A health care provider is getting an informed consent to start a patient on intravenous heparin therapy. Which of the following statements is most accurate regarding the action of heparin?
a. Heparin inactivates clotting factors that are currently circulating in the blood-stream.
b. Heparin dilates the arterial lumen so blood can flow around a site of injury.
c. Heparin prevents clots from forming by making blood vessel walls slippery.
d. Heparin interferes with production of vitamin K-dependent clotting factors in the liver.
ANS: A
Unfractionated heparin (UFH) is comprised of long-chain molecules that form tertiary complexes with antithrombin III, allowing for inactivation of IIa (thrombin) as well as factors Xa and XIa. In low dosages, UFH prevents the conversion of prothrombin to thrombin by its effects on factor Xa. In higher doses, UFH neutralizes thrombin, thus preventing the conversion of fibrinogen to fibrin.
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