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U N I T 3
Hematopoietic Function
Hemostasis, which refers to the stoppage of blood flow, is divided into five stages:
(1) vessel spasm, (2) formation of the platelet plug, (3) development of a blood clot
(coagulation cascade), (4) clot retraction, and (5) clot dissolution.This multistep process
involves the interaction of substrates, enzymes, protein cofactors, and calcium ions that
circulate in the blood or are released from platelets and cells in the vessel wall.
U N D E R S T A N D I N G
Hemostasis
Vessel Spasm.
Injury to a blood
vessel causes vascular smooth mus-
cle in the vessel wall to contract and
thus instantaneously reduce blood
flow. Both local neural reflexes
and local humoral factors such as
thromboxane A
2
(TXA
2
), which is
released from platelets, contribute
to the vasoconstriction.
1
Injury to
endothelium
Smooth muscle
Platelets
Injury
Vessel
spasm
Formation of the Platelet
Plug.
Seconds after vessel injury,
von Willebrand factor, released from
the endothelium, binds to platelet
receptors, causing adhesion of the
platelets to the exposed collagen
fibers (
inset
). As the platelets adhere
to the collagen fibers on the damaged
vessel wall, they become activated
and release adenosine diphosphate
(ADP) and TXA
2
. The ADP and
TXA
2
attract additional platelets,
leading to platelet aggregation.
2
Collagen
Endothelium
Subendothelium
Platelet
aggregation
ADP
TXA
2
Factor
VIII
von Willebrand
factor (vWf)
Collagen
Platelet
Smooth muscle
