Tunica media

The tunica media is the middle layer of a blood-vessel wall, composed chiefly of smooth muscle cells and elastic components. Its structure varies with vessel type and helps regulate vessel diameter and withstand pressure.

Vascular smooth muscle: Contractile cells in blood-vessel walls that regulate vessel diameter and blood flow. These cells form the tunica media’s main contractile component.
Vasoconstriction: The narrowing of a blood vessel caused by contraction of its smooth muscle. Media contraction narrows the vessel lumen through this process.
Elastic artery: A large artery whose media contains abundant elastic lamellae, allowing it to expand and recoil with each heartbeat. Its elastic-rich media smooths the pressure pulses leaving the heart.
Cystic medial degeneration: A pattern of medial extracellular-matrix breakdown and smooth-muscle loss in the aorta. It weakens the aortic media and is associated with aneurysm and dissection.
Elastic fiber: A connective-tissue fiber that stretches under force and recoils when the force is removed. Elastic fibers let the media stretch with each pulse and recoil afterward.
Vasodilation: The widening of a blood vessel caused by relaxation of its smooth muscle. Relaxation of medial smooth muscle enlarges the lumen through this process.
Muscular artery: A medium-sized artery whose media contains many smooth muscle cells and regulates blood distribution to organs. Its muscle-rich media adjusts regional blood flow by changing vessel diameter.
Aortic dissection: A condition in which blood enters and separates layers of the aortic wall through a tear in the intima. The dissection plane often extends through the media, splitting its layers.
Collagen: A structural protein that forms fibers providing tensile strength in connective tissues. Collagen in the media helps limit excessive vessel-wall stretching.
Vascular tone: The partial, ongoing contraction of vascular smooth muscle that sets vessel diameter at rest. The media’s baseline contraction establishes vascular tone.

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