Veins, like arteries, have three layers: tunica intima, tunica media, and tunica externa. The tunica media is the middle layer and contains a thinner layer of smooth muscle and elastic fibers compared to arteries. This muscle layer helps regulate the internal diameter of the vein, which contributes to blood flow.
While veins have a muscle layer, it's less prominent than in arteries. This is because veins primarily rely on the pressure created by the heart and the contraction of skeletal muscles to propel blood back to the heart.
