Section II. FUNCTION OF THE BLOOD IN TRAUMA-HEMOSTASIS
Hemostasis (the arrest of bleeding) has three separate mechanisms--platelet
clumping (agglutination), contracting of blood vessels, and forming a fibrin clot.
When either a vein or an artery (not a capillary) is cut or damaged, platelets
begin to accumulate rapidly at the site of injury and cling to the interior blood vessel
wall. These form a temporary plug that can stop the bleeding.
At the same time, the muscles of the damaged vessel constrict. Shortly after the
first plug forms, platelets fuse into a dense mass. This mass forms a temporary solid
seal at the injury site. As the platelets agglutinize (clump), a second type of constriction
occurs. The platelets release a chemical called "serotonin." This chemical causes the
second type of constriction of the injured and neighboring blood vessels. After this
series of platelet changes, the clotting mechanism occurs.
The soluble protein "fibrinogen" is transformed into the insoluble "fibrin" in a very
complex, but rapid, process. Besides the fibrinogen, the clotting process requires
prothrombin (which is converted into thrombin), vitamin K, calcium, thromboplastin, and
many other factors that participate in the clotting process. When the platelets stick to
the surface of the damaged vessel, calcium ion, and active thromboplastin are released
from the cells and split prothrombin into two parts. One of these parts is thrombin.
When thrombin is released from the prothrombin, it transforms the fibrinogen into fibrin.
a. The fibrin becomes a network of fibers or threads that trap blood cells,
platelets, and plasma to form the blood clot. Calcium ions appear necessary for the
operation of the sequence that results in the release of thrombin from prothrombin.
b. The synthesis of prothrombin and some of the other clotting process factors
takes place in the liver. Vitamin K is needed for the liver to produce these factors. If
any of these factors are in short supply or absent, the person would have a tendency to
bleed. This is characteristic of a genetically transmitted disease called "hemophilia."