Describe the internal structure of an artery and a vein in short.

(N/A) Blood is circulated in a definite pathway by blood vessels,primarily arteries and veins. Both consist of three layers:
$(1)$ Tunica Intima: The innermost layer,composed of a lining of squamous endothelium.
$(2)$ Tunica Media: The middle layer,composed of smooth muscles and elastic fibers. This layer is significantly thicker in arteries than in veins.
$(3)$ Tunica Externa: The outermost layer,composed of fibrous connective tissue with collagen fibers.