 In arthropods, the open circulatory system is a system in which a fluid in a cavity called the hemocoel bathes the organs directly with oxygen and nutrients and there is no distinction between blood and interstitial fluid; this combined fluid is called hemolymph or haemolymph.Muscular movements by the animal during locomotion can facilitate hemolymph movement, but diverting flow from one area to another is limited.When the heart relaxes, blood is drawn back toward the heart through open-ended pores (ostia).