Arterial endothelium and atherothrombogenesis. I--Intact endothelium in vascular and blood homeostasis.

Normal endothelium constitutes a physical and biological barrier between the blood and the vascular wall, and also acts as a sensor and transducer of various endogenous and exogenous factors that modulate the blood circulation. Endothelial activity in a given individual at any particular moment reflects the balance between cardiovascular risk factors, genetic predisposition and vascular protection mechanisms. The availability and activity of endothelium-derived nitric oxide (NO) is a major factor in these mechanisms. Further, vasoactive substances synthesized by the vascular wall and/or by blood cells may affect the behavior of the blood-endothelium interface. Vasomotricity is dependent on the balance between vasodilator substances (particularly prostacylin) and vasoconstrictor products (mainly endothelin-1 and angiotensin II). The coagulation-anticoagulation or fibrinolysis balance is also affected by various different proteins. The mechanisms of these factors, on which blood fluidity depends under normal conditions and with intact endothelium, are discussed, along with mention of potential abnormalities, which will be examined in the second part of this review.