The arteries and veins of humans and most animals are made up of several different layers, or “tunics.” The tunica media is the middle layer. It is surrounded on top by the tunica adventitia or tunica externa, and underneath by the tunica intima. This middle layer contains soft elastic tissues and, in the largest of the arteries, it houses nerves. It is usually the thickest of the layers, and is the most flexible, making it an essential component to effective and consistent blood transportation.
Blood is constantly pumped through the heart, distributed to all parts of the body, and returned to the heart in a systematic circuit. The circuit is made possible by veins and arteries, each of which acts as a series of tubes or pipes for blood. A cross-section of any given tube reveals several layers of tissue and muscle, the middle-most of which is the tunica media.
Arteries and veins vary in size. Some are very small, like those in the fingers and toes; these arteries are usually referred to as arterioles. Others, particularly those close to the heart, can be quite large. They can be primarily muscular or primarily elastic in function. Regardless of size, the walls of each are composed nearly identically.
There is more to transporting blood than simply carrying it — changes in blood pressure, adrenaline, and stress can all cause blood to course faster or slower through the body. Speed of blood flow is regulated in part by the rigidity and constriction of the arteries and veins through which the blood passes. The middle layer is primarily responsible for constricting actions.
All layers of arteries and veins contain elastic fibers, but the bulk of elastic and mobile properties are located in the tunica media. In arteries, the tunica media is composed primarily of smooth muscle and connective tissue, which contains both elastic and collagenous fibers. Veins do not generally contain muscle fibers, but the middle layer of veins is composed of similarly elastic fibers and pliable cells. The thrust of vein and artery constriction and movement is controlled in the middle layer.
The tunica media is most pronounced, and most powerful, in the larger arteries surrounding the heart, such as the aorta. These arteries are primarily muscular, while smaller arteries and arterioles are generally primarily elastic. Larger arteries need more constriction and relaxation power than smaller channels of blood transportation, and need to be strong enough to contain and distribute large quantities of blood. An even control of blood flow directly into and out of the heart’s chambers is also essential to heart health and blood pressure consistency.
Larger arteries also can contain nerves that carry electrical signals to the surrounding tissues and tunics, instructing them when to constrict. These nerves are housed within the smooth muscle of the middle layer. Nerve control is just one more way that the tunica media can help to maintain a consistent blood flow throughout the body’s pulmonary system of arteries and veins.