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.
Section III. BLOOD TYPES OR GROUPS
Before being used for a transfusion, blood must be typed and cross-matched to
assure compatibility between the blood of the person who gets the transfusion and the
type of blood being used. The most well known of the groupings is the A-B-O group.
Each person has either type A, B, AB, or O. The most common types in America
are type 0 (45 percent) of the population and type A (41 percent). The blood type
indicates the presence or absence of certain antigens. These antigens stimulate the
body to produce antibodies. The antibodies react with their antigens to form
agglutination or little clusters (clumps) in the blood. When a person has type A blood,
his blood contains antibodies for type B, but none for type A. The opposite is true if he
has type B blood. That is, the person would have antibodies for type A, but none for
type B. If a person has type AB blood, he has neither type A nor type B antibodies and
can receive type A, B or AB blood. Type O blood causes no antibodies to be formed
and thus can be used to infuse any A-B-O group person. Figure 1-5 shows these
Figure 1-5 Blood types and compatible transfusions.