Pathology of the hemostatic system: thrombocytopenias and thrombocytopathies

Thrombocytopenias: causes and mechanisms of development

Thrombocytopenia is a typical form of pathology in which the number of blood platelets per unit volume drops below the physiological norm.

This condition is caused by factors of various origins:

• Physical: high doses of ionizing radiation, which suppress platelet hemopoiesis.
• Chemical: exposure to cytostatics and alkylating agents.
• Biological: the effect of exotoxins and endotoxins of various microorganisms.

The development of the pathology is facilitated by mutations in hematopoietic stem cells, a deficiency of thrombopoietin, and an excess of fibrinogen, which provokes massive agglutination and aggregation. In addition, cells can be destroyed under the attack of autoaggressive immunoglobulins (Ig) or immunocytes.

Pathogenesis includes four main mechanisms:

1. Suppression of the platelet germ line in the bone marrow.
2. Increased destruction of circulating platelets.
3. Massive consumption of cells during generalized thrombus formation (consumption thrombocytopenia).
4. Excessive sequestration (pooling) in the spleen.

The consequence of these processes is a drop in the activity of platelet coagulation factors, prolongation of bleeding time, poor blood clot retraction, and, as a result, the formation of hemorrhagic syndrome.

Thrombocytopathies: qualitative defect

Thrombocytopathy represents an impairment of key platelet properties: adhesive, aggregatory, and coagulant. The main difference between this condition and quantitative deficiency lies in the presence of stable, long-term defects of the cells themselves. These changes can affect their morphology, biochemistry, or functional activity, which naturally leads to hemostatic disorders.

To correct the consequences (bleeding, hemorrhages, and microhemocirculation disorders), a symptomatic treatment principle is applied:

• Administration of solutions: the use of plasma and plasma expanders to restore normal blood rheological properties.
• Stop bleeding: implementation of adequate hemostatic therapy.
• Elimination of post-hemorrhagic conditions: combating resulting hypovolemia, anemia, and other consequences of blood loss.

Physiology and typical disorders of hemostasis

In a narrow sense, hemostasis is the process of stopping bleeding, the biological role of which is to prevent blood loss in case of damage to blood vessels or the heart. Normally, it ensures protein coagulation, as well as adhesion, aggregation, and activation of formed blood elements to form a thrombus.

The hemostatic system relies on three categories of mechanisms:

• Coagulation system: responsible for the coagulation of plasma proteins and the creation of a clot.
• Anticoagulant system: blocks or inhibits the coagulation process.
• Fibrinolytic system: performs lysis (dissolution) of the formed fibrin.

The initiation of coagulation occurs via two pathways. The intrinsic pathway starts with the activation of factor XII, while the extrinsic pathway is initiated by the release of tissue factor and the activation of factor VII. Both cascades ultimately converge on a common stage — the activation of factor X.

There are three groups of typical pathologies of the hemostatic system:

1. Hypercoagulation (thrombotic syndrome): an inadequate, excessive increase in clotting with the formation of blood clots, which leads to ischemia of organs and tissues. Causes may include vascular wall damage (trauma, vasculitis, atherogenesis, angiopathies) or blood cell pathologies (thrombocytopathies, increased erythrocyte hemolysis, hyperaggregation).
2. Hypocoagulation (hemorrhagic syndrome): a pathological decrease in clotting and impaired clot formation.
3. Thrombohemorrhagic syndromes: conditions characterized by phased impairment of the hemostatic system.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanisms of suppression of platelet hemopoiesis in radiation injury.
• The role of autoaggressive immunoglobulins in platelet destruction.
• Pathogenesis of hemorrhagic syndrome in hypocoagulation.
• Thrombohemorrhagic syndromes: phases of hemostatic disorders.
• Effect of high fibrinogen concentration on platelet aggregation.

Other topics in "Chapter 21. Typical Pathology Forms of the Blood System"

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