SECONDARY HEMOSTASIS:
Coagulation Pathway
Althea Marie Mamaril, RMT
Coagulation Cascade
● Includes 2 important pathways—INTRINSIC SYSTEM & EXTRINSIC SYSTEM. Both
leads to the activation of the COMMON PATHWAY.
○ INTRINSIC PATHWAY - activated in vivo by contact of coagulation proteins with
subendothelial tissue
○ EXTRINSIC PATHWAY - initiated with the release of TF (Tissue Factor is a
lipoprotein released from cell membranes into plasma when there is vascular
injury.)
● The start of the coagulation or clot forming process is signaled by the activation of
either the extrinsic or intrinsic pathway. The end point of each pathway which is the
activation of FX will initiate the common pathway that would eventually lead to the
formation of the fibrin clot.
Extrinsic Coagulation Pathway
The extrinsic pathway is initiated by the entry of tissue thromboplastin into the
circulating blood. Tissue thromboplastin is derived from phospholipoproteins and
organelle membranes from disrupted tissue cells.
These membrane lipoproteins, termed tissue factors, are normally extrinsic to the
circulation. Platelet phospholipids are not necessary for activation of the extrinsic
pathway because tissue factor supplies its own phospholipids.
Factor VII binds to these phospholipids in the tissue cell membranes and is
activated to factor VIIa, a potent enzyme capable of activating factor X to Xa in the
presence of ionized calcium.
The activity of the tissue factor–factor VII complex seems to be largely dependent
on the concentration of tissue thromboplastin. The proteolytic cleavage of factor VIIa by
factor Xa results in inactivation of factor VIIa. Factor VII participates only in the
extrinsic pathway. Membranes that enter the circulation also provide a surface for the
attachment and activation of factors II and V. The final step is the conversion of
fibrinogen to fibrin by thrombin.
Intrinsic Coagulation Pathway
The intrinsic pathway involves the contact activation factors prekallikrein,
HMWK, factor XII, and factor XI. These factors interact on a surface to activate factor
IX to IXa. Factor IXa reacts with factor VIII, PF 3, and calcium to activate factor X to Xa.
In the presence of factor V, factor Xa activates prothrombin (factor II) to thrombin,
which in turn converts fibrinogen to fibrin.
Strong negatively charged solids that can participate in the activation of factor XII
include glass and kaolin in vitro as well as elastin, collagen, platelet surfaces,
kallikrein, plasmin, and high–molecular-weight kininogen in vivo. Collagen exposed
by blood vessel injury greatly influences the rate of reaction. FXIIa is cleaved into
fragments called FXIIf, which increases their roles in coagulation (Turgeon, 2018).
Intrinsic Coagulation Pathway
• Physiologic significance of FXIIa and XIIf (Steine-Martin et al., 1998):
1. FXIIa initiates the intrinsic pathway. In the presence of HMWK, FXIIa converts
FXI to FXIa.
2. Factors XIIa and XIIf can initiate the extrinsic pathway by activating the TF:FVII
to TF:FVIIa complex.
3. FXIIa and FXIIf initiate fibrinolysis by activating plasminogen to plasmin. Plasmin
is the most important fibrinolytic agent.
4. FXIIf, together with HMWK, initiates the kinin and complement systems by
converting prekallikrein to kallikrein leading to activation of HMWK to kinins. The
plasmin formed as a result of kallikrein initiates the complement system.
Intrinsic Coagulation Pathway
• Physiologic significance of kallikrein (Steine-Martin et al., 1998):
1. Perpetuates FXII activation.
2. Initiates kinin system.
3. Initiates fibrinolytic system.
• Physiologic significance of plasmin (Steine-Martin et al., 1998):
1. Promotes dissolution of the fibrin clot.
2. If plasmin is not destroyed by antiplasmins, it cleaves FXIIa to FXIIf resulting to
inhibition of further contact activation.
3. Activates kinin and complement systems.
Common Coagulation Pathway
Once factor X is activated to Xa, the extrinsic and intrinsic pathways enter a
common pathway. Factor II, prothrombin, is activated to thrombin (factor IIa), which
normally circulates in the blood as an inactive factor. Following the activation of factor
Xa, it remains platelet bound and activates factor V.
The complex of factors Xa and Va on the platelet surface is formed near
platelet-bound factor II molecules. In turn, the platelet-bound Xa/Va complex cleaves
factor II into thrombin, factor IIa. The stage is accelerated by factor V and ionized
calcium (Turgeon, 2018).
Final Fibrin Clot Formation
Clotting is the visible result of the conversion of plasma fibrinogen into a stable
fibrin clot. Thrombin plays a major role in converting factor XIII to XIIIa and in
converting fibrinogen to fibrin. Fibrin formation occurs in three phases: proteolysis,
polymerization, and stabilization (Turgeon, 2018):
1. Initially, thrombin, a protease enzyme, cleaves fibrinogen, which results in a
fibrin monomer, fibrinopeptide A, and fibrinopeptide B fragments.
2. In the second step, the fibrin monomers spontaneously polymerize end-to-end
due to hydrogen bonding.
3. Finally, the fibrin monomers are linked covalently by factor XIIIa into fibrin
polymers. These polymers form a meshy network, and the final fibrin solution is
converted to a gel when more than 25% of the fibrinogen is converted to fibrin.
This stabilizes the insoluble fibrin clot.
Final Fibrin Clot Formation
The insoluble fibrin clot formed is more resistant to fibrinolysis than soluble fibrin.
When plasmin lyses insoluble fibrin, stable fibrin degradation products (FDPs) are
formed such as the D-dimer which are not normally found when plasmin lyses soluble
fibrin or fibrinogen. The stabilized clot is insoluble in 5 M urea and other weak acids
(Stiene-Martin et al., 1998).
Factor XIII is converted to the active form, factor XIIIa, in two steps. In the first
step, thrombin cleaves a peptide from each of the two alpha chains of factor XIII with
formation of an inactive intermediate form of factor XIII. In the second step, calcium
ions cause factor XIII to dissociate, forming factor XIIIa (Turgeon, 2018).
Thrombin Feedback Mechanisms
1. Procoagulant Activity – Thrombin induces platelet activation and aggregation. It also
activates VIII to VIIIa, I to Ia, and XIII to XIIIa, V to Va (Turgeon, 2018) and possibly XI to
Xia (Stiene-Martin et al., 1998). It can also convert prothrombin to thrombin via
autocatalysis (Turgeon, 2018).
2. Coagulation Inhibitor – In high concentrations, thrombin has an opposite effect to
FV and FVIII. It also binds AT-III to inhibit serine proteases and binds to
thrombomodulin to activate Protein C which inactivates FV and FVIII. It can also initiate
the endothelial cell release of tissue Plasminogen Activator (TPA) which activates
plasminogen to plasmin causing fibrin and fibrinogen degradation (Turgeon, 2018).
3. Tissue Repair – Thrombin induces cellular chemotaxis and stimulation of the
proliferation of smooth muscle and endothelial cells (Turgeon, 2018).
Coagulation Regulatory Mechanisms
• Tissue factor pathway inhibitor (TFPI) - is synthesized in endothelial cells and is
present in plasma and platelets. It blocks the extrinsic coagulation pathway after its
initial activation by binding first to factor Xa. This binary factor Xa/TFPI complex then
docks onto and inactivates factor VIIa/tissue factor complexes, shutting down the
extrinsic pathway after a small amount of factor Xa, and subsequently thrombin, have
been generated. This is sufficient to activate factor VIII and factor V, enabling the
intrinsic pathway to provide a burst of procoagulant activity (Bunn & Furie, 2016).
• Antithrombin - (also referred to as antithrombin III) is the most important of a
number of plasma serine protease inhibitors (serpins) in regulating hemostasis.
Antithrombin is also known as heparin cofactor. Upon binding to endogenous
glycoconjugates on the surface of endothelial cells or the anticoagulant heparin,
antithrombin undergoes a change in conformation that vastly increases its activity as a
serine protease inhibitor. It inactivates not only thrombin but also factors VIIa, IXa, Xa,
and XIa as they are carried in the blood away from the developing thrombus (Bunn &
Furie, 2016).
Coagulation Regulatory Mechanisms
• Protein C - in the presence of its cofactor, protein S, is activated by thrombin bound
to thrombomodulin on the surface of normal endothelial cells. Protein C and protein S
both undergo vitamin K–dependent carboxylation and share other structural features
with factors VII, IX, X, and prothrombin. Activated protein C attenuates the coagulation
cascade by specifically inactivating factors Va and VIIIa (Bunn & Furie, 2016).
ASSIGNMENT:
Enumerate other “serpins”
and describe their mechanisms of
inhibition.
Quiz on Friday!
ASSIGNMENT: Modules 6&7 (CFU portion only)
