Plasma coagulation factors
Plasma factors form the basis of the coagulation cascade. Historically, they have been assigned Roman numerals. Key representatives:
- Factor I (Fibrinogen): A protein synthesized in the liver. Under the action of thrombin, it converts into insoluble fibrin, the strands of which form the basis of the blood clot. It also participates in platelet aggregation.
- Factor II (Prothrombin): Produced in the liver with the participation of vitamin K. Under the action of the prothrombinase complex, it turns into an active enzyme — thrombin.
- Factor III (Tissue thromboplastin): A cell membrane phospholipoprotein. Released upon tissue destruction and triggers coagulation via the extrinsic pathway.
- Factor IV (Calcium ions): Required at all stages of coagulation.
- Factor V (Proaccelerin, Ac-globulin): A liver protein, the precursor of accelerin (Va), which is necessary for the formation of prothrombinase.
- Factor VI: Excluded from modern nomenclature.
- Factor VII (Proconvertin): A vitamin-K-dependent glycoprotein required for the formation of tissue prothrombinase and the activation of factor X.
- Factor VIII (Antihemophilic globulin A): A glycoprotein required for the formation of blood prothrombinase.
- Factor IX (Christmas factor, antihemophilic globulin B): A glycoprotein. Interacting with other components, it participates in the activation of factor X.
- Factor X (Stuart–Prower factor, thrombotropin): A glycoprotein involved in the formation of both tissue and blood prothrombinase.
- Factor XI (Rosenthal factor): A glycoprotein that, after interacting with factor XII, activates factor IX.
- Factor XII (Hageman factor): Initiates the intrinsic pathway of coagulation upon contact with damaged tissues.
- Factor XIII (Fibrinase): Stabilizes the formed fibrin.
In addition to the numbered ones, Fletcher factor (plasma prekallikrein) and Fitzgerald factor (high-molecular-weight kininogen) are distinguished. Both are proteins involved in the activation of factors XI, XII, and the plasminogen system.
Platelet and other cellular factors
Platelets contain substances necessary for blood clotting and wound healing, including various growth factors. A total of 12 platelet factors are known. The most significant of them are:
- Factor 3 (Platelet thromboplastin): A phospholipid of the blood platelet membrane. Released upon their destruction and plays a crucial role in initiating the formation of blood prothrombinase.
- Factor 4 (Antiheparin factor): Responsible for binding and neutralizing heparin.
- Factor 5 (Platelet fibrinogen): Necessary for platelet aggregation. Under the influence of thrombin, it transforms into fibrin, which strengthens the platelet plug.
- Factor 8 (Thrombosthenin): A specific complex of contractile proteins that ensures clot retraction.
- Vasoconstriction factors: Vasoconstrictor factor (factor 9) and serotonin.
- Aggregation stimulants: Aggregation factor (factor 12) and ADP.
During platelet destruction or activation, thromboxane $A_2$ is also abundantly released — an extremely potent vasoconstrictor and aggregation stimulant.
Other cells are also involved in hemostasis. For example, erythrocytes and leukocytes also contain clotting factors, with erythrocytes serving as an important source of erythrocytic thromboplastin.
Tissue factors and the role of vitamin K
Damaged body tissues supply their own substances regulating hemostasis into the bloodstream. These include highly active tissue thromboplastin and endothelial von Willebrand factor, which is absolutely essential for the adhesion (attachment) of platelets to the site of vascular injury. Tissues also contain antiheparin factor, platelet aggregation stimulants, fibrinolysis activators, and compounds structurally similar to plasma factors (V, VII, and X).
Vitamin K is of immense clinical importance in the hemostatic system. Based on dependence on it, all factors are divided into two groups:
- Vitamin-K-dependent: factors II, VII, IX, X.
- Vitamin-K-independent: all other factors.
Vitamin K deficiency leads to severe consequences: inhibition of blood clotting processes, increased bleeding tendency, as well as the occurrence of subcutaneous and internal hemorrhages.
Briefly: answers to topical questions
What components make up the prothrombinase complex?
The enzymatic prothrombinase complex consists of four mandatory components. It includes:
- Factor Xa — activated factor X, which is the central component of the complex.
- Factor Va — the activated form of factor V (plasma ac-globulin), acting as an activator protein.
- Calcium ions ($Ca^{2+}$) — factor IV.
- Phospholipids — negatively charged components of destroyed cell membranes, acting as a matrix for assembly.
The complex is formed via both the extrinsic and intrinsic pathways of coagulation and catalyzes the conversion of prothrombin to thrombin.
What is the exact biochemical role of calcium ions (factor IV) in the coagulation cascade?
Calcium ions (factor IV) are required for binding clotting factors to the phospholipid matrix. They ensure the attachment of proteolytic enzymes and their cofactors to the negatively charged phospholipids of cell membranes.
The formation of a bond via $Ca^{2+}$ allows the assembly of membrane complexes (prothrombinase, tenase). This ensures close contact between factors, increases their proteolytic activity, and accelerates enzyme activation by more than 10,000 times. Due to this, calcium ions participate in all phases of blood coagulation.
Which factors are involved in activating the intrinsic blood coagulation pathway?
The intrinsic coagulation pathway is triggered upon contact with subendothelial collagen. Its activation involves:
- Factor XII (Hageman factor) — upon contact with collagen, it converts to XIIa.
- Prekallikrein / kallikrein and high-molecular-weight kininogen (HMWK, Fitzgerald factor) — participate in the contact phase: the kallikrein–HMWK complex activates factor XII, and the XIIa–HMWK complex activates factor XI.
- Factor XI (Rosenthal factor) — after activation to XIa, it activates factor IX.
- Factor IX (Christmas factor) — after activation to IXa, it forms a complex with factor VIIIa, Ca2+, and phospholipids.
- Factor VIIIa — a component of the intrinsic tenase complex IXa + VIIIa + Ca2+ + phospholipids, which activates factor X.
For quick memorization of vitamin-K-dependent factors, the "1972 year rule" is used: its digits hide the numbers of factors 10 (implying 1), 9, 7, and 2.
Class and exam questions
What are the differences between hemophilia A, B, and C?
These diseases are caused by the hereditary deficiency of various plasma glycoproteins. Hemophilia A occurs due to a lack of factor VIII, hemophilia B due to factor IX, and hemophilia C due to factor XI.
What is the function of von Willebrand factor?
This is a tissue (endothelial) factor that is critically necessary to ensure adhesion — the primary "sticking" of platelets to the damaged vascular wall.
What factors cause vascular spasm (vasoconstriction)?
Thromboxane A2, serotonin, and the specific platelet vasoconstrictor factor (factor 9) possess potent vasoconstrictive effects.
What else to study on this topic
- Mechanisms of tissue and blood prothrombinase formation
- Differences between the extrinsic and intrinsic pathways of blood coagulation
- Clot retraction process and the role of the thrombosthenin complex
- Involvement of the endothelium in the synthesis and release of von Willebrand factor
- Physiological functions and activation of plasminogen
Master the entire topic in the "Physiology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on «Blood clotting factors» and 4,128 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 114 video lessons on the subject
Other topics in the "Blood" section
Free video lessons: PhysiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.