Blood from the capillary now carrying carbon dioxide and other cellular waste drains into larger vessels called venues that empty into still larger veins. Veins provide a low- resistance pathway by which blood can return to the heart. The walls of veins are much thinner and more expandable than those of the arteries, although both contain a layer of smooth muscle. Because blood pressure in the veins is low, the contractions of skeletal muscle during exercise and breathing must assist in the return of the blood to the heart. These muscular movements squeeze the veins, forcing blood through them. When veins are compressed, you might predict that blood would be forced away from the heart as well as toward it. To prevent this, veins are eqipped with one-way valves that allow blood flow only toward the heart. When you sit or stand for long periods, the lack of muscle activity allows blood to accumulate in the veins of the lower legs. This accumulation accounts for swollen feet often experienced by airplane passengers. Long periods of inactivity can also contribute to varicose veins, in which the valves become stretched and weakened. If blood pressure should fall, for instance after extensive bleeding, veins can help restore it. The sympathetic nervous system automatically stimulates contraction of the smooth muscles in the vein walls. This action decreases the internal volume of the veins and raises blood pressure, speeding up the return of the blood to the heart.