1-4 INTRODUCTION

Hemostasis (the arrest of bleeding) has three separate mechanisms – platelet clumping (agglutination), contracting of blood vessels, and forming a fibrin clot.

1-5. TEMPORARY PLUG

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.

1-6. MUSCLE CONSTRICTION

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.

1-7. CLOT FORMATION

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.”

c. This clotting process is the same one that causes a thrombus (blood clot) to form in the blood vessels of the leg or arm. Normally, the body contains the factor “heparin” in the tissues. This substance is produced in many organs of the body, and it prevents clotting in the absence of blood vessel damage. If there is a breakdown in this system, the drug “dicumarol” can be used clinically as an anticoagulant. It blocks the stimulating effect of vitamin K on the liver. Therefore, the liver produces less prothrombin, and the blood loses its tendency to form thromboses.