Arteries have three concentric layers of tissue:
the intima the media the adventitia
The internal elastic membrane borders the intima and the external elastic membrane separates the media from the adventitia.
The intima surrounds the lumen of all blood vessels and is a single continuous layer of endothelial cells.
The endothelial cells synthesise regulators of thrombosis including prostacyclin, plasminogen activator and heparin like molecules.
Endothelial cells produce prothrombotic molecules including Von Willebrand factor, and plasminogen activator inhibitor.
The intima modulates blood flow and vascular reactivity through synthesis of vasoconstrictors:
Intima synthesizes vasodilators
Intima regulates immune and inflammatory reactions, elaborates interleukins, adhesion molecules and histocompatibility antigens
The Media’s smooth muscle cells dilate and constrict to accommodate blood flow
The Intima is perfused by small arterioles called vasa vasorum
Adventitia is the outer layer of the artery made up of connective tissue, perfused by vasa vasorum.