Arteries have three main layers or tunics known as the adventitia (outer), media (middle) and intima (inner). The outer layer is made of connective tissue with collagen fibers, the middle layer consists of smooth muscle and elastic fibers, and the innermost layer is composed of specialized squamous cells supported by the basement membrane of connective tissue.
The tunica adventitia, or outer layer, is the strongest of the three layers of an artery. The elastic collagen fibers allow arteries to stretch to prevent over-expansion due to higher pressures in these blood vessels.
The tunica media, or middle layer, has smooth muscle fibers in addition to elastic fibers that can stretch. This layer is thicker in arteries than in veins. The muscle layer is surrounded on either side by elastic fibers.
The tunica intima, or inner layer, features a thin membrane lining and smooth epithelial tissue that is covered by elastic tissue. A thin layer of connective tissue anchors the innermost part of arteries to the walls of the blood vessels.
Arteries range in size from 25 millimeters (mm) to 0.3 mm as they diminish in diameter the further the arteries get away from the heart. The smallest arteries are called capillaries, and they have walls only one cell thick to allow for cellular exchanges of nutrients and waste material.