In some thromboembolic conditions, it is desirable to delay the coagulation process. Various anticoagulants have been developed for this purpose. The ones most useful clinically are heparin and the coumarins.

HEPARIN AS AN INTRAVENOUS ANTICOAGULANT

Commercial heparin is extracted from several different animal tissues and prepared in almost pure form. Injection of relatively small quantities, about 0.5 to 1 mg/kg of body weight, causes the blood-clotting time to increase from a normal of about 6 minutes to 30 or more minutes. Furthermore, this change in clotting time occurs instantaneously, thereby immediately preventing or slowing further development of a thromboembolic condition.

The action of heparin lasts about 1.5 to 4 hours. The injected heparin is destroyed by an enzyme in the blood known as heparinase.

COUMARINS AS ANTICOAGULANTS

 When a coumarin, such as warfarin, is given to a patient, the amounts of active prothrombin and Factors VII, IX, and X, all formed by the liver, begin to fall. Warfarin causes this effect by inhibiting the enzyme VKOR c1. As discussed previously, this enzyme converts the inactive, oxidized form of vitamin K to its active, reduced form. By inhibiting VKOR c1, warfarin decreases the available active form of vitamin K in the tissues. When this de crease occurs, the coagulation factors are no longer carboxylated and are biologically inactive. Over several days the body stores of the active coagulation factors degrade and are replaced by inactive factors. Although the coagulation factors continue to be produced, they have greatly decreased coagulant activity.

After administration of an effective dose of warfarin, the coagulant activity of the blood decreases to about 50 percent of normal by the end of 12 hours and to about 20 percent of normal by the end of 24 hours. In other words, the coagulation process is not blocked immediately but must await the degradation of the active prothrombin and the other affected coagulation factors already present in the plasma. Normal coagulation usually returns 1 to 3 days after discontinuing coumarin therapy.

PREVENTION OF BLOOD COAGULATION OUTSIDE THE BODY

 Although blood removed from the body and held in a glass test tube normally clots in about 6 minutes, blood collected in siliconized containers often does not clot for 1 hour or more. The reason for this delay is that preparing the surfaces of the containers with silicone prevents contact activation of platelets and Factor XII, the two principal factors that initiate the intrinsic clotting mechanism. Conversely, untreated glass containers allow contact activation of the platelets and Factor XII, with rapid development of clots.

Heparin can be used for preventing coagulation of blood outside the body, as well as in the body. Heparin is especially used in surgical procedures in which the blood must be passed through a heart lung machine or artificial kidney machine and then back into the person.

Various substances that decrease the concentration of calcium ions in the blood can also be used for pre venting blood coagulation outside the body. For instance, a soluble oxalate compound mixed in a very small quantity with a sample of blood causes precipitation of calcium oxalate from the plasma and thereby decreases the ionic calcium level so much that blood coagulation is blocked.

Any substance that deionizes the blood calcium will prevent coagulation. The negatively charged citrate ion is especially valuable for this purpose, mixed with blood usually in the form of sodium, ammonium, or potassium citrate. The citrate ion combines with calcium in the blood to cause an unionized calcium compound, and the lack of ionic calcium prevents coagulation. Citrate anti coagulants have an important advantage over the oxalate anticoagulants because oxalate is toxic to the body, whereas moderate quantities of citrate can be injected intravenously. After injection, the citrate ion is removed from the blood within a few minutes by the liver and is polymerized into glucose or metabolized directly for energy. Consequently, 500 milliliters of blood that has been rendered incoagulable by citrate can ordinarily be transfused into a recipient within a few minutes without dire consequences. However, if the liver is damaged or if large quantities of citrated blood or plasma are given too rapidly (within fractions of a minute), the citrate ion may not be removed quickly enough and the citrate can, under these conditions, greatly depress the level of calcium ion in the blood, which can result in tetany and convulsive death.

اخر الاخبار

اخبار العتبة العباسية المقدسة

العتبة العباسية المقدسة: نحو 5 آلاف مستفيد يوميًّا من خدمات المطبخ المركزي في لبنان

العتبة العباسية المقدسة تعلن أسماء الفائزين بمسابقة ذكرى ولادة السيدة فاطمة المعصومة (عليها السلام)

قسم العلاقات يناقش مع المديرية العامّة لتربية كربلاء سبل تعزيز التعاون المشترك

وفد ثقافي عراقي يطّلع على مقتنيات متحف الكفيل ونفائسه ويشيد بطرائق عرضها

المزيد

الآخبار الصحية

عدد مرات التبرز يكشف أسرارا عن صحة الأمعاء

علامات جينية طويلة الأمد تتركها الرضاعة الطبيعية في دم الطفل

هل يمكن إعادة برمجة الجهاز المناعي لإنتاج أجسام مضادة نادرة؟

دراسة: أدوية الزهايمر "لا تحدث فرقا يذكر لدى المرضى"

المزيد

الآخبار التكنلوجية

روسيا تطور تقنية جديدة لتعديل خصائص البلاديوم

الصين: طاقم مهمة شينتشو-21 سيبقى في الفضاء شهرا آخر

"أبل" تهدد بحذف تطبيق "غروك" من متجرها وتكشف السبب

"أبل" تهدد بحذف تطبيق "غروك" من متجرها وتكشف السبب

المزيد

مواضيع ذات صلة

The Rationale and Methodology for Hepcidin-Lowering Agents

Therapy of Iron Deficiency

Intestinal Iron Absorption

Liver Regulation of Systemic Iron Homeostasis and Iron Storage

Recycling of Erythrocyte Iron by Macrophages

Use of Iron For Erythropoiesis

Essential Features OF Red Blood Cell Homeostasis

sentinel lymph node biopsy (SLNB, Lymphoscintigraphy)

salivary gland nuclear imaging (Parotid gland nuclear imaging)

pulmonary angiography (Pulmonary arteriography)

Fever of Unknown Origin

Biological Markers of Mental Illness