5
Blood (continued)
fibrin monomers. It should be noted that thrombin does not
exist as such in circulating blood, but is generated during clot-
ting from an inactive precursor, prothrombin (factor II). Interest-
ingly, the coagulation of blood can be induced by certain snake
venoms, which either promote the formation of thrombin or
clot fibrinogen directly, accounting in part for their toxicity.
Bacterial enzymes, including staphylocoagulase (derived from
Staphylococcus aureus), may also induce clotting.
Platelets
Besides furnishing phospholipids for the clotting process,
platelets help to stanch the flow of blood from injured blood
vessels by accumulating at the point of injury, forming a plug.
Platelets participate in the phenomenon of clot retraction, in
which the blood clot shrinks, expelling liquid serum. Although
the function of retraction is unknown, individuals in whom this
process is impaired have a bleeding tendency.
Hereditary defects
Hereditary deficiencies of the function of the various
protein-clotting factors have been described, including classic
hemophilia (in which antihemophilic factor is present in a non-
functional form). Most of the hereditary functional deficiencies
are associated with a bleeding tendency. Therapy for bleeding
due to deficiencies of clotting factors often includes the trans-
fusion of blood plasma or fractions of plasma rich in particular
substances that the individual may lack.
Heparin
Heparin, a polysaccharide–sulfuric acid complex found par-
ticularly in the liver and lungs, impairs coagulation by interfer-
ing with the formation of the prothrombin-converting principle
and with the action of thrombin. Both coumarin and heparin
are used clinically to impede coagulation in thrombotic states,
including thrombophlebitis and coronary heart disease.
Fig. 5: Colorized scanning electron micrograph of blood clotting in an ovarian follicle.
The red blood cells (erythrocytes; colorized red) are trapped in filaments of fibrin protein
(colorized gray).
(Credit: Science Photo Library/Alamy Stock Photo)
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