The network of blood vessels is divided into systemic and pulmonary circulation. What term describes this two-circuit flow?

• A. Single circulation
• B. Pulmonary circulation
• C. Double circulation
• D. Systemic circulation

Answer: (C)

The concept being tested is why the heart supports two separate circuits in one overall circulation. In mammals and other higher vertebrates, blood travels through two distinct loops: a pulmonary loop that carries blood to the lungs for oxygenation, and a systemic loop that delivers oxygen-rich blood to the rest of the body. This arrangement is described as double circulation, because the blood makes two major passes through the heart as part of one complete cycle: through the right side to the lungs and back, then through the left side to the body and back. This two-circuit setup differs from a single circuit, where blood passes through the heart only once per cycle (as seen in some fish), and from terms that refer to just one of the loops (pulmonary or systemic) which don’t capture the presence of both loops in the circulatory system.