alleryThe double-circulation system of mammals means:
ATwo hearts in series
BTwo lungs working separately
CTwo veins per artery
DBlood passes through the heart TWICE per circuit (lungs + body)
Answer & Solution
Correct answer: D. Blood passes through the heart TWICE per circuit (lungs + body)
1. Mammals (and birds) have a 4-chambered heart that completely separates oxygenated and deoxygenated blood.
2. Per body circuit, blood passes the heart TWICE: pulmonary loop (lungs) and systemic loop (body).
3. Hence the name 'double circulation' — efficient for warm-blooded animals with high metabolic rate.
_Source: syllabus Science Ch 5 §5.3.2 Heart and Double Circulation_
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