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Definition: A system in tetrapod vertebrates for distributing blood through two separate circuits. One circuit carries deoxygenated blood from the heart through the lungs for oxygenation and back to the heart as oxygenated blood; a separate circuit carries oxygenated blood to tissues where oxygen is released and then back to the heart in deoxygenated form. Double circulation features various mechanisms for separating the alternative circuits.
Source: Hickman, C.P., et al. 2011. Integrated Principles of Zoology. Fifteenth Edition. McGraw-Hill Higher Education.