Hemostasis refers to the process by which bleeding is stopped.

It consists of three main stages:

  1. Local vasoconstriction
  2. Formation of a platelet plug
  3. Blood coagulation (clotting)

Figure 1. General outline of hemostasis and the coagulation cascade

Description

Generally, hemostasis is divided into three distinct stages:

  1. Local vasoconstriction: vasospasm
  2. Primary hemostasis: platelet activation
  3. Secondary hemostasis: clot formation

Local vasoconstriction

Once a blood vessel is injured, an immediate response to the injury involves local vasoconstriction to reduce blood flow and a subsequent blood loss from the site of injury.

The vasoconstriction is primarily mediated by endothelin-1, a potent vasoconstrictor, which is synthesized by the damaged endothelium.

Formation of a platelet plug

Platelets circulate through the blood and become activated once they reach a site of vascular injury.

  • Their main functions include:
    • Adhesion
    • Aggregation
    • Release mediators
  • They carry numerous receptors:
    • Collagen

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