Heparin is an injectable anticoagulant, nowadays usually made synthetically. The injectable form of heparin is commonly derived from porcine intestine. It is used both as an anticoagulant in people, and in various medical devices such as test tubes and extracorporeal circulation devices such as renal dialysis machines. more...
Native heparin is a glycosaminoglycan with a molecular weight ranging from 6 kDa to 40 kDa. The average molecular weight of most commercial heparin preparations is in the range of 12 kDa to 15 kDa. Heparin consists of alternating units of sulfated D-glucosamine and D-glucuronic acid. Because of its ester and amide groups of sulfuric acid, it exists as the anion at physiologic pH and is usually administered as the sodium salt.
Heparin was originally isolated from liver cells, hence its name (hepar or "ηπαρ" is Greek for "liver"). Scientists were looking for an anticoagulant that could work safely in humans, and Jay McLean, a second-year medical student from Johns Hopkins University working under the guidance of William Henry Howell, found a compound extracted from liver that acted as an anticoagulant.
Mechanism of action
Heparin works by potentiating the action of antithrombin III, as it is similar to the heparan sulfate proteoglycans that are naturally present on the cell membrane of the endothelium. Because antithrombin III inactivates many coagulation proteins, the process of coagulation will slow down.
The effects of heparin are measured in the lab by the partial thromboplastin time (aPTT), (the time it takes the blood plasma to clot).
Heparin has to be adminstered parenterally: It is digested when taken by mouth. It can be injected intravenously, into a muscle, or subcutaneously (under the skin). Because of its short biologic half-life of approximately one hour, heparin must be given frequently or as a continuous infusion.
If long-term anticoagulation is required, heparin is often only used to commence anticoagulation therapy until the oral anticoagulant warfarin is working effectively.
When given parenterally, heparin acts as an anticoagulant, preventing the formation of clots and extension of existing clots within the blood. While heparin does not break down clots that have already formed, it allows the body's natural clot lysis mechanisms to work normally to break down clots that have already formed. Heparin is used for anticoagulation for the following conditions:
- Acute coronary syndrome, e.g., myocardial infarction
- Atrial fibrillation
- Deep-vein thrombosis/pulmonary embolism.
Test tubes, Vacutainers, and capillary tubes that use lithium heparin as an anticoagulant are usually marked with green stickers and green tops. Heparin has the advantage over EDTA as an anticoagulant, as it does not affect levels of ions (such as calcium). Heparin can interfere with some immunoassays, however. As lithium heparin is usually used, a person's lithium levels cannot be obtained from these tubes; for this purpose, royal-blue topped Vacutainers containing sodium heparin are used.
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