Both blood vessels have three layers: (C) Tunica Externa, (D) Tunica media, and (E) Tunica intima, also known as the endothelium.

The tunica externa (tunica adventitia in some references) is the outer layer of the blood vessel that is usually composed of connective tissue that provides basic structural support for the blood vessels.

The tunica media is the middle layer of a blood vessel that is made up of thin, cylindrical, smooth muscle cells and elastic tissue. It also supports the blood vessels and is essential to help regulate blood flow and pressure within the blood vessels by changing vessel diameter (through constriction and dilation). This layer also makes up the bulk of arterial walls.

The endothelium is a thin monolayer of endothelial cells. This layer is directly in contact with blood and plays a role in keeping the blood moving smoothly throughout the body.

The arteries are noticeably thicker and bulkier-looking than veins. This is because arteries need to handle the high-pressure, oxygenated blood leaving the heart, while veins have thinner walls as they transport lower-pressure, deoxygenated blood back to the heart.