What distinguishes a double circulatory system?

A double circulatory system is characterized by the presence of two distinct circuits that work together to circulate blood throughout the body. These two circuits are the systemic circuit and the pulmonary circuit. In the systemic circuit, oxygen-rich blood is pumped from the heart to the rest of the body to deliver oxygen and nutrients to tissues. Once the blood has delivered its oxygen, it returns to the heart depleted of oxygen. In the pulmonary circuit, this deoxygenated blood is then sent to the lungs, where it releases carbon dioxide and gains oxygen before returning to the heart to enter the systemic circuit again.

This structural organization allows for more efficient transport of oxygen and nutrients than a single circulatory system, which would only have one circuit functioning for both the lungs and the body simultaneously. In contrast, options that suggest a single circuit, multiple connections, or reliance solely on the lungs do not accurately capture the essence of a double circulatory system's efficient design and functionality.