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
Hemostasis: The Prevention of Blood Loss
I. The initial response to blood vessel damage is
vasoconstriction and the sticking together of the opposed
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
b. This process is also enhanced by von Willebrand factor,
secreted by the endothelial cells, and by thromboxane
, 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
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
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
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
ﬁ brinolytic system
hematopoietic growth factor
platelet factor (PF)
pluripotent hematopoietic stem
tissue factor pathway inhibitor
tissue plasminogen activator
von Willebrand factor
SECTION F CLINICAL TERMS
Desmodus rotundus salivary
iron deﬁ ciency
iron-deﬁ ciency anemia
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.