Mammals' Double Circulatory System Defined

Steps:

• Recall that circulation in mammals separates pulmonary and systemic circuits for efficient oxygenation.
• Identify key feature: blood flows from body to heart (right side), to lungs, back to heart (left side), then to body.
• Compare to single circulation (e.g., fish), where blood passes heart once per circuit.
• Match definition to options: only B describes blood passing through heart twice.

Why B is correct:

• Double circulation means blood completes a full body circuit by passing through the heart twice—once for deoxygenated blood to lungs and once for oxygenated blood to body—as per mammalian cardiovascular anatomy.

Why the others are wrong:

• A: Describes a closed circulatory system, not specific to double circulation.
• C: Refers to vessel types like arteries and veins, unrelated to double paths.
• D: Inaccurate; mammalian hearts have four chambers, not two.

Final answer: B