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Circulation-is the movement of substances such as nutrients and gases within blood vessels and cavities throughout the body.
Not all animals have a circulatory system. Poriferans, cnidarians, platyhelminthes and nematodes (nematodes have pseudocoelom fluid but no vessels) are avascular animals. Echinoderms do not have true circulatory systems either.
In animals that do not contain a circulatory system, the transport of substances occurs by cell to cell diffusion.
Blood is a fundamental means of substance transport for larger animals since, in these animals, tissues are distant from each other and from the environment thus making diffusion impossible.
Circulatory systems can be classified into open circulatory systems and closed circulatory systems.
An open circulatory system is one in which blood does not circulate only inside blood vessels but also flows into cavities that irrigate tissues. In open circulatory systems, blood pressure is low and generally the blood (called hemolymph) has a low level of cellularity.
Arthropods, molluscs (cephalopods are exception) and protochordates have open circulatory systems.
A closed circulatory system is one in which blood circulates only inside blood vessels. For this reason, the blood pressure is higher in animals with closed circulatory systems. The cellularity of the blood is also higher, with many specific blood cells.
Closed circulatory systems are a feature of annelids, cephalopod molluscs and vertebrates
A closed circulatory system is more efficient. Since blood circulates only inside blood vessels, it has a higher pressure and, as a result, can travel greater distances to the organs where hematosis happens and to peripheral tissues. In addition, the higher circulatory speed increases the animal’s capacity to distribute large supplies of oxygen to tissues that consume it in large amounts, such as muscle tissues, which can then perform faster movements. Animals with an open circulatory system (with the exception of insects, which carry out gas exchange separately from circulation) are generally slower and have a low metabolic rate.
In insects, the circulatory system is open but this system does not participate in the gas exchange process or in oxygen supply to tissues. Gases enter and exit through the independent tracheal system, which allows for the direct contact of cells with the ambient air. Therefore, an insect can supply the large oxygen demand of its fast-beating wing muscles even though it has an open circulatory system.
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