Tunica media

Tunica media

The tunica media is the middle layer of the walls of blood vessels, particularly arteries and veins. It is primarily composed of smooth muscle cells and elastic tissue, which allow the vessel to regulate its diameter and maintain blood pressure.

Structure

The tunica media is situated between the tunica intima and the tunica externa. In arteries, the tunica media is typically thicker than in veins, reflecting the higher pressures that arteries must withstand. The smooth muscle cells in the tunica media are arranged in concentric layers, and the amount of elastic tissue varies depending on the type of vessel.

Diagram of artery wall structure, showing the tunica media.

Arteries

In arteries, the tunica media is rich in elastic fibers, especially in large elastic arteries such as the aorta. This elasticity allows the arteries to expand and recoil with each heartbeat, helping to maintain a consistent blood flow. The smooth muscle in the tunica media can contract or relax to change the diameter of the artery, a process known as vasoconstriction or vasodilation, respectively.

Cross-section of an artery, showing the tunica media.

Veins

In veins, the tunica media is thinner and contains fewer smooth muscle cells and elastic fibers compared to arteries. This reflects the lower pressure in the venous system. The reduced muscular and elastic content means veins are more compliant and can hold more blood.

Diagram of a vein, showing the tunica media.

Function

The primary function of the tunica media is to regulate the diameter of the blood vessel, which in turn controls blood flow and pressure. The smooth muscle cells respond to various stimuli, including neural and hormonal signals, to either contract or relax. This regulation is crucial for maintaining homeostasis and ensuring adequate blood supply to tissues.

Pathology

Diseases affecting the tunica media can have significant impacts on cardiovascular health. One such condition is atherosclerosis, where the buildup of plaques can lead to the thickening and hardening of the arterial walls, affecting the tunica media.

Calcification in atherosclerosis affecting the tunica media.

Related pages

• Tunica intima
• Tunica externa
• Vasoconstriction
• Vasodilation
• Atherosclerosis

References

• Guyton, A.C., & Hall, J.E. (2006). Textbook of Medical Physiology. Elsevier Saunders.
• Ross, M.H., & Pawlina, W. (2011). Histology: A Text and Atlas. Lippincott Williams & Wilkins.