In order for blood to always flow unidirectionally through a closed circulatory system, the

Closed Circulatory system in which the blood is confined to vessels and is kept separate from the intestinal fluid. Closed systems are more efficient at transporting circulatory fluids to tissues and cells.  chemical exchanges occurs between blood and intestinal fluid, as well as between intestinal fluid and body cells.

segmentation=endoskeleton=backbone=coelom=tissues