CDS (II) 2021 General Knowledge · Question 17 of 120

Correct — A, Oxygenated blood reaches the left side of the heart from the lungs. Follow the circuit as the textbook lays it out. Oxygen-rich blood coming back from the lungs enters the thin-walled upper chamber on the left, the left atrium, which relaxes to collect it and then contracts to pass it to the left ventricle. The muscular left ventricle then pumps that blood out to the whole body. Blood returning from the body, now de-oxygenated, arrives in the right atrium, is passed to the right ventricle, and is sent from there to the lungs to be oxygenated. Left side handles oxygenated blood, right side handles de-oxygenated blood, and the blood passes through the heart twice in one complete circuit — which is what the phrase double circulation names. Keeping the two sides separate is the point of the design: it prevents mixing and allows a highly efficient supply of oxygen, which birds and mammals need because they spend energy constantly to hold their body temperature steady.

• (b)Blood in the left side of the heart is poor in oxygen and is brought to the right side of the heart. — Reverses the roles of the two sides and invents a direct left-to-right transfer. The two sides are separated by a septum precisely so that their contents do not mix.
• (c)Deoxygenated blood from the left side of the heart is brought to the lungs for oxygenation. — The journey to the lungs is real but starts on the wrong side. De-oxygenated blood leaves the right ventricle, not the left.
• (d)Oxygenated blood from the right side of the heart is sent around the body. — Again the correct destination with the wrong chamber. The systemic circuit begins at the left ventricle, which is why its wall is the thickest of the four.

In double circulation the blood passes twice through the heart in one complete round of the body. The pulmonary circuit runs right ventricle to lungs to left atrium; the systemic circuit runs left ventricle to the body and back to the right atrium. Atria are the thin-walled receiving chambers and ventricles the thick-walled pumping chambers, with the left ventricle the most muscular because it drives blood the furthest. Valves between and beyond the chambers stop backflow when the muscle contracts.

Every option names a real part of the circuit and misplaces one term, so the item is best attacked by fixing one anchor and deriving the rest. The anchor most people can hold is that blood coming back from the lungs must be oxygen-rich, and that it enters on the left. From there everything else follows: if oxygenated blood is on the left, the left ventricle must supply the body, the right atrium must receive the used blood, and the right ventricle must send it to the lungs. The vocabulary can also mislead — pulmonary arteries carry de-oxygenated blood and pulmonary veins carry oxygenated blood, the single exception to the usual rule that arteries carry oxygen-rich blood. Fish, with a two-chambered heart, pass blood through the heart only once per circuit, so double circulation is not universal among vertebrates.

• Oxygen-rich blood from the lungs enters the left atrium; de-oxygenated blood from the body enters the right atrium.
• The left ventricle pumps blood to the whole body and has the thickest muscular wall; the right ventricle pumps blood to the lungs.
• Valves ensure that blood does not flow backwards when the atria or the ventricles contract.
• Keeping the two sides separate prevents oxygenated and de-oxygenated blood from mixing, which allows a highly efficient supply of oxygen — important for birds and mammals, which use energy constantly to maintain body temperature.
• Pulmonary arteries carry de-oxygenated blood and pulmonary veins carry oxygenated blood, reversing the usual artery-vein rule.

• Swapping the two sides; the left side always carries oxygenated blood in the mammalian heart.
• Applying the usual artery-vein rule to the pulmonary vessels, where it is reversed.
• Assuming all vertebrates have double circulation; fishes with a two-chambered heart do not.

As a which-statement-is-true item on the circuit, as a chamber-to-function match, or as a reasoning question on why double circulation suits birds and mammals.

• practice — not a real PYQ

  Which chamber of the human heart pumps blood into the pulmonary artery?

  • (a)Left atrium
  • (b)Left ventricle
  • (c)Right atrium
  • (d)Right ventricle

  Answer(d) Right ventricle — it sends de-oxygenated blood to the lungs, while the left ventricle supplies the body.

• practice — not a real PYQ

  The wall of the left ventricle is thicker than that of the right ventricle because it has to

  • (a)receive blood from the lungs
  • (b)pump blood to the entire body
  • (c)store more blood
  • (d)prevent backflow through the valves

  Answer(b) pump blood to the entire body — the systemic circuit is far longer and needs a higher pressure than the pulmonary one.