438
Chapter 12
III. The leukocytes include three classes of polymorphonuclear
granulocytes (neutrophils, eosinophils, and basophils),
monocytes, and lymphocytes.
IV. Platelets are cell fragments essential for blood clotting.
V. Blood cells are descended from stem cells in the bone
marrow. Hematopoietic growth factors control their
production.
Hemostasis: The Prevention of Blood Loss
I. The initial response to blood vessel damage is
vasoconstriction and the sticking together of the opposed
endothelial surfaces.
II. The next events are formation of a platelet plug followed by
blood coagulation (clotting).
III. Platelets adhere to exposed collagen in a damaged vessel and
release the contents of their secretory vesicles.
a. These substances help cause platelet activation and
aggregation.
b. This process is also enhanced by von Willebrand factor,
secreted by the endothelial cells, and by thromboxane
A
2
, produced by the platelets.
c. Fibrin forms the bridges between aggregating platelets.
d. Contractile elements in the platelets compress and
strengthen the plug.
IV. The platelet plug does not spread along normal endothelium
because the latter secretes prostacyclin and nitric oxide,
both of which inhibit platelet aggregation.
V. Blood is transformed into a solid gel when, at the site
of vessel damage, plasma fi
brinogen is converted into
fi brin molecules, which then bind to each other to form
a mesh.
VI. This reaction is catalyzed by the enzyme thrombin, which
also activates factor XIII, a plasma protein that stabilizes the
fi brin meshwork.
VII. The formation of thrombin from the plasma protein
prothrombin is the end result of a cascade of reactions
in which an inactive plasma protein is activated and then
enzymatically activates the next protein in the series.
a. Thrombin exerts a positive feedback stimulation of
the cascade by activating platelets and several clotting
factors.
b. Activated platelets, which display platelet factor and
binding sites for several activated plasma factors, are
essential for the cascade.
VIII. In the body, the cascade usually begins via the extrinsic
clotting pathway when tissue factor forms a complex with
factor VIIa. This complex activates factor X, which then
catalyzes the conversion of small amounts of prothrombin
to thrombin. This thrombin then recruits the intrinsic
pathway by activating factor XI and factor VIII, as well
as platelets, and this pathway generates large amounts of
thrombin.
IX. The liver requires vitamin K for the normal production of
prothrombin and other clotting factors.
X. Clotting is limited by three events: (1) Tissue factor
pathway inhibitor inhibits the tissue factor–factor VIIa
complex; (2) protein C, activated by thrombin, inactivates
factors VIIIa and Va; and (3) antithrombin III inactivates
thrombin and several other clotting factors.
SECTION F KEY TERMS
albumin
425
antithrombin III
436
basophil
429
bilirubin
426
blood coagulation
432
bone marrow
426
clot
432
clotting
432
eosinophil
429
erythropoiesis
427
erythropoietin
427
extrinsic pathway
433
ferritin
426
fi brin
432
fi brinogen
425
fi brinolytic system
436
folic acid
426
globulin
425
hematopoietic growth factor
(HGF)
429
hemoglobin
425
hemostasis
431
heparin
436
intrinsic factor
427
intrinsic pathway
433
iron
426
lymphocyte
429
megakaryocyte
429
monocyte
429
neutrophil
429
nitric oxide
432
plasma protein
425
plasmin
436
plasminogen
436
plasminogen activator
436
platelet activation
431
platelet aggregation
431
platelet factor (PF)
433
platelet plug
431
pluripotent hematopoietic stem
cell
429
polymorphonuclear
granulocyte
429
prostacyclin (PGI
2
)
432
protein C
436
prothrombin
433
reticulocyte
426
serum
425
thrombin
433
thrombomodulin
435
thromboxane A
2
431
thrombus
432
tissue factor
435
tissue factor pathway inhibitor
(TFPI)
435
tissue plasminogen activator
(t-PA)
436
transferrin
426
vitamin B
12
427
vitamin K
435
von Willebrand factor
(vWF)
431
SECTION F CLINICAL TERMS
anemia
428
aspirin
436
Desmodus rotundus salivary
plasminogen activator
(DSPA)
437
hematoma
431
hemochromatosis
426
hemophilia
435
hemorrhage
428
hypercoagulability
435
iron defi ciency
426
iron-defi ciency anemia
428
malaria
428
oral anticoagulants
437
pernicious anemia
427
polycythemia
428
recombinant t-PA
437
sickle-cell anemia
428
streptokinase
437
thrombolytic therapy
437
SECTION F REVIEW QUESTIONS
1. Give average values for total blood volume, erythrocyte
volume, plasma volume, and hematocrit.
2. Which is the most abundant class of plasma protein?
XI. Clots are dissolved by the fi brinolytic system.
a. A plasma proenzyme, plasminogen, is activated by
plasminogen activators to plasmin, which digests fi brin.
b. Tissue plasminogen activator is secreted by endothelial
cells and is activated by fi
brin in a clot.
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