General concept and main participants
Fibrinolysis is a critically important mechanism for restoring vascular patency after a thrombus has fulfilled its function of stopping bleeding. At the center of this biochemical cascade is the conversion of a proenzyme into its active form.
Initially, an inactive protein — plasminogen — circulates in the blood plasma. To initiate clot dissolution, it must transform into plasmin. This reaction occurs under the influence of specific substances: plasma and tissue activators. As soon as plasmin is formed, it begins to directly affect the thrombus (networks of insoluble fibrin), cleaving it into degradation products — fibrinopeptides.
Phase I: Formation of activators
Fibrinolysis proceeds in three stages. The first phase consists of the formation and activation of plasminogen stimulants. Notably, the natural trigger for the start of this stage is hemocoagulation (blood clotting) itself.
There are three main activation systems:
- Extrinsic system. Represented by tissue activators produced by vascular endothelial cells. A classic example is urokinase — an activator synthesized by the epithelium of the urinary tract.
- Intrinsic system. Based on blood plasma components. Here, the most important stimulant is activated Hageman factor (coagulation factor XIIa).
- Cellular system. Includes activators produced directly by formed elements of the blood: erythrocytes, leukocytes, and platelets.
Phase II: Transformation into plasmin
The second phase involves the key biochemical reaction — the conversion of plasminogen into active plasmin under the influence of accumulated activators.
The mechanism of this process is strictly ordered:
- Activated Hageman factor (XIIa) acts on prekallikrein and releases the enzyme kallikrein from it.
- Kallikrein, working together with kininogen, catalyzes the transition of plasminogen to plasmin.
An important feature of this phase is its localization. Plasminogen has the ability to adsorb onto fibrin threads. Due to this, the entire cascade of plasmin formation unfolds not diffusely in the bloodstream, but directly inside the fibrin clot.
Phase III: Cleavage of the thrombus
The final stage is characterized by the direct destruction of the thrombus. Under the influence of active plasmin, peptide bonds in insoluble fibrin molecules are broken. As a result, the thrombus disintegrates into soluble products — individual peptides and amino acids.
In addition to lysing fibrin itself, plasmin possesses broad proteolytic activity. It is capable of additionally cleaving:
- Fibrinogen;
- Coagulation factor V;
- Coagulation factor VIII.
Such combined action ensures not only the removal of the already formed clot, but also the prevention of excessive formation of new thrombi in this vascular segment.
Briefly: answers to topical questions
Which fibrinolysis inhibitors (antiplasmins) are present in blood plasma?
According to their sources, natural limiters of fibrinolysis include:
- Antiplasmins — circulate in the blood and block the action of plasmin; specifically named is α2-antiplasmin, which rapidly inactivates plasmin and prevents systemic fibrinogenolysis.
- Specific plasminogen activator inhibitors — neutralize plasminogen activators; specifically named is plasminogen activator inhibitor type 1 (PAI-1), which suppresses fibrinolysis.
Which specific tissue plasminogen activators (besides urokinase) are synthesized by the endothelium?
In addition to the urokinase activator, endothelial cells secrete another specific activator.
- Tissue plasminogen activator (tPA) — secreted by endothelial cells (in particular, when they maintain a normal phenotype or under the influence of heparin) and participates in the paracrine regulation of thrombus resorption.
To easily remember which coagulation factors (besides fibrin and fibrinogen) are destroyed by plasmin, use the "Five-Eight" rule: plasmin cleaves factors V and VIII.
Class and exam questions
Where exactly does the conversion of plasminogen to plasmin occur?
This process occurs directly within the fibrin clot, as plasminogen is pre-adsorbed onto fibrin.
What acts as the natural stimulant of fibrinolysis?
The natural trigger for the formation of plasminogen activators is the hemocoagulation (blood clotting) process itself.
What role does urokinase play?
Urokinase is a representative of the extrinsic fibrinolysis activation system (a tissue activator) produced by the epithelium of the urinary tract.
What else to study on this topic
- The role of Hageman factor (XIIa) in the intrinsic activation system
- The interaction of kallikrein and kininogen
- Properties and functions of fibrinopeptides
- Mechanisms of blood formed elements participation in cellular activation
- The effect of plasmin on coagulation factors V and VIII
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 "Fibrinolysis" and 4,128 other subject pages
- 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.