Overview of hemostasis

Hemostasis is the balance between bleeding and clotting (thrombosis). It is achieved by the following components in the blood:
1. Blood vessel wall
2. Platelets
3. Clotting proteins or factors
4. Fibrinolytic system
5. Naturally occurring anticoagulants.

Introduction

The purpose of the blood clotting or coagulation system is two-fold:

1. Keep blood in the fluid state such that it circulates.

2. Prevent leakage of blood whenever there is an injury to the blood vessel wall or from normal wear and tear, by sealing defects in the vessel wall or endothelium.

When a blood vessel is injured, it narrows (vasoconstriction) thereby diverting and decreasing the amount of blood flow. At the site of the injury, blood is exposed to the subendothelium (the layer below the endothelium); bleeding is then rapidly stopped by a process known as hemostasis. There are three components of hemostasis:

First the platelets, a disc shaped tiny blood cells, stick or adhere to the damaged blood vessel and then to each other (aggregation) forming a platelet clump that can plug and stop minor bleeding. A glue like protein called von Willebrand factor is produced by the endothelium as well as the platelets and binds the platelet to the site of injury.

As the platelet adhere and aggregate, they change shape (like starfish) and are called “activated”. Such platelets release chemicals such as serotonin that further potentates vasoconstriction, thromboxaneA2 that causes platelet aggregation. Activated platelets also express on their surface certain proteins (receptors) that attach to von Willebrand factor, fibrinogen (glycoprotein IIb-IIIa), collagen (glycoprotein Ia) and a crucial clotting protein called thrombin. A “platelet plug” is sometimes sufficient to stop minor bleeding.

Finally the activated platelets and the damaged tissue initiate blood clotting by activating a number of clotting proteins. A clot often stops major bleeding. Excess clot formation is prevented or removed by proteins called anticoagulants (shown here as the revolving blade termed plasmin). Platelet release a factor called platelet derived growth factor (PDGF) that promotes wound repair and healing.