In closed circulatory systems, blood vessels separate blood from interstitial fluid. Blood is pumped through this vascular system by one or more hearts, and cellular and macromolecular components of the blood never leave the vessels. Closed circulatory systems characterize vertebrates and some invertebrate groups, among them annelids.
A simple example of a closed circulatory system is that of the earthworm (Figure 49.1C). One large ventral blood vessel carries blood from the worm’s anterior end to its posterior end. Smaller vessels branch off and transport the blood to even smaller vessels serving the tissues in each body segment. In the smallest vessels, respiratory gases, nutrients, and metabolic wastes diffuse between the blood and interstitial fluid. The blood then flows from these vessels into larger vessels that lead into one large dorsal vessel that carries the blood from the posterior to the anterior end of the body. Five pairs of muscular vessels connect the large dorsal and ventral vessels in the anterior end, thus completing the circuit. The dorsal vessel and the five connecting vessels serve as hearts for the earthworm; their contractions keep the blood circulating. The direction of circulation is determined by one-
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Closed circulatory systems have several advantages compared with open systems:
Fluid can flow more rapidly through vessels than through intercellular spaces and can therefore transport things to and from tissues more rapidly.
By changing the diameter (and hence the resistance) of specific vessels, closed systems can control the flow of blood to selective tissues and organs to match their needs.
Specialized cells and large molecules that aid in transporting hormones, nutrients, and oxygen can be kept in the vessels but can drop their cargo in the tissues where it is needed.
With all of these “advantages” of closed circulatory systems, you might wonder how some species with open circulatory systems can sustain such high levels of activity. In the case of insects, the answer is clear: insects do not depend on their circulatory systems for respiratory gas exchange (see Figure 48.4).