Some aquatic annelids have thin-walled, feathery gills through which gases are exchanged between the blood and the environment. However, most annelids have no special organs for gas exchange, and respiration occurs directly through the body wall.

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For these reasons they are considered to be more complex than Phylum Nematoda and Phylum Platyhelminthes due to evolution. The aforementioned phyla do not have specified cells for transporting oxygen through the bloodstream to the rest of the body. The ability to pass oxygen through the bloodstream, which features multiple heart-like pumps throughout the body, allows for larger organisms that aren't very thin (Annelid).

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Characteristics of the circulatory system vary within the phylum. The blood usually contains hemoglobin, a red oxygen-carrying pigment; some annelids have a green oxygen-carrying pigment, and others have unpigmented blood. The circulatory system is usually closed, i.e., confined within well-developed blood vessels; in some polychaetes and leeches the circulatory system is partly open, with blood and coelomic fluid mixing directly in the sinuses of the body cavity. Blood flows toward the head through a contractile vessel above the gut and returns to the terminal region through vessels below the gut; it is distributed to each body compartment by lateral vessels. Some of the lateral vessels are contractile and serve as hearts, i.e., pumping organs for driving the blood.