In order for blood to always flow unidirectionally through a closed circulatory system, the in order for blood to always flow unidirectionally through a closed circulatory system, the capillaries must have a thick endothelium pressure in all vessels must be equal arteries must be elastic blood vessels farthest from the heart must have valves

The  blood vessels farthest from the heart must have valves.
Blood flow through the body system by the action of the pumping heart and it is usually pushed under high velocity and pressure away from the heart, toward the main artery and  the aorta. The flow of blood through the capillary beds is usually regulated by the body needs which is directed by the nerves and hormone signals. However, for blood to always flow unidirectionally through a closed circulatory system, the  blood vessels farthest from the heart must have valves.