Hemostasis Notes

Hemostasis Definition

Hemostasis is defined as arrest or stoppage of bleeding.

Stages Of Hemostasis

When a blood vessel is injured, the injury initiates a series of reactions resulting in hemostasis. It occurs in three stages:

1. Vasoconstriction
2. Platelet plug formation
3. Coagulation of blood.

Read And Learn More: Medical Physiology Notes

1. Vasoconstriction

• Immediately after injury, the blood vessel constricts and decreases the loss of blood from the damaged portion. Usually, arterioles and small arteries constrict.
• The vasoconstriction is purely a local phenomenon. When the blood vessels are cut, the endothelium is damaged and the collagen is exposed
• The platelets adhere to this collagen, and get activated.
• The activated platelets secrete serotonin and other vasoconstrictor substances which cause constriction of the blood vessels.
• The adherence of platelets to the 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.

2. Formation Of Platelet Plug

• The platelets get adhered to the collagen of ruptured blood vessel and secrete ADP and thromboxane A₂. These two substances attract more and more platelets and activate them.
• AM these platelets aggregate together and form a loose temporary platelet plug or temporary hemostatic plug, which closes the vessel and prevents further blood loss. The platelet aggregation is accelerated by the platelet-activating factor (PAF)

3. Coagulation Of Blood: During this process, the fibrinogen is converted into fibrin. The fibrin threads get attached to the loose platelet plug, which blocks the ruptured part of blood vessels and prevents further blood loss completely.