For sponges and diploblasts, the water in which they live provides the medium
for transport. Water, propelled by ciliary, flagellar, or body movements, passes
through channels or compartments to facilitate the movement of food,
respiratory gases, and wastes. True circulatory system – containing vessels
through which blood moves – are essential to animals so large or so active that
diffusional processes alone cannot supply their oxygen needs.
Flattened and leaflike acoelomate flatworms survive without a circulatory
system because the distance of any body part from the surface is short;
respiratory gases and metabolic wastes transfer by simple diffusion even though
many are relatively large animals.
A circulatory system having a full set of components – pump, arterial distribution
system, capillaries that interface with cells, various reservoir and return system
– is fully recognizable in annelid worms. In earthworms, there are two main
vessels, a dorsal vessel carrying blood anteriorly, and a ventral vessel delivering
blood posteriorly throughout the body by way of segmental vessels and a dense
tissue capillary network. The dorsal vessel drives the blood forward by
peristalsis and thus serves as a heart. Five aortic arches that connect the dorsal
and ventral vessels laterally are contractile and serve as accessory hears to
maintain a steady flow of blood into the ventral vessel and to the head region,
which has its own afferent and efferent blood vessels. Many smaller segmental
vessels that deliver blood to tissue capillaries are actively contractile as well. We
see, then, that there is no localized pump pushing the blood through a system of
passive tubes; instead, the power of contraction is widely distributed throughout
the vascular system.
Open and closed circulatory system
The system just described is a closed circulation because the circulating fluid,
blood, is confined to vessels throughout its journey through the vascular system.
Many invertebrates have an open circulation in which there are no small blood
vessels or capillaries interfacing with cells or connecting arteries with veins. In
insects and other arthropods, in most molluscs, and in many smaller
invertebrate groups, blood sinuses, collectively called a hemocoel, replace
capillary beds. The hemocoel comprises the primary body cavity (persistent
blastocoel) and secondary coelomic cavities through which blood (hemolymph)
freely circulates. Since there is no separation of the extracellular fluid into blood
plasma and lymph (as there is in a close circulation), a blood volume is large and
may constitute 20-40% of body volume. By contrast, blood volume in animals
with closed circulations is only about 5-10% of body volume.
In arthropods, the heart and all viscera lie in the hemocoel, bathed by blood.
Blood enters the heart through valve opening, the ostia, and the heart’s
contractions, which resemble a forward-moving peristaltic wav, propel blood
into a limited arterial system. Blood travels to the head and other organs, then
flows into the hemocoel. It is routed through the body and appendages by a
system of baffles and longitudinal membranes (septa) before returning to the
heart. Because blood pressure is very low in open systems, many arthropods
have auxillary hearts or contractile vessels to boost blood flow. Insects and many other terrestrial arthropods do not use their circulatory
system for respiratory gas transport. Instead, a separate respiratory system has
evolved for this purpose: a tracheal system in insects and some other terrestrial
arthropods, pseudotrachae in terrestrial pill bugs, and book lungs in some
Closed systems have certain features in common. A heart pumps blood into
arteries that branch and narrow into arterioles and then into a vast system of
capillaries that interfaces with cells in body tissues. Blood leaving capillaries
flows into venules and then veins that return the blood to the heart. Capillary
walls are thin, permitting rapid transfer of materials between blood and tissues.
Closed systems are more suitable for large and active animals because blood
moves rapidly to the most active tissues. In addition, flow to various organs can
be readjusted to meet changing needs by varying the diameters of blood vessels.
Because blood pressures are much higher in closed than in open systems, fluid is
constantly moving through capillary walls into the surrounding tissue spaces.
Most of this fluid is drawn back into capillaries by osmosis. The remainder is
recovered by a lymphatic system, which has evolved to separately but in
conjunction with the high-pressure system of vertebrates.
Plan of vertebrates circulatory system
In vertebrates the principal differences in the blood-vascular system involve the
gradual separation of the heart into two separate pumps as vertebrates evolved
from aquatic life with gill breathing to full terrestrial life with lung breathing.
A fish heart contains 2 chambers in series, an atrium and a ventricle. The atrium
is preceded by an enlarged chamber, the sinus ventricle, which collects blood
from the venous system to assure a smooth delivery of blood to the heart.