In an open circulatory system, the circulatory fluid (hemolymph) is not entirely confined to blood vessels. Instead, it is pumped by the heart into a primary body cavity known as the haemocoel (or hemocoel). In this cavity, the fluid directly bathes the internal organs, allowing for the direct exchange of nutrients and gases. This system is characteristic of most invertebrates, such as arthropods (insects, spiders) and most molluscs.

A. vein, B. artery, and C. arteriole: These are specialized tubular blood vessels that form a continuous network in a closed circulatory system. For instance,Veins return blood to the heart, arteries carry blood away from the heart, arterioles are smaller branches of arteries that lead to capillaries.