Endothelium

The endothelium separates the blood from the extravascular space. It is comprised of dynamic endothelial cells (EC) that are flattened, and have diverse functions. Tight junctions between the cells prevent diffusion of substances.
1. Endothelial cells produce substances that prevent blood clotting and ensure smooth blood flow. It exhibits both anticoagulant and when injured, thrombotic properties.
2. Endothelial cells lead the formation of new blood vessel as may occur during wound healing, angiogenesis, revascularization
3. Endothelial cells stimulated by cytokines such as interleukin-1 (IL-1) or tumor necrosis factor a (TNF-a) express cell adhesion molecules called P-selectin that allow for capturing and tethering the leukocytes (neutrophil adhesion). Other adhesion molecules attract lymphocytes.
4. Endothelial cells allow transport of substances (glucose) and regulate passage of fluids.
5. It regulates vascular tone.

Substances produced by endothelium Functions
Procoagulant properties
Von Willebrand factor (vWF) Secreted by the endothelium, stored in the Weibel-Palade bodies. Participates in platelet adhesion and coagulation
Secretes adhesive proteins: collagen, basement membrane, vitronectin, fibronectin, laminin Promote platelet adhesion and coagulation
Endothelins Vasoconstriction
Prostacyclin (PGI2) and Nitric oxide also called endothelium-derived relaxing factor (EDRF) Vasodilatation
Plasminogen Activator inhibitor (PAI-1) Inactivates tissue plasminogen activator (tPA) and urokinase PA (uPA).
Platelet activating factor (PAF) Activates platelets and neutrophils.
Tissue factor expression Only seen in cultured Endothelium when exposed to toxins
Effector cell protease receptor-1 (EPR-1) Facilitates binding of coagulation factor and allowing activation to occur.
Anticoagulant properties
Thrombomodulin and Heparan Sulfate Binds to thrombin. Heparan activates Antithrombin
Tissue plasminogen activator (tPA) and urokinase PA (uPA) Activates plasminogen
Prostacyclin (PGI2) and Nitric oxide Inhibits platelet aggregation
Synthesizes Tissue Factor Pathway Inhibitor (TFPI) Inhibits tissue factor (TF)
Protein S A cofactor for Protein C that helps inactivate FVa and FVIIIa
Annexin V Anticoagulant shield on the surface