Most veins are 1-9 mm in diameter with a well developed adventitia. Small and medium sized veins have valves to prevent backflow within their lumens. The valves consist of two folds of the tunica intima composed of elastic tissue and covered with endothelium that project into the lumen. Veins with valves are prevalent in the extremities where the contraction of skeletal muscle helps to propel the blood toward the heart . Large veins such as those in the abdomen contain smooth muscle in the adventitia that can contract peristalticly to propel blood toward the heart against gravity.
Cardiac muscle is present in the vena cava and pulmonary veins as they join the heart.