Single-celled animals rely on transport system, for instance passive transport, for the exchange of nutrients and metabolic wastes between the cell and the environment. However, for multicellular organisms, organ systems are necessary to allow circulation and maintain homeostasis. One of these transport systems is the circulatory system.
The circulatory system is the organ system that is involved primarily in internal transport. There are two forms of circulatory systems in animals: (1) open circulatory system and (2) closed circulatory system.
In closed circulatory system, the blood circulates within blood vessels throughout the body. Therefore, there is a distinction between blood and lymph. Examples of animals with closed circulatory system are annelids, few mollusks, and vertebrates. The vertebrates have a cardiovascular system comprised of heart and blood vessels where blood circulates and another system called lymphatic system that circulates lymph.