AI-generated practice question — model-generated for extra practice, not a previous-year CBSE board question.
(D) Left atrium
Oxygen-rich blood returning from the lungs first enters the left atrium — the thin-walled upper chamber on the left side of the heart.
Source: Life Processes, Section 5.4.1
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The passage clearly states: "Oxygen-rich blood from the lungs comes to the thin-walled upper chamber of the heart on the left, the left atrium." Students often confuse left and right sides. Remember: lungs → left atrium → left ventricle → body; and body → right atrium → right ventricle → lungs. The word "first enters" is the key — the left atrium receives it before the left ventricle pumps it out.