What are the various layers of blood vessels?

Blood vessels have three layers of tissue:

Tunica Intima

The innermost and thinnest layer of arteries and veins, tunica intima is in direct contact with blood, surrounding the blood as it flows through the body. Tunica intima serves multiple roles. It keeps the blood flowing smoothly, regulates blood pressure, protects blood from toxins, and prevents blood clots.  

Tunica Media

The middle layer of arteries and veins, tunica media is made up of elastic fibers that helps the blood vessels contract and expand, and maintains the unidirectional flow of blood. 

Tunica Externa

Tunica externa, also known as tunica adventitia, is composed of collagen and is the outermost layer of arteries and veins. It is located adjacent to the tunica media. Its collagen content gives blood vessels their structure and support. Tiny vessels and nerves in the tunica externa help to deliver oxygen and nutrients from the blood to the cells while also removing waste for removal from the body.