Which of the following is the correct sequence of blood flow through heart chambers?

Correct — A. Blood returning from the body enters the heart at the right atrium.

Right atrium → right ventricle, which pumps it to the lungs.
Oxygen-rich blood comes back from the lungs to the left atrium.
Left atrium → left ventricle, which pumps it out to the body.

That is right atrium → right ventricle → left atrium → left ventricle → option (a).

Why the others are wrong

• (b)
  Left atrium → Left ventricle → Right atrium → Right ventricle — Left atrium first follows the same one-way loop from the lungs' side: left atrium → left ventricle → body → right atrium → right ventricle. SSC's key starts instead where blood returns from the body, at the right atrium.
• (c)
  Right ventricle → Right atrium → Left ventricle → Left atrium — Ventricle → atrium runs backwards. Atria receive blood and pass it down to the ventricles, and valves stop it flowing back, so right ventricle → right atrium reverses the flow.
• (d)
  Left ventricle → Left atrium → Right ventricle → Right atrium — Left ventricle → left atrium runs backwards through the valve between them. The left ventricle pumps blood out to the body, so this sequence is the whole loop in reverse.

The heart keeps two circuits apart. The right side sends carbon dioxide-rich blood to the lungs, and the left side sends oxygen-rich blood to the body.

On each side, blood enters a thin-walled upper chamber, the atrium, and passes to a lower chamber, the ventricle, which pumps it out. Valves stop it flowing backwards.

Blood passes through the heart twice on one trip round the body, which NCERT calls double circulation.

Key facts

• Atria receive blood and pass it to the ventricles, which pump it out of the heart.
• The right ventricle pumps blood to the lungs, and the left ventricle pumps it to the rest of the body.
• Ventricles have thicker muscular walls than atria because they pump blood into various organs.
• Fishes have a two-chambered heart, so blood passes through it only once in each circuit of the body.

Common traps

• Starting at a ventricle: blood enters the heart at an atrium, so a sequence that opens with a ventricle runs backwards.
• Treating (a) and (b) as different routes, when they are the same loop and the key starts at the right atrium.

No directly related past PYQ was found.