and if it were inside a vessel it would stimulate more
and unnecessary clotting, which might eventually
obstruct blood flow.
Prevention of Abnormal Clotting
Clotting should take place to stop bleeding, but too
much clotting would obstruct vessels and interfere
with normal circulation of blood. Clots do not usually
form in intact vessels because the endothelium(sim-
ple squamous epithelial lining) is very smooth and
repels the platelets and clotting factors. If the lining
becomes roughened, as happens with the lipid depo-
sits of atherosclerosis, a clot will form.
Heparin, produced by basophils, is a natural antico-
agulant that inhibits the clotting process (although
heparin is called a “blood thinner,” it does not “thin”
or dilute the blood in any way; rather it prevents a
chemical reaction from taking place). The liver pro-
duces a globulin called antithrombin, which combines
with and inactivates excess thrombin. Excess thrombin
would exert a positive feedback effect on the clotting
cascade, and result in the splitting of more prothrom-
bin to thrombin, more clotting, more thrombin
formed, and so on. Antithrombin helps to prevent this,
as does the fibrin of the clot, which adsorbs excess
thrombin and renders it inactive. All of these factors
are the external brake for this positive feedback mech-
anism. Together they usually limit the fibrin formed to
what is needed to create a useful clot but not an
obstructive one.
Thrombosis refers to clotting in an intact vessel;
the clot itself is called a thrombus. Coronary throm-
bosis, for example, is abnormal clotting in a coronary
artery, which will decrease the blood (oxygen) supply
to part of the heart muscle. An embolismis a clot
or other tissue transported from elsewhere that lodges
in and obstructs a vessel (see Box 11–7: Dissolving
Clots).
SUMMARY
All of the functions of blood described in this chap-
ter—transport, regulation, and protection—contri-
bute to the homeostasis of the body as a whole.
However, these functions could not be carried out if
the blood did not circulate properly. The circulation
of blood throughout the blood vessels depends upon
the proper functioning of the heart, the pump of the
circulatory system, which is the subject of our next
chapter.268 Blood
BOX11–7 DISSOLVING CLOTS
Abnormal clots may cause serious problems in
coronary arteries, pulmonary arteries, cerebral
vessels, and even veins in the legs. However, if
clots can be dissolved before they cause death of
tissue, normal circulation and tissue functioning
may be restored.
One of the first substances used to dissolve
clots in coronary arteries was streptokinase,
which is actually a bacterial toxin produced
by some members of the genus Streptococcus.
Streptokinase did indeed dissolve clots, but its
use created the possibility of clot destruction
throughout the body, with serious hemorrhage a
potential consequence.
Safer chemicals called third-generation
thrombolytics are now used (thrombo “clot”
and lytic “to lyse” or “split”). In a case of coro-
nary thrombosis, if a thrombolytic can be admin-
istered within a few hours, the clot may be
dissolved and permanent heart damage pre-
vented. The same procedure is also used to pre-
vent permanent brain damage after strokes
(CVAs) caused by blood clots.STUDY OUTLINE
The general functions of blood are trans-
portation, regulation, and protection.
Characteristics of Blood
- Amount—4 to 6 liters; 38% to 48% is cells; 52% to
62% is plasma (Fig. 11–1). - Color—arterial blood has a high oxygen content
and is bright red; venous blood has less oxygen and
is dark red.- pH—7.35 to 7.45; venous blood has more CO 2 and
a lower pH than arterial blood. - Viscosity—thickness or resistance to flow; due to
the presence of cells and plasma proteins; con-
tributes to normal blood pressure.