Vascular systems
While ciliary respiratory currents are sufficient to
supply the requirements of animals with simple epithelial
tissues and low metabolic rates, most species whose
bodies contain a number of organ systems require a more
efficient circulatory system. Many invertebrates andall
vertebrates have a closed vascular system in which the
circulatory fluid is totally confinedwithin a series
of vessels consisting of arteries, veins, and fine linking
capillaries. Insects, most crustaceans, and many mollusks,
however, have an open system in which the circulating
fluid passes somewhat freely among the tissues before
being collected and recirculated.
The distinction between open and closed circulatory
systems may not be as great as was once thought; some
crustaceans have vessels with dimensions similar to
those of vertebrate capillaries before opening into
tissue sinuses. The circulatory fluid in open systems
is strictly hemolymph, but the term “blood” is commonly
used to denote the transporting medium in both open
and closed systems. Compared with closed systems, open
circulatory systems generally work at lower pressures,
and the rate of fluid return to the heart is slower.
Blood distribution to individual organs is not regulated
easily, and the open system is not as well-adapted for
rapid response to change.
