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21.3: Circulatory System Architecture

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    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 free in a cavity. Circulatory systems may be open (mixed with the interstitial fluid) or closed (separated from the interstitial fluid).

    Closed Circulatory Systems

    In a closed circulatory system, blood is contained inside blood vessels and circulates unidirectionally from the heart around the systemic circulatory route, then returns to the heart again, as illustrated in Figure 1a.

    Illustration A shows the closed circulatory system of an earthworm. Dorsal and ventral blood vessels run along the top and bottom of the intestine, respectively. The dorsal and ventral blood vessels are connected by ring-like hearts. Hearts are also associated with the dorsal blood vessel. These hearts pump blood forward, and the ring-like hearts pump blood down to the ventral vessel, which returns blood to the back of the body. Illustration B shows the open circulatory system of a bee. The dorsal blood vessel, which contains multiple hearts, runs along the top of the bee. Blood exits the dorsal blood vessel through an opening in the head, into the body cavity. Blood reenters the blood vessels through openings in the hearts called ostia.
    Figure 1. In (a) closed circulatory systems, the heart pumps blood through vessels that are separate from the interstitial fluid of the body. Most vertebrates and some invertebrates, like this annelid earthworm, have a closed circulatory system. In (b) 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 like this bee and most mollusks have open circulatory systems.

    Open Circulatory Systems

    As opposed to a closed system, arthropods—including insects, crustaceans, and most mollusks—have an open circulatory system, as illustrated in Figure 1b. In an open circulatory system, the blood is not enclosed in the blood vessels but is pumped into a cavity called a hemocoel and is called hemolymph because the blood mixes with the interstitial fluid. As the heart beats and the animal moves, the hemolymph circulates around the organs within the body cavity and then reenters the hearts through openings called ostia. This movement allows for gas and nutrient exchange. An open circulatory system does not use as much energy as a closed system to operate or to maintain; 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 outcompeted 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 get food faster and possibly to prey on the insects.

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