Open circulatory system
The circulatory system is an organ system responsible for transporting blood, nutrients, gases, and other molecules throughout the body. The two types of circulatory systems are the open circulatory system and the closed circulatory system. The difference lies on the mode of transporting blood in the body of an animal. In open circulatory system, the blood is not confined within blood vessels. The blood bathes the organs and tissues directly. Because of this the blood and the interstitial fluid have no distinction. Thus, the term hemolymph is used. The hemolymph is pumped by the heart through the vessels into the sinuses and then to the surrounding cells where exchange of materials occur. The hemolymph is taken back to the heart through the ostia that close via the valves during heart contraction.
The hemolymph is composed of a fluid plasma (containing water, organic compounds, and inorganic salts such as Na+, Cl-, K+, Mg2+, and Ca2+). There are also free-floating cells called hemocytes that are involved in the immune function.