Circulatory systems may be open (mixed with the interstitial fluid) or closed (separated from the interstitial fluid). Closed circulatory systems are a characteristic of vertebrates; however, there are significant differences in the structure of the heart and the circulation of blood between the different vertebrate groups due to…

In most animals, the circulatory system is used to transport blood through the body. However, complex organisms use the circulatory system to carry gases, nutrients, and waste through the body.

circulatory system — used to transport blood through the body. closed circulatory system — system in which the blood is separated from the bodily interstitial fluid and contained in blood vessels. double circulation — flow of blood in two circuits: the pulmonary circuit through the lungs and the systemic circuit through the organs and body. unidirectional circulation — flow of blood in a single circuit; occurs in fish where the blood flows through the gills, then past the organs and the rest of the body, before returning to the heart.