Platelets and Hemostasis: How They Stop Bleeding

When a blood vessel is injured, platelets orchestrate hemostasis through a coordinated cascade:

1. Vasoconstriction: Injured vessel spasms, reducing blood flow.
2. Platelet Adhesion: Platelets bind exposed collagen/von Willebrand factor at injury site.
3. Platelet Activation: Shape change, pseudopodia formation, granule release (ADP, thromboxane A₂).
4. Platelet Aggregation: Activated platelets clump, forming a primary plug.
5. Clot Retraction: Platelets contract, tightening the plug.
6. Fibrin Formation: Platelets provide phospholipid surface for coagulation cascade → fibrin mesh reinforces plug.
7. Wound Healing: Platelet-derived growth factors promote tissue repair.
8. Fibrinolysis: Plasmin dissolves clot after healing.

Platelets are essential for rapid bleeding control and initiating repair.