261
emostasis
is a multistep process that maintains the
integrity of a closed high-pressure circulatory system
after vessel injury. The normal process of hemostasis is
regulated by a complex array of activators and inhibi-
tors that maintain blood fluidity and prevent blood from
leaving the vascular compartment. Hemostasis is normal
when a blood vessel is sealed to prevent blood loss and
hemorrhage. It is deemed abnormal when inappropriate
blood clotting occurs or when clotting is insufficient to
stop the flow of blood from the vascular compartment.
Disorders of hemostasis fall into two main categories:
the inappropriate formation of clots within the vascular
system (thrombosis) and the failure of blood to clot in
response to an appropriate stimulus (bleeding).
Hemostasis and Blood
Coagulation
Hemostasis preserves vascular integrity by balancing
the processes that maintain blood in a fluid state and
prevent excessive bleeding following injury. The process
involves the transformation of blood into a semisolid
clot with erythrocytes trapped in its fibrin meshwork at
the site of injury (Fig. 12-1).
Components of Hemostasis
Hemostasis is a multistep process that involves platelets,
plasma clotting factors, naturally occurring anticoagu-
lants, and the inherent properties of the endothelial lin-
ing of blood vessels.
Platelets
Platelets, also called
thrombocytes,
are large fragments
from the cytoplasm of bone marrow cells called
mega-
karyocytes.
1–5
There are normally 150,000 to 400,000
platelets in each microliter (
μ
L) of blood
1
with an aver-
age platelet life span of 8 to 9 days. Platelets do not
leave the blood as white blood cells do, but at any time
about one third of them are stored in blood-filled spaces
in the spleen and can be released into the circulation as
needed. Platelet production is controlled by a protein
H
Disorders of
Hemostasis
12
C h a p t e r
Hemostasis and Blood Coagulation
Components of Hemostasis
Platelets
The Coagulation System
Endothelium
Clot Formation and Dissolution
Vessel Spasm
Platelet Plug Formation
The Coagulation Cascade
Clot Retraction
Clot Dissolution
Hypercoagulability States
Increased Platelet Function
Increased Clotting Activity
Inherited Disorders
Acquired Disorders
Bleeding Disorders
Platelet Disorders
Thrombocytopenia
Impaired Platelet Function
Coagulation Disorders
Inherited Disorders
Acquired Disorders
Bleeding Associated with Vascular Disorders
Disseminated Intravascular Coagulation
