Hemostasis

Hemostasis

Definition Stages Vasoconstriction Platelet Plug Coagulation Sequence

1 Definition

Hemostasis is defined as a process during which bleeding from an injured blood vessel is arrested or stopped.

2 Stages of Hemostasis

  • During injury of a blood vessel, a series of reactions takes place leading to hemostasis.
  • Injury itself initiates the reactions.

During Injury

  • Endothelium of the blood vessel is damaged.
  • Collagen is exposed.
  • Platelets adhere to the exposed collagen and get activated.
  • Adherence of platelets to collagen is accelerated by von Willebrand factor.
  • This factor acts as a bridge between a specific glycoprotein present on the surface of platelet and collagen fibrils.
  • Activated platelets induce hemostasis.

Hemostasis Occurs in 3 Stages

1. Vasoconstriction
2. Platelet Plug Formation
3. Coagulation of Blood

3 Vasoconstriction

  • When a blood vessel is injured, it constricts and decreases the loss of blood.
  • Normally, arterioles and small arteries constrict.
  • Vasoconstriction is purely a local phenomenon.

When Blood Vessels Are Cut

  1. Endothelium is damaged.
  2. Collagen is exposed.
  3. Platelets adhere to the collagen and get activated.
  4. Activated platelets secrete serotonin and other vasoconstrictor substances.
  5. These substances cause constriction of the blood vessels.

4 Formation of Platelet Plug

Platelets adhered to collagen of the ruptured blood vessel secrete:
  • Adenosine diphosphate (ADP)
  • Thromboxane A₂

Actions of ADP and Thromboxane A₂

Attract more platelets
Activate them
Promote platelet aggregation

All the platelets aggregate together and form a:

  • Loose temporary platelet plug
  • Temporary hemostatic plug

Functions of the Platelet Plug

  • Closes the vessel.
  • Prevents further blood loss.
Platelet aggregation is accelerated by platelet-activating factor (PAF).
If platelet plug is unable to stop bleeding completely, the third stage, coagulation of blood, starts.

5 Coagulation of Blood

  • During this process, fibrinogen is converted into fibrin thread.
  • Fibrin threads get attached to the loose platelet plug.
  • This blocks the ruptured part of the blood vessel.
  • It prevents further blood loss completely.

6 Sequence of Hemostasis

Injury to blood vessel
↓
Damage of endothelium
↓
Exposure of collagen
↓
Von Willebrand factor
↓
Adherence of platelets to collagen
↓
Activation of platelets
↓
Stage 1
Secretion of serotonin
↓
Vasoconstriction
↓
Stage 2
Secretion of ADP and thromboxane A₂
↓
PAF
↓
Aggregation of platelets
↓
Platelet plug formation
↓
Stage 3
Formation of prothrombin activator
↓
Blood coagulation

7 Abbreviations

ADP

Adenosine diphosphate

PAF

Platelet-activating factor

Hemostasis