Willebrand disease : most common

hereditary bleeding disorder . Def of vWF, that mediates glycoprotein Ib (GPIb) binding to collagen.

Thrombosis is the pathological development of blood clots. May form an embolus or cause occlusionocclusion by embolus or thrombus leads to ischemic necrosis of tissue.

Most cases of thrombosis are due to acquired extrinsic problems (surgery, cancer, immobility, obesity,)

A small proportion of people harbor predisposing conditions known collectively as Thrombophilia (antiphospholipid syndrome, factor V leiden).

Procoagulants Adsorbent chemicals, such as zeolites, used for

use in sealing severe injuries quickly (traumatic bleeding secondary to gunshot wounds).

Thrombin and fibrin glue are used surgically to treat bleeding and to thrombose aneurysms.

Desmopressin is used to improve platelet function Prothrombin complex concentrate, cryoprecipitate

and fresh frozen plasma Recoombinant activated human factor VII is

increasingly popular in the treatment of major bleeding.

Tranexemic acid and aminocaproic acid inhibit fibrinolysis.

Anticoagulants

Anticoagulants and anti-platelets. Anti platelet agents : aspirin, clopidogrel, dipyridamole and

ticlopidine; the parenteral glycoprotein Iib/IIIa inhibitors are used during angioplasty

Anticoagulants, Warfarin (and related coumarins) and Heparin are the most commonly used.

Warfarin affects the vitamin K-dependent clotting factors (II, VII, IX,X).

Heparin and related compounds increase the action of antithrombin on thrombin and factor Xa.

A newer class of drugs, the direct thrombin inhibitors, is under development; some members are already in clinical use (such as lepirudin).

compounds that interfere directly with the enzymatic action of particular coagulation factors (e.g., rivaroxaban).

The only non mammalian animal known to use serine proteases for blood coagulation is the horseshoe crab.

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Thankyou