Hemostasis pt 2

Intrinsic (aPTT)

Time required for plasma to clot after adding:

⚬ contact activator

⚬ phospholipid- will not work without phospholipid; must be bound to a cell

⚬ Ca- needed to activate the next thing

Extrinsic (PT)

(prothrombin time)

Time required for plasma to clot after adding tissue factor (III) and Ca

Coag proteins

most circulate as zymogens

⚬ named in order of discovery, NOT rxn sequence

⚬ roman numeral followed by “a” indicates activated

form

Common pathway

X marks the spot

• V fits into X = V is cofactor for X

Intrinsic (TENENT)

the aPTT is a basic TENENT of coagulation studies

⚬ twelve

⚬ eleven

⚬ nine

⚬ eight

⚬ ten (beginning of common)

extrinsic

3 + 7 = 10

• “PT” has less letters than “aPTT”, shorter pathway,

fewer steps to remember, that’s lucky, lucky number 7

• III = tissue factor

vit K dependent group

PROTIENS THAT NEED VIT K

II, VII, IX, X

• Protein C, Protein S

⚬ (inhibit coag)

• synthesized by liver

• all found in serum EXCEPT II (prothrombin)

• Vitamin K enables Ca²⁺-dependent binding to activated

platelet membranes

fibrinogen group

PROTIENS THAT NEED FIBRINOGEN

thrombin-sensitive

• I, V, VIII, XIII

⚬ I (fibrinogen) → fibrin

⚬ XIII = fibrin stabilizer

• not found in serum

contact group

activated by contact with negatively charged surfaces

⚬ collagen in basement membrane

⚬ initiate intrinsic pathway (aPTT)

• XII, XI, PK, HK

⚬ PK = prekallikrein

⚬ HK = high-molecular- weight-kininogen

⚬ deficiencies in all except XI are asymptomatic but cause prolonged

PTT

• found in serum in reduced amount

• also play role in fibrinolysis, activation of complement, inflammation

Summary of contact group, Vit K and fibrinogen

coagulation cascade

complex formation on phospholipid membranes

⚬ localizes clotting to site of injury

⚬ clotting factors bind to membrane of activated plts

￭ complex = enzyme + cofactor + substrate

coagulation cascade complex formation

on phospholipid membranes

⚬ extrinsic Xase

￭ starts coagulation

⚬ intrinsic Xase

￭ amplifies factor X activation

⚬ prothrombinase

￭ receives Xa from either complex

￭ converts prothrombin to thrombin

coagulation cascade intrinsic pathway - aPTT

contact factors

￭ XII, XI, PK, HK

￭ activated when exposed to negatively

charged surfaces

• glass, celite (silica), ellagic acid

• does not require Ca

deficiency in XII, PK, HK

￭ prolonged aPTT

￭ no bleeding phenotype

⚬ deficiency in XI

￭ prolonged aPTT

￭ variable bleeding phenotype

coagulation cascade intrinsic pathway - aPTT (FACTOR DETAILS)

XII = Hageman factor

￭ activated by collagen in vivo

￭ activated by glass, silica in vitro

⚬ XI

￭ only contact factor associated with abnormal

bleeding (variable)

⚬ IX = Christmas factor

￭ vit K dependent

⚬ VIII = antihemophilic factor

￭ deficiency = hemophilia A

￭ circulates as complex with vWF

coagulation cascade extrinsic pathway - PT

vessel injury → nonvascular cells such

as fibroblasts, smooth muscle cells with

tissue factor (TF, III) are exposed to

blood

⚬ TF (III) binds VII in the presence of Ca

￭ complex activates X and IX

coagulation cascade extrinsic pathway - PT FACTOR DETAILS

TF (III) = thromboplastin

￭ transmembrane glycoprotein

￭ cofactor for VII

⚬ VII

￭ vit K dependent

coagulation cascade common pathway

begins with factor X

￭ vit K dependent

￭ activated via extrinsic or intrinsic Xase

⚬ V

￭ 20% in plt alpha granules

￭ positive feedback loop with thrombin:

• Va helps make thrombin, thrombin

activates more V

coagulation cascade common pathway FACTORS

prothrombin (II)

￭ vit K dependent

￭ cleaved by prothrombinase complex to

⚬ thrombin (IIa)

￭ procoagulant

￭ anticoagulant

￭ antifibrinolytic

coagulation cascade common pathway thrombin (IIa)

procoagulant

• Fibrinogen → fibrin

• Activates V, VIII, XI, XIII

• Activates platelets

• Stimulates endothelial release of vWF and

expression of TF

(coagulation cascade common pathway) fibrinogen (I)

￭ acute phase reactant

￭ most abundant coag protein

⚬ 3 stages → fibrin

proteolysis = cleaved by thrombin into fibrin

monomers

polymerization = self-assemble into fibrin polymers

fibrin polymers stabilized by XIIIa

• creates covalent bond crosslinks between fibrin

• fibrin mesh stabilizes primary plt plug

• = secondary hemostasis

fibrinolytic system

digesting and removing fibrin

• activated in response to

⚬ coag cascade activation

⚬ fibrin formation

• limited to area of fibrin formation

• excessive fibrinolysis = bleeding

• inadequate fibrinolysis = excessive clotting

(fibrinolytic system) plasminogen (PLG)

synthesized by liver

⚬ activated by plasminogen activators (PA) to plasmin (PLN)

⚬ plasmin substrates=breakdown:

￭ fibrin (Ia)

￭ fibrinogen (I)

￭ V, VIII, vWF

￭ complement

￭ plt surface receptors-stops receiving signals

(fibrinolytic system) 2 major PAs in circulation

⚬ tPA = tissue plasminogen activator(t for tissue)

￭ main activator for dissolving fibrin clots inside vessels

• most released by endothelial cells

￭ binds to fibrin => much more effective

• = localizes/concentrates fibrinolysis to clot

⚬ uPA = urokinase-type plasminogen activator

￭ fibrin- independent

￭ functions mainly in tissues (tissue remodeling/cell movement)

• other activators produced during contact phase- start plasminogen

⚬ kallikrein- don’t care about

⚬ XIIa

⚬ XIa

plasmin (fibrinolytic system)

⚬ responsible for degradation of fibrin (and fibrinogen)

￭ = fibrinolysis (fibrinogenolysis)

￭ → fibrin degradation products (FDPs)

￭ DDimer specific for fibrin

D (DDimer) and E fragments

Plasmin cannot break down D fully

Plasmin breaks down fibrinogen, which then can break down D

Elevated D-dimer most likely indicates a clot but does not confirm it

fibrinolytic system inhibitors of fibrinolytic system

prevent systemic proteolysis

⚬ action:

￭ at the PLG activation step

• PAI1 = plasminogen activator inhibitor 1

⚬ primary inhibitor of tPA and uPA

• TAFI = thrombin-activatable fibrinolysis inhibitor

⚬ activated by thrombin/thrombomodulin complex

⚬ inhibits plasminogen → plasmin

￭ directly on plasmin

fibrinolytic system inhibitors of fibrinolytic system PT2

prevent systemic proteolysis

⚬ action:

￭ at the PLG activation step

• PAI1

• TAFI

￭ directly on plasmin

• α₂-antiplasmin

⚬ principle inhibitor of plasmin

⚬ cross-linked to fibrin by XIIIa(13)

• α₂-macroglobulin

⚬ backup inhibitor for plasmin

control of hemostasis

physical control = blood flow

⚬ vasoconstriction

⚬ return to normal blood flow → limits coagulation by

dilution of activated factors

physical control: liver clearance

⚬ liver produces many clotting factors

⚬ removes activated factors complexed with inhibitors,

and FDPs

⚬ liver disease may cause hemorrhage

￭ reduced production of coag factors

￭ if unable to remove activated proteases =contribute to systemic fibrinolysis or thrombosis

control of hemostasis feedback control positive

positive feedback amplification of coagulation

￭ thrombin(II)-activates fibrinogen to fibrin(I to Ia)

• major activator of plts

• promotes release of plt Va

• activates Va, VIIIa, XIa, XIIIa

￭ Xa

• activates VIIa and VIIIa

control of hemostasis negative feedback inhibition

control of hemostasis

• feedback control

⚬ negative feedback inhibition

￭ thrombin

• activate OR inactivate Va and VIIIa

￭ fibrin

• has strong affinity for thrombin, limiting amount of

thrombin available to convert fibrinogen to fibrin

￭ FDPs = inhibit fibrin formation

control of hemostasis biochemical inhibitors pt1

⚬ antithrombin

￭ most important inhibitor of serine proteases

￭ inactivates:

• thrombin (IIa)

• Xa

• IXa, XIa, XIIa

￭ activity enhanced by

• endothelial heparan sulfate

• pharmaceutical heparin

control of hemostasis biochemical inhibitors pt 2

biochemical inhibitors

⚬ antithrombin

⚬ protein c system = protein c and protein s

￭ vit K dependent

￭ synthesized primarily by liver

￭ thrombin - thrombomodulin activates protein c

￭ activated protein c + protein s

• inactivate Va and VIIIa

control of hemostasis biochemical inhibitors pt 3

antithrombin

⚬ protein c system = protein c and protein s

⚬ thrombomodulin

￭ binds thrombin on intact endothelial cells

￭ complex activates protein c

￭ (Free thrombin promotes clotting;

thrombomodulin-bound thrombin helps limit

clotting)

control of hemostasis biochemical inhibitors pt 4

antithrombin

⚬ protein c system = protein c and protein s

⚬ thrombomodulin

⚬ TFPI = tissue factor pathway inhibitor

￭ inhibits extrinsic/tissue factor pathway

￭ binds Xa

￭ TFPI-Xa complex inhibits TF-VIIa

physiologic pathway = cell-based model

initiation

⚬ vascular injury exposes cells expressing TF

⚬ TF-VIIa activates X and IX

⚬ small amount of thrombin generated

• amplification

⚬ thrombin activates plts and factors V, VIII, XI

• propagation

⚬ coagulation complexes assemble on activated plts

⚬ thrombin burst → fibrin formation

⚬ XIIIa stabilizes fibrin

• termination

⚬ natural anticoagulants limit coagulation to injury site

Vit K required

C. XI, XII, PK, HK

1972

Which proteins are Vitamin K-dependent anticoagulants?

Protein C and Protein S

Vitamin K allows clotting factors to bind to:

Platelet phospholipid membranes

Fibrinogen Group

I

V

VIII

XIII

1,5,8,13

VIII deficiency
→ Hemophilia A

IX deficiency
→ Hemophilia B

Contact Group

XII

XI

Prekallikrein (PK)

High molecular weight kininogen (HK)

Activated by:

• Collagen

• Glass

• Silica

• Negatively charged surfaces

Deficiency of:

XII

PK

HK

Produces:

✔ Prolonged aPTT

❌ No bleeding

Only XI deficiency may produce bleeding.

Common Pathway

Starts at:

Factor X

↓

Xa + Va

↓

Prothrombinase

↓

II → IIa

↓

Fibrinogen

↓

Fibrin

↓

XIIIa stabilizes clot

Plasmin

Digests:

Fibrin

Fibrinogen

Factor V

Factor VIII

vWF

Produces:

FDPs

Main plasmin inhibitor

α₂-antiplasmin

Main coagulation inhibitor

Antithrombin