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Hemostasis

*Haemostasis*

For medical students2 min readUpdated 2026-10-10

The hemostatic system performs a dual function: it maintains blood in a fluid state for tissue perfusion while simultaneously ensuring rapid arrest of bleeding upon vascular injury. Dysfunction of these mechanisms leads to pathological thrombosis.

Blood RheologyMaintaining blood in a fluid state to ensure adequate tissue perfusion.
VasospasmImmediate protective response to injury in small vessels.
ThrombosisIntravascular formation of thrombi leading to lumen occlusion.
Platelet AggregationClumping of platelets into aggregates upon endothelial injury.

Dual Function of the Hemostatic System

To understand the pharmacology of drugs affecting the blood, it is essential to highlight two primary tasks of this system:

Mechanisms of Bleeding Arrest

The physiological process of bleeding cessation following trauma occurs in stages:

  1. Vasospasm: an immediate response to injury, effective primarily in minor trauma of small vessels.
  2. Thrombus formation: the creation of a hemostatic plug as a protective body response that mechanically seals the defect and prevents blood loss.

Pathological Thrombosis

Sometimes this protective mechanism fails, turning into pathology—intravascular thrombosis without external bleeding. This causes vessel occlusion and impairs normal blood flow.

Three main triggers of thrombosis:

Key Stages of Thrombus Formation

All thrombus formation—physiological or pathological—relies on two basic processes:

The initial stage starts with endothelial injury. At this point, platelets form aggregates via adhesion: they bind directly to collagen and other subendothelial proteins, where von Willebrand factor plays a crucial bridging role.

Mnemonic

Hemostasis is a balance: fluid blood (rheology) on one side, and a platelet plug (hemostasis) on the other.

Frequently asked questions

What diseases or conditions lead to endothelial injury of the vascular wall?

Various pathological conditions and external influences lead to vascular endothelial injury. The primary etiologic factors causing destruction of the vessel wall and exposure of the subendothelial layer include:

  • Atherosclerosis
  • Hypertension
  • Mechanical trauma
  • Infections

These factors lead to reduced synthesis of protective substances (prostacyclin and nitric oxide) and loss of the endothelial antithrombotic properties.

Which subendothelial proteins bind platelets during adhesion?

During adhesion, platelets bind to the following subendothelial matrix proteins:

  • Collagen — provides direct attachment of platelets via specific membrane receptors (GP Ia).
  • Von Willebrand factor — acts as a mediator («bridge») between the vessel wall and the platelet, binding to the GP Ib receptor.

Other subendothelial proteins may also participate in the process, interacting with platelets at the site of injury.

What are the two main physiological functions of the hemostatic system?

First, maintaining blood in a fluid state for organ perfusion; second, rapidly stopping bleeding upon vascular injury.

In which cases is vasospasm effective?

Vasospasm is effective only in minor injuries of small vessels as an immediate response to trauma.

What triggers platelet aggregation?

The process is triggered by vascular endothelial injury, when platelets adhere to collagen and subendothelial proteins with the participation of von Willebrand factor.

What triggers intravascular thrombosis?

Vascular wall pathology, hemodynamic disturbances, and a blood factor imbalance due to a deficiency of natural anticoagulants.

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