Open and Closed Circulations
Open and Closed
Circulations
The system just described is a closed circulation because the circulating medium, 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 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 found in animals with closed systems. During development of the body cavity in these groups, the blastocoel is not completely obliterated by the expanding mesoderm. This space be-comes the hemocoel, which is nothing more than the primary body cavity (persistent blastocoel) through which blood (also called hemolymph) freely circulates (bottom diagrams in Figure 33-8). Since there is no separation of the extracellular fluid into blood plasma and lymph (as there is in a closed circulation,) the blood volume is large and may constitute 20% to 40% of body volume. By contrast, blood volume in animals with closed circulations (vertebrates, for example) is only about 5% to 10% of body volume.
In arthropods, the heart and all viscera lie in the hemocoel, bathed by blood (Figure 33-8). Blood enters the heart through valved openings, the ostia, and the heart’s contractions, which resemble a forward-moving peristaltic wave, propel blood into a limited arterial system. Blood is distributed to the head and other organs, then escapes 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 the blood pressure is very low in open systems, seldom exceeding 4 to 10 mm Hg, many arthropods have auxiliary hearts or contractile vessels to boost blood flow (Figure 33-9).
During embryonic development of animals with closed circulatory systems (most annelids, cephalopod molluscs, and all vertebrates) the coelom increases in size to obliterate the blastocoel and forms a secondary body cavity (top diagrams in Figure 33-8). A system of continuously connected blood vessels develops within the mesoderm. All 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. Blood leaving capillaries enters venules and then veins that return the blood to the heart. Capillary walls are thin, permitting rapid rates of transfer of materials between blood and tissues. Closed systems are more suitable for large and active animals because blood can be moved rapidly to tissues needing it. 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 filtered across capillary walls into the surrounding tissue spaces. Most of this fluid is drawn back into capillaries by osmosis. The remainder is recovered by the lymphatic system which has evolved in parallel with the high-pressure system of vertebrates.
The system just described is a closed circulation because the circulating medium, 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 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 found in animals with closed systems. During development of the body cavity in these groups, the blastocoel is not completely obliterated by the expanding mesoderm. This space be-comes the hemocoel, which is nothing more than the primary body cavity (persistent blastocoel) through which blood (also called hemolymph) freely circulates (bottom diagrams in Figure 33-8). Since there is no separation of the extracellular fluid into blood plasma and lymph (as there is in a closed circulation,) the blood volume is large and may constitute 20% to 40% of body volume. By contrast, blood volume in animals with closed circulations (vertebrates, for example) is only about 5% to 10% of body volume.
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| Figure 33-8 Diagrams showing how open and closed circulatory systems develop. The principal body cavity of arthropods is the persistent blastocoel which becomes a hemocoel; the true coelom remains mostly undeveloped. |
In arthropods, the heart and all viscera lie in the hemocoel, bathed by blood (Figure 33-8). Blood enters the heart through valved openings, the ostia, and the heart’s contractions, which resemble a forward-moving peristaltic wave, propel blood into a limited arterial system. Blood is distributed to the head and other organs, then escapes 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 the blood pressure is very low in open systems, seldom exceeding 4 to 10 mm Hg, many arthropods have auxiliary hearts or contractile vessels to boost blood flow (Figure 33-9).
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| Figure 33-9 Circulatory system of an insect. Although the circulatory system is open, blood is directed through the appendages in channels formed by longitudinal septa. Arrows indicate the course of circulation. |
During embryonic development of animals with closed circulatory systems (most annelids, cephalopod molluscs, and all vertebrates) the coelom increases in size to obliterate the blastocoel and forms a secondary body cavity (top diagrams in Figure 33-8). A system of continuously connected blood vessels develops within the mesoderm. All 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. Blood leaving capillaries enters venules and then veins that return the blood to the heart. Capillary walls are thin, permitting rapid rates of transfer of materials between blood and tissues. Closed systems are more suitable for large and active animals because blood can be moved rapidly to tissues needing it. 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 filtered across capillary walls into the surrounding tissue spaces. Most of this fluid is drawn back into capillaries by osmosis. The remainder is recovered by the lymphatic system which has evolved in parallel with the high-pressure system of vertebrates.
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