The Blood 313
Vessel cutAggregation
Hemorrhage^ of platelets^ProthrombinThromboplastinFibrinogen (^) Red cells
Fibrin^
Thrombin
Platelets^
Figure 13- 5 The stages of blood clotting.
Red cells
enmeshed
in fibrin
(^) ®
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we call the clot. As the clot forms, blood cells and plate-
lets get enmeshed in the fibrin threads and the wound stops
bleeding. Clot retraction or syneresis (sih-NER-eh-sis) is
the tightening of the fibrin clot in such a way that the
ruptured area of the blood vessel gets smaller and smaller,
thus decreasing the hemorrhage. The clear yellowish liquid
that is seen after the clot forms is called serum. Serum is
blood plasma without the clotting fac-tors. Now that the
hemorrhage is stopped, blood vessel tissues repair
themselves by mitotic cellular division.
Once the tissue is -repaired, fibrinolysis (fi-brih-NOL-ih-
sis) or dissolution of the blood clot occurs. This is caused
by a plasma protein that digests the fibrin threads and other
proteins associated with the formation of the clot. Table 13-
1 summarizes the chemical reactions in blood clotting.
Occasionally, unwanted clotting may occur in an
undamaged blood vessel. This is brought about by a
cholesterol-containing mass called plaque (PLAK) that
adheres to the smooth walls of blood vessels. This
Table 13- 1 The Chemical Reactions in Blood Clotting (^)
(^) Ca+ Ca+ (^)
Injured tissue thromboplastin Ca + prothrombin activator (^)
Prothrombin activator + Ca+ (^)
Prothrombin thrombin
®^
Learning^(^)
- Soluble fibrinogen thrombin fibrin threads
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