There are two basic types of circulation. The open circulatory system of invertebrates is characterized by a system of body cavities or coeloms connected with vascular conduits, both of which lack the endothelial lining. This simple arrangement allows the circulating fluid, the hemolymph, to bathe the inner organs directly, thus fulfilling the dual function of tissue fluid and blood. Functionally, it resembles the early embryonic circulation of vertebrates before the formation of a continuous vascular endothelium. In contrast, the closed circulatory system of the vertebrates is phylogenetically younger and consists of a network of continuous vessels in which the blood does not come in direct contact with the surrounding tissues. An overlap exists at the level of local circulations and individual organs within the vertebrate and invertebrate species.
In primitive vertebrates, such as the lancelet, the circulating fluid moves without a heart as the central organ of circulation. In fishes’ single-circuit system, the gills and the heart are placed in series. The two-chambered heart is in the venous limb, downstream from the gut and liver, and supplies the gills with pressures that exceed those in the arteries. Largely devoid of gravity, fish depend on water for respiration, fluid balance, thermoregulation, reproduction, and fin development. The amphibians are adapted to life in water only during early stages of their development. Transition to land is marked by loss of fins and gills, and the emergence of tail and limbs. Adaptation to air respiration introduces a fundamental change in the structure of the cardiovascular system. The heart and the lung are joined by a newly formed pulmonary circulation placed in parallel with the systemic circulation. In contrast to fish, the circulatory loops cross and assume the shape of a lemniscate. The heart acquires a new chamber, the left atrium, while a common ventricle is shared between the pulmonary and systemic loops. Amphibians continue to depend for temperature, reproduction, and part of their respiratory needs on water (skin respiration). Through the development of sophisticated organ systems such as thermoregulation, respiration, excretion, inner reproduction, and locomotion, mammals have attained a high degree of environmental emancipation. The cardiovascular system consists of two anatomically separate, but functionally unified, parts—the systemic and pulmonary circulations—placed in series. In addition to an independent inner watery environment, mammals have developed an “inner atmosphere,” reflected primarily in the partial pressure of oxygen and nitrogen in the blood that parallels the atmospheric pressure. The essential new feature of the mammalian circulation is a pressurized arterial compartment. The similarity of arterial pressure across the mammalian species suggests that the pressure as such does not serve the blood propulsion.
Circulation in invertebrates Circulation in vertebrates Amphibian circulation Circulation in insects Circulation in lancelet Fish circulation Bird circulation Cross-current gas exchange Lymph circulation Open circulation Closed circulation In series circulation In parallel circulation
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