Overview
This lecture explains the three main steps of blood clotting (hemostasis), detailing how the body stops bleeding after blood vessel injury.
Step 1: Vascular Spasm
- Blood vessel injury causes smooth muscle in the vessel wall to constrict.
- This constriction is called vascular spasm and reduces blood flow at the injury site.
- The goal is to limit blood loss through the damaged vessel.
Step 2: Platelet Plug Formation
- Damage exposes collagen fibers in the vessel's connective tissue.
- Platelets (cell fragments from megakaryocytes) circulate in blood and recognize exposed collagen.
- Platelets stick to collagen and become activated within 30-60 seconds.
- Activated platelets release prothrombin activator.
Step 3: Coagulation Phase
- Prothrombin activator converts prothrombin (a liver-produced, vitamin K-dependent protein) into thrombin.
- Thrombin acts as molecular scissors to cut fibrinogen into fibrin.
- Fibrin forms protein threads that reinforce the platelet plug, stabilizing the clot.
- The vessel wall is now closed off and blood loss stops.
- Connective tissue remodeling helps repair the vessel wall, aided by fibrin.
Key Terms & Definitions
- Hemostasis — The process of stopping bleeding; also called blood clotting.
- Vascular Spasm — Smooth muscle constriction of a blood vessel after injury.
- Platelet Plug — Temporary aggregate of platelets at the injury site.
- Prothrombin Activator — Substance released by activated platelets to trigger clotting.
- Prothrombin — Protein made in the liver, converted into thrombin during clotting.
- Thrombin — Enzyme that converts fibrinogen into fibrin.
- Fibrinogen — Soluble protein in blood plasma, converted into fibrin.
- Fibrin — Insoluble protein threads that form the framework of a blood clot.
Action Items / Next Steps
- Review the three phases of hemostasis and the role of each component.
- Understand the sequence of activation from vessel injury to clot stabilization.