Blood circulation in a mammal is said to be double because?

Step 1: Analyze the question The question asks why mammalian blood circulation is called "double circulation."

Step 2: Explain the concept In mammals, the heart is divided into a right side and a left side, which keeps oxygenated and deoxygenated blood separate. Blood travels through two circuits:

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

So in one complete circuit of the body, blood passes through the heart twice: once on the right side and once on the left.

Step 3: Evaluate the options

• A. It passes twice through the heart in a complete circuit. Correct. This is the standard definition of double circulation.
• B. It moves in both arteries and veins. Fish and other animals with single circulation also have arteries and veins, so this does not explain "double."
• C. It circulates in both the heart and other organs. This is too vague and applies to any circulatory system.
• D. The heart contains auricles and ventricles. This describes heart structure, not the pathway of blood. Fish hearts also have an auricle and a ventricle but have single circulation.

Step 4: Answer The correct option is A.

Exam tip: Think "double = two trips through the heart per full circuit." Compare this with a fish, where blood passes through the heart only once per circuit (single circulation). If a question asks about double circulation, look for the option mentioning the heart being passed through twice, or lungs plus body.