Blood circulation in a mammal is said to be double because

Analyze the question: It asks why mammalian blood circulation is described as "double."

Concept: In mammals, the heart is divided into a right side and a left side, which keeps oxygenated and deoxygenated blood separate. This produces two circuits:

1. Pulmonary circulation: Right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium (blood picks up oxygen).
2. Systemic circulation: Left ventricle → aorta → body tissues → vena cava → right atrium (blood delivers oxygen to the body).

In one complete trip around the body, blood therefore passes through the heart twice: once on the right side and once on the left. This is what "double circulation" means.

Checking the options:

• A. It passes twice through the heart in a complete circuit. Correct. This is the definition of double circulation.
• B. It moves in both arteries and veins. Fish also have arteries and veins, and this doesn't explain "double."
• C. It circulates in both the hands and other organs. This is meaningless as an explanation.
• D. The heart contains auricles and ventricles. This describes the heart's structure, not the number of circuits. Fish hearts also have an atrium and a ventricle, yet fish have single circulation.

Answer: A

Exam tip: Think "double = two circuits = through the heart twice." Fish have single circulation (the heart is passed once per circuit), while mammals and birds have double circulation. If you see "passes twice through the heart," that is almost always the right choice.