Veins have valves that allow blood to flow only towards the heart.
Breathing also helps return blood to the heart since the pressure change in the thoracic cavity during inhalation cause the vena cavae and large veins near the heart to expand and fill. Capillaries in the brain, heart, kidneys, and liver usually carry a full load of blood. Two mechanisms regulate the distribution of blood in capillary beds. Both are controlled by nerve signals and hormones. One mechanism involves the contraction and relaxation of the smooth muscle layer in the walls of arterioles.