The O₂ carried by hemoglobin in red blood cells is transported to tissues throughout the body by the circulatory system (Fig. 39.16). Generally, smaller animals, such as insects and many mollusks, have open circulatory systems that contain few blood vessels: Most of the circulating fluid, the hemolymph, is contained within the animal’s body cavity. The hemolymph bathes the animal’s tissues and organs (Fig. 39.16a). Open circulatory systems have limited control of where the fluid moves. Muscles that are active in locomotion can assist circulation of the hemolymph, and some invertebrates have simple hearts with openings that help pump fluid between different regions of the animal’s body cavity.

The closed circulatory systems of larger animals are made up of a set of internal vessels and a pump—the heart—to transport the blood to different regions of the body (Fig. 39.16b). Closed circulatory systems have two conflicting requirements: They must produce enough pressure to carry the circulating blood to all the tissues, but, once the blood reaches smaller vessels that supply the cells within the tissues, the blood pressure and flow rate must not be too high. High pressure would push fluid through the walls of the smaller vessels and cause the blood to flow so quickly that there would not be enough time to exchange gases.

About 500 million years ago, segmented worms first evolved closed circulatory systems that allow them to expand individual body segments to burrow underground. About 480 million years ago, the first cephalopods (squid and octopus) evolved closed circulatory systems that enabled them to be early successful predators in Paleozoic oceans. A closed circulatory system delivers O₂ at high rates to exercising tissues, necessary to enable their mitochondria to provide the energy needed to chase prey. In contrast, open circulatory systems generally operate under low pressure and have limited transport capacity. As a result, animals with open circulatory systems are less active. Their ability to control the delivery of respiratory gases and metabolites to specific tissues and regions is also limited. However, insects can be very active despite having an open circulatory system because they obtain O₂ by their tracheal system independently of the circulatory system.

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Closed circulatory systems can control blood flow to specific regions of the body by varying the resistance to flow. For example, wading birds reduce blood flow to their legs when they are in cold water to reduce heat loss, and vertebrates can increase blood flow to their muscles to deliver O₂ and nutrients during exercise. In order to pump blood through a set of closed interconnected vessels, a muscular heart is needed to produce sufficient pressure to overcome the flow resistance of the vessels.