| Blood coagulation factors interact to form the secondary, fibrin-rich, hemostatic plug in small vessels, and the secondary fibrin thrombus in arteries and veins
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| Plasma coagulation factors are identified by roman numerals: they are listed in Table 6.2, together with some of their properties. Tissue factor was formerly known as factor III, calcium ion as factor IV; factor VI does not exist. Congenital deficiencies of other coagulation factors (I-XIII) result in excessive bleeding, which illustrates their physiologic importance in hemostasis. The exception is factor XII deficiency, which does not increase the bleeding tendency, despite prolonging blood clotting times in vitro; the same is true for its cofactors, prekallikrein or high-molecular-weight kininogen (HMWK). A possible explanation for this is given below.
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Figure 6.4 illustrates the currently accepted scheme of blood coagulation. Since the early 1960s, this has been accepted as a 'waterfall' or 'cascade' sequence of interactive pro-enzyme to enzyme conversions, each enzyme activating the next pro-enzyme in the sequence(s). Activated factor enzymes are designated by the letter 'a' - for example, factor XIa. Traditionally, the scheme has been divided into three parts:
- the intrinsic pathway,
- the extrinsic pathway,
- the final common pathway.
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Table 6-2.
Coagulation factors and some of their properties. |
| Body_ID: None |
| Coagulation factors and their properties |
| Body_ID: T006002.50 |
| Factor | Synonyms | Molecular weight (Da) | Plasma concentration (mg/dL) |
| Body_ID: T006002.100 |
| I | Fibrinogen | 340000 | 200-400 |
| Body_ID: T006002.150 |
| II | Prothrombin | 70000 | 10 |
| Body_ID: T006002.200 |
| III | Tissue factor (thromboplastin) | 44000 | 0 |
| Body_ID: T006002.250 |
| IV | *Calcium ion | 40 | 9-10 |
| Body_ID: T006002.300 |
| V | Proaccelerin, labile factor | 330000 | 1 |
| Body_ID: T006002.350 |
| VII | Serum prothrombin conversion accelerator (SPCA), stable factor | 48000 | 0.05 |
| Body_ID: T006002.400 |
| VIII | Antihemophilic factor (AHF) | 330000 | 0.01 |
| Body_ID: T006002.450 |
| (vWF) | | (250000)n | 1 |
| Body_ID: T006002.500 |
| IX | Christmas factor | 55000 | 0.3 |
| Body_ID: T006002.550 |
| X | Stuart-Prower factor | 59000 | 1 |
| Body_ID: T006002.600 |
| XI | Plasma thromboplastin | 160000 | 0.5 |
| Body_ID: T006002.650 |
| | antecedent (PTA) | | |
| Body_ID: T006002.700 |
| XII | Hageman factor | 80000 | 3 |
| Body_ID: T006002.750 |
| XIII | Fibrin-stabilizing factor (FSF) | 320000 | 1-2 |
| Body_ID: T006002.800 |
| Prekallikrein | Fletcher factor | 85000 | 5 |
| Body_ID: T006002.850 |
| High-molecular-weight kininogen (HMWK) | Fitzgerald, Flaujeac; or Williams factor, contact activation cofactor | 120000 | 6 |
| Body_ID: T006002.900 |
|
| Body_ID: T006002.950 |
*To convert calcium ion to mmol/L multiply by 0.2495
n indicates number of subunits.
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| page 69 |  | | page 70 |
| Figure 6.4 Blood coagulation: activation of coagulation factors. After the initiation of blood coagulation, the coagulation factor pro-enzymes are activated sequentially: activated factor enzymes are designated by the letter 'a'. The purple box indicates contact factors that have no apparent function in in vivo hemostasis. Phospholipids are supplied in vivo by platelets. HMWK, high-molecular-weight kininogen. |
| page 70 | | | page 71 |
These are described below. They are distinguished on the basis of the nature of the initiating factor and its corresponding test in the clinical hemostasis laboratory; hence, three tests of coagulation are performed in clinical laboratories on citrated, platelet-poor plasma:
- activated partial thromboplastin time (APTT),
- prothrombin time,
- thrombin
 time.
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| Platelet-poor plasma is used in these tests because the platelet count influences clotting time results. To obtain the platelet-poor plasma, citrate anticoagulant is added to blood to sequester calcium ions reversibly, and the blood is centrifuged at 2000 g for 15 minutes. The coagulation time tests are initiated by adding calcium and appropriate initiating agents.
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