[Functions of the endothelium]

The venous endothelium, acting as an interface between the blood and the layer of smooth muscle cells, plays a major role in many essential functions of vessel physiology. The endothelium prevents and inhibits pathological activation of the coagulation system and plays an active role in the regulation of venous tone. Mediators include the prostacyclin which has vasodilating and antiagregating properties and nitric oxide (NO), a vasodilator and inhibitor of thrombosis, adhesion and cellular proliferation. To maintain endothelial integrity and tissue cohesion, endothelial cells express adhesion factors which regulate cell to cell and cell to extracellular matri interactions. Finally, the endothelium plays a role in vasotrophism by synthesizing, or activating growth factors which act directly on cell renewal of the endothelium itself and proliferation of adjacent tissues. Thus, the venous endothelium cannot be considered simply as a biologically inert barrier but rather as an active cell layer capable of intercell signalling and interactions constantly modified by the pathophysiological environment.