Haemostasis
The immediate response to damage of the blood vessel wall is
vasoconstriction, which reduces blood flow. This is followed by a sequence of
events leading to sealing of the wound by a clot. Collagen in the exposed
subendothelial matrix binds von Willebrand factor (vWF), which in turn
binds to glycoprotein Ib (GPIb) receptors on platelets, the first stage
of platelet adhesion. This initial tethering
promotes binding of platelet integrin α2β1 and GPVI receptors directly to collagen. Binding of
receptors initiates activation, partly by increasing intracellular Ca2+.
Platelets change shape, put out pseudopodia and make thromboxane A2
(TXA2) via cyclooxygenase (COX). TXA2
releases mediators from platelet dense granules, including serotonin (5-HT)
and adenosine diphosphate (ADP), and from α granules vWF, factor
V (see below) and agents that promote vascular repair. TXA2 and 5-HT
also promote vasoconstriction. ADP
activates more platelets via P2Y12 purinergic receptors,
causing activation of fibrinogen (GPIIb/IIIa) receptors and exposure of phospholipid
(PLD) on the platelet surface. Plasma fibrinogen binds to GPIIb/IIIa
receptors causing the platelets to aggregate (stick together) forming a
soft platelet plug (Figure 7a). This is stabilized with fibrin during
clotting. Note that thrombin (see below) is also a potent platelet
activator.
The final stage of blood clotting (coagulation) is formation of the clot
– a tight mesh of fibrin entrapping platelets and blood cells. The
process is complex, involving sequential conversion of proenzymes to active
enzymes (factors; e.g. factor X → Xa). The ultimate purpose is to
produce a massive burst of thrombin (factor IIa), a protease that cleaves fibrinogen to fibrin. The cell-based
model of clotting (Figure 7b) has replaced the older extrinsic and
intrinsic pathways. Most of the action in this model occurs on the cell surface
(hence its name).
The initial phase of clotting is initiated when cells in the subendothelial
matrix that bear tissue factor (TF; thromboplastin) are exposed
to factor VIIa from plasma. Such cells include fibroblasts and monocytes, but
damaged endothelium and circulating cell fragments containing TF
(microparticles) can also initiate clotting. TF forms a complex with factor
VIIa (TF:VIIa) which activates factor X (and IX, see below). Factor
Xa with its cofactor Va then converts prothrombin
(factor II) to thrombin; activation of both factor X and prothrombin
require Ca2+. Comparatively little thrombin is produced at this time, but sufficient
to initiate the amplification phase. Activity of these processes is
normally sup- pressed by tissue factor pathway inhibitor (TFPI), which
inhibits and forms a complex with factor Xa, which then inhibits TF:VIIa; however, the influx of plasma factors
after damage overwhelms this suppression.
The amplification phase (sometimes viewed as part of the propagation
phase) takes place on platelets (Figure 7b). Thrombin produced in the initial
phase activates further platelets, and mem- brane-bound factor V which is
released from platelet α granules. Factor VIII is normally bound to circulating
vWF, which protects it from
degradation. Thrombin cleaves factor VIII from vWF and activates it, when it
binds to the platelet membrane.
The scene is now set for the propagation phase. Either factor XIa
(itself activated by thrombin) or TF:VIIa can activate factor IX, which binds and forms a complex with
factor VIIIa on the platelet membrane called tenase; this is a much more
powerful activator of factor X than TF:VIIa. Factors Xa and Va then bind to
form prothrombinase on the platelet membrane. This process leads to a
massive burst of thrombin production, 1000-fold greater than in the initial
phase and localized to activated platelets.
Factor XII (Hageman factor, not shown) is probably of limited
significance, as deficiency does not lead to bleeding. It is activated by
negative charge on glass and collagen, and can activate factor XI. It may be involved in pathological
clotting in the brain.
Thrombin cleaves small fibrinopeptides from fibrinogen to form fibrin
monomers (Figure 7c), which spontaneously polymerize. This polymer is cross-linked
by factor XIIIa (activated by thrombin in the presence of Ca2+)
to create a tough network of fibrin fibres and a stable clot. Retraction
of entrapped platelets contracts the clot by ∼60%, making it tougher and
assisting repair by drawing the edges
of the wound together.
Inhibitors of haemostasis and fibrinolysis Inhibitory mechanisms
are vital to prevent inappropriate clotting (thrombosis). Prostacyclin
(PGI2) and nitric oxide from undam- aged endothelium impede
platelet adhesion and activation. Anti-thrombin inhibits thrombin,
factor Xa and IXa/tenase; its activity is strongly potentiated by heparin,
a polysaccharide. Heparan on endothelial cells is similar. TFPI has
already been mentioned. Thrombomodulin on endothelial cells binds
thrombin and prevents it cleaving fibrinogen; instead, it activates protein
C (APC) which with its cofactor protein S inactivates
cofactors Va and VIIIa, and hence tenase and prothrombinase (Figure 7d).
Fibrinolysis is the process by which a clot is broken down by plasmin,
a protease (Figure 7d). This creates soluble fibrin degradation products (FDPs)
including small D-dimers. Plasmin is formed from fibrin-bound plasminogen
by tissue plasminogen activator (tPA), released from damaged
endothelial cells in response to bradykinin, thrombin and kallikrein. Urokinase
(uPA) is similar. APC inactivates an inhibitor of plasminogen
activator inhibitor (tPA; PAI-1 and 2), and so promotes fibrinolysis
(Figure 7d).
Plasmin is itself inactivated by α2-antiplasmin, and
inhibited by thrombin activated
fibrinolysis inhibitor (TAFI).
Defects in haemostasis
The most common hereditary disorder is haemophilia A, a deficiency
of factor VIII sex linked to males. Christmas disease is a deficiency of
factor IX, and von Willebrand disease a deficiency of vWF. The latter
leads to defective platelet adhesion and reduced availability of factor VIII,
which is stabilized by vWF. The liver requires vitamin K for correct
synthesis of prothrombin and factors VII, IX and X. As vitamin K is obtained
from intestinal bacteria and food, disorders of fat absorption or liver disease
can result in deficiency and defective clotting. Factor V Leiden is
brought about by a mutant factor V that cannot be inactivated by APC. Five per
cent carry the gene, which causes a fivefold increase in the risk of
thrombosis. Antiphospholipid syndrome is caused by phospholipid- binding
antibodies (e.g. cardiolipin, lupus anticoagulant) which may inhibit APC and
protein S, or facilitate cleavage of pro- thrombin. It is associated with
recurrent thrombosis and linked to 20% of strokes in people under 50 years,
more common in females.
