Blood Clotting Controls Bleeding
LEARNING OBJECTIVES
1.
Describe
the three mechanisms that stop bleeding.
2.
identify
the steps involved in forming a blood
clot.
3.
Explain
how your body minimizes blood clots
on a daily basis.
W
hile shaving in the morning, you nick your
face, leg, or underarm with a razor. You begin
to bleed. You instinctively apply pressure to
the wound to stop the bleeding, and your body
does something that eventually seals the wound. But how
exactly does your body prevent you from bleeding to death?
Hemostasis Helps Minimize Blood Loss
The
series
of physiological
re-
sponses
to
stop
bleeding when
blood vessels are injured is called
hemostasis
. The major hemostatic
mechanisms that stop large amounts of blood loss, or
hem-
orrhage
(HEM-o-rij), include the following (
Figure 10.7
):
Vascular spasm
, or
vasospasm ,
restricts blood flow in
damaged vessels.
Platelets assemble to form a
platelet plug
to seal the
damaged blood vessel. (This works well for small holes.)
Blood clotting
, or
coagulation
, is a more complex
mechanism that ultimately closes up the wound. This
process requires chemicals called
clotting factors
and
fibrinogen
.
Depending on the extent of damage, these mechanisms
take seconds to minutes to seal the wound.
hemostasis
(he-MO-
sta-sis) The stoppage
of bleeding.
Fibrinolysis Creates Space for the Final
Repair of a Damaged Vessel
Once a blood clot has sealed a wound, platelets within the
clot pull on the fibrin threads and contract the clot. The
contracting clot removes excess fluid and brings the dam-
aged surfaces close together so that permanent wound
CONCEPT CHECK
1.
What
are the steps in the process of platelet
plug formation?
2.
What
does prothrombinase do?
294 CHAPTER 10
The Cardiovascular System: Blood
healing can occur. Later, clots get
dissolved in a process called
fi-
brinolysis
that makes room for a
more permanent patch of connec-
tive tissue to be placed on the ves-
sel and a new endothelium to be
formed. Fibrinolysis involves the following steps:
1.
When a clot forms, an inactive plasma enzyme called
plasminogen
incorporates into the clot.
2.
Various factors from blood and tissues, such as thrombin,
factor XII, and tissue plasminogen activator (tPA),
convert inactive plasminogen into active
plasmin
.
3.
Plasmin digests fibrin threads and dissolves the clot
after the damage is repaired.
Occasionally a small clot called a
thrombus
(plural
is
throm bi)
will form in an unbroken vessel and dissolve
spontaneously. However, if the thrombus remains intact,
it can interfere with blood flow to the organ “downstream”
from the clot. Sometimes a clot can break off and travel
through the cardiovascular system, eventually blocking
small blood vessels. This moving
clot is called an
embolus
. Em-
boli can lodge in various places
and cause problems, such as pul-
monary embolism (lungs), heart
attack (heart), stroke (brain), or
kidney failure (kidney). To treat
such embolic events, physicians
try to dissolve the clots by inject-
ing tissue plasminogen activator (tPA) into the patient. If
this is done soon after the event begins, blood flow can be
re-established through the vessel, and the damage to the
blocked organ can be minimized.
embolus
A blood
clot, a bubble of air or
fat from broken bones,
a mass of bacteria, or
other debris or foreign
material transported
by the blood. The
plural is
e m b o li.
fibrinolysis
(fT-br
i
-
NOL-i-sis) Dissolution
of a blood clot by the
action of a protein-
digesting enzyme.
3.
H
ow
can tissue plasminogen activator (tPA) be
used to treat a heart attack?
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