Arteries, capillaries and veins form a continuous system lined by endothelium. The endothelial layer is low friction to allow smooth blood flow. The endothelium forms the innermost layer of arteries; | Arteries, capillaries and veins form a continuous system lined by endothelium. The endothelial layer is low friction to allow smooth blood flow. The endothelium forms the innermost layer of arteries; |