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40.1B: Open and Closed Circulatory Systems

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  • The circulatory system can either be open or closed, depending on whether the blood flows freely in a cavity or is contained in vessels.


    Summarize circulatory system architecture


    Key Points

    • A closed circulatory system, found in all vertebrates and some invertebrates, circulates blood unidirectionally from the heart, around the body, and back to the heart.
    • An open circulatory system, found in arthropods, pumps blood into a cavity called a hemocoel where it surrounds the organs and then returns to the heart(s) through ostia (openings).
    • The blood found in arthropods, a mix of blood and interstitial fluid, is called hemolymph.

    Key Terms

    • ostium: a small opening or orifice, as in a body organ or passage
    • hemolymph: a circulating fluid in the bodies of some invertebrates that is the equivalent of blood
    • hemocoel: the system of cavities between the organs of arthropods and mollusks through which the blood circulates

    Circulatory System Architecture

    The circulatory system is effectively a network of cylindrical vessels (the arteries, veins, and capillaries) that emanate from a pump (the heart). In all vertebrate organisms, as well as some invertebrates, this is a closed-loop system in which the blood is not moving freely in a cavity. In a closed circulatory system, blood is contained inside blood vessels, circulating unidirectionally (in one direction) from the heart around the systemic circulatory route, then returning to the heart again.


    Closed and open circulatory systems: (a) In closed circulatory systems, the heart pumps blood through vessels that are separate from the interstitial fluid of the body. Most vertebrates and some invertebrates, such as this annelid earthworm, have a closed circulatory system. (b) In open circulatory systems, a fluid called hemolymph is pumped through a blood vessel that empties into the body cavity. Hemolymph returns to the blood vessel through openings called ostia. Arthropods, such as this bee and most mollusks, have open circulatory systems.

    In contrast to a closed system, arthropods (including insects, crustaceans, and most mollusks) have an open circulatory system. In an open circulatory system, the blood is not enclosed in the blood vessels, but is pumped into a cavity called a hemocoel. The blood is called hemolymph because it mixes with the interstitial fluid. As the heart beats and the animal moves, the hemolymph circulates around the organs within the body cavity, reentering the heart through openings called ostia (singular: ostium). This movement allows for gas and nutrient exchange. An open circulatory system does not use as much energy to operate and maintain as a closed system; however, there is a trade-off with the amount of blood that can be moved to metabolically-active organs and tissues that require high levels of oxygen. In fact, one reason that insects with wing spans of up to two feet wide (70 cm) are not around today is probably because they were outmatched by the arrival of birds 150 million years ago. Birds, having a closed circulatory system, are thought to have moved more agilely, allowing them to obtain food faster and possibly to prey on the insects.