Small Veins and Muscular Arteries
Small veins and arteries also travel in pairs and share their outer tunic (adventitia). They usually have three tunics: an intima (endothelium) media (smooth muscle) and adventitia (connective tissue). Arteries are more thick-walled than veins and veins are often supplied with valves to prevent backflow of blood. Lymphatic vessels closely parallel blood vessels, are exceedingly thin-walled (one layer of endothelium) and are often collapsed in tissue sections.