What defines a double circulatory system?

A double circulatory system is characterized by the separation of blood flow into two distinct circuits: the pulmonary circuit and the systemic circuit. In this system, blood flows in a loop from the heart to the lungs (for oxygenation) and then returns to the heart before being pumped out to the rest of the body. This design allows for more efficient oxygen transport and delivery to tissues, as the oxygen-rich and oxygen-poor blood are kept separate.

The systemic circuit is responsible for delivering oxygenated blood from the heart to the rest of the body, while the pulmonary circuit carries deoxygenated blood from the heart to the lungs for re-oxygenation. This two-loop configuration is a defining characteristic of a double circulatory system, commonly found in mammals and birds, which helps to support the higher metabolic demands of these organisms compared to those with a single circulatory system.