A double circulatory system, often called a double-loop circulatory system refers to two distinct and separate circulatory pathways for blood flow. In a first order circulatory circuit, often called the pulmonary circuit, blood is pumped to the lungs, thus acquiring oxygen while simultaneously releasing carbon dioxide. Fully oxygenated blood then enters the second order circuit, called the systemic circuit, going to the brain and body. The second order systemic output blood is selectively deoxygenated by the needs of the peripheral organs, returning once again to the heart where it returns back to the lungs to become oxygenated again. A double-loop system is also a closed circulatory system because blood usually travells in the body through distinct blood vessels.