Open Circulatory System

The Open Circulatory System is a system in which fluid (called hemolymph) 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 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).

Hemolymph fills all of the interior hemocoel of the body and surrounds all cells.

Hemolymph is composed of water, inorganic salts (mostly Na⁺, Cl^-, K⁺, Mg²⁺, and Ca²⁺), and organic compounds (mostly carbohydrates, proteins, and lipids).

The primary oxygen transporter molecule is hemocyanin.