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The cells of all living things must receive nutrients and eliminate waste from their metabolism.

In more complex animals that have systems that specialize in the transport of numerous substances, there is a heart that pumps circulating fluid to cells at a certain frequency. Circulating fluid may be colorless, called hemolymph, present in insects, or colored and in this case is called blood. Color is determined by the existence of pigments, as is the case with hemoglobin present in many invertebrates and in all vertebrates, which contain iron atoms responsible for the reddish coloration of the blood.

How is the circulation in the different animal phyla?

Phylum How is the circulation
Porifers Water circulation through the atrium, mobile amoebocytes in the gelatinous layer of the body wall.
Cnidarian Gastrovascular cavity - food digestion and circulation of water and dissolved substances.
Flatworms Branched digestive cavity (gastrovascular cavity).
Annelids onwards Circulatory System - Vessels favor the continuous flow of material dissolved in water.

The two types of circulatory systems

In animals, there are two types of circulatory system: open system and closed system. In the open circulatory system, the fluid pumped through the heart periodically leaves the vessels and falls into body gaps. In these cavities, the exchange of substances between the liquid and the cells is slow. Slowly, the fluid returns to the heart, which again pumps it to the tissues. This system is found among the arthropods and in most mollusks. The slow transport of materials is a limiting factor to the size of the animals. Also, because it is an open system, the pressure is not large, just enough for the blood to reach short distances.

The grasshopper has open circulation

At the closed system, the blood never leaves the vessels. In place of the body gaps, there is a large network of thin-walled vessels, the capillaries, through which substance exchange occurs between blood and tissues. In such systems, the circulating liquid is constantly moving, circulation is fast. The pressure developed by the heart pump is high and the blood can reach great distances. The size of the animals may be larger. This type of circulatory system is found in the annelids, in some agile clams (squid and octopus) and in all vertebrates.

Closed circulation of an annelid.