UPPSC 2024 Prelims GS Paper-I · Question 68 of 150

Correct — A, Pulmonary vein. The whole question rests on getting one definition right: a vein is a vessel that carries blood towards the heart, and an artery is a vessel that carries blood away from the heart. Oxygen content is not part of the definition — it is only a rule of thumb that happens to hold everywhere in the systemic circulation. In the pulmonary circulation the rule of thumb breaks. Deoxygenated blood is pushed out of the right ventricle into the pulmonary trunk, which divides into the right and left pulmonary arteries and delivers it to the lungs; there, in the alveolar capillaries, it takes up oxygen and gives up carbon dioxide. That freshly oxygenated blood is collected by the four pulmonary veins — two from each lung — which return it to the left atrium of the heart, from where it passes through the bicuspid (mitral) valve into the left ventricle and out through the aorta to the body. So the pulmonary vein carries oxygen-rich blood and is still a vein, because it is heading towards the heart. It has an exact mirror image worth learning in the same breath: the pulmonary artery is the only artery that carries deoxygenated blood. Both exceptions live in the same place — the lesser, or pulmonary, circulation — and for the same reason. One honest refinement for a careful student: in the foetus the umbilical vein also carries oxygenated blood, from the placenta to the developing baby, but that vessel closes at birth and becomes the ligamentum teres of the liver. The word 'only' in this question describes the body after birth, and among the four options given, the pulmonary vein is unambiguously the answer.

• (b)Cystic vein — The cystic vein drains the gall bladder, emptying into the portal venous system on its way to the liver. Like every other vessel of the systemic drainage it carries blood that has already given up its oxygen to the tissue it supplied — in this case the wall of the gall bladder. Nothing about the gall bladder oxygenates blood; the gall bladder merely stores and concentrates the bile that the liver makes.
• (c)Cardiac vein — The cardiac veins — the great, middle and small cardiac veins — drain the heart's own muscle, the myocardium, and empty through the coronary sinus into the right atrium. They are the return half of the coronary circulation whose supply half is the coronary arteries. In fact this is about the most deoxygenated blood in the body: the myocardium extracts an unusually high fraction of the oxygen delivered to it, because a muscle that never rests cannot afford to hold anything in reserve. So a cardiac vein is the opposite of what the question asks for.
• (d)Hepatic Portal vein — This is the strongest distractor, because it is the one vein students remember as carrying 'rich' blood — and it does, but rich in nutrients, not in oxygen. The hepatic portal vein collects blood from the capillaries of the stomach, small and large intestine, spleen and pancreas, laden with the products of digestion, and delivers it to the liver for processing before it ever reaches the general circulation. That blood has already passed through one capillary bed and given up its oxygen; it then enters a second capillary bed, the liver sinusoids, which is exactly what makes it a 'portal' vein. Nutrient-rich is not oxygen-rich — separate the two ideas and this option falls away.

Human circulation is a double circulation: blood passes through the heart twice in one complete circuit. The pulmonary (lesser) circuit runs right ventricle → pulmonary trunk and arteries → lungs → pulmonary veins → left atrium, and its job is to pick up oxygen. The systemic (greater) circuit runs left ventricle → aorta → body tissues → superior and inferior venae cavae → right atrium, and its job is to deliver that oxygen. Arteries and veins are named by direction of flow relative to the heart, not by what they carry, which is why the pulmonary circuit produces the two famous exceptions — an oxygenated vein and a deoxygenated artery.

The trap is a definition students absorb informally in school — 'arteries carry oxygenated blood, veins carry deoxygenated blood' — and then never revise. Replace it with the directional definition and the answer is immediate. A second trap is planted in option (d): 'hepatic portal vein' triggers the memory of blood that is rich in something, and a hurried candidate converts 'nutrient-rich' into 'oxygen-rich'. The safest habit is to ask, for any named vessel, two separate questions — which way is it flowing relative to the heart, and has it just come from a gas-exchange surface? Only a vessel returning from the lungs can answer yes to the second. Historically, this is the insight that William Harvey established in De Motu Cordis (1628) when he showed that blood circulates in a closed loop rather than being consumed at the periphery.

• Artery = carries blood away from the heart; vein = carries blood towards the heart. Oxygen content is not part of either definition.
• Four pulmonary veins, two from each lung, return oxygenated blood from the alveolar capillaries to the left atrium — the only oxygen-rich veins in the body after birth.
• The mirror-image exception is the pulmonary artery (from the pulmonary trunk out of the right ventricle), the only artery carrying deoxygenated blood; both exceptions belong to the pulmonary circulation.
• The hepatic portal vein carries nutrient-rich but deoxygenated blood from the stomach, intestines, spleen and pancreas to the liver, linking two capillary beds — nutrient-rich is not oxygen-rich.
• Cardiac veins drain the myocardium into the coronary sinus and thence into the right atrium; the cystic vein drains the gall bladder into the portal system.
• In the foetus the umbilical vein also carries oxygenated blood, from the placenta; it closes after birth, which is why 'the only vein' is stated of the post-natal body.

• Defining artery and vein by oxygen content instead of by direction of flow. That single error is the entire question.
• Reading 'nutrient-rich' as 'oxygen-rich' for the hepatic portal vein — it is rich in absorbed food materials and poor in oxygen.
• Forgetting the paired exception. Examiners flip the same fact and ask for the only artery carrying deoxygenated blood, where the answer is the pulmonary artery.

UPPSC asks this as a one-line 'the only vein / the only artery' recall item in its biology block. UPSC prefers to test the same anatomy indirectly — naming a vessel by its function (the vessel that brings nutrient-rich blood to the liver) or by its territory (the arteries supplying the heart itself) — so learn each vessel with its source, destination and cargo, not just its name.

• practice — not a real PYQ

  Which one of the following is the only artery in the human body that carries deoxygenated blood?

  • (a)Aorta
  • (b)Carotid artery
  • (c)Pulmonary artery
  • (d)Coronary artery

  Answer(c) Pulmonary artery — it carries deoxygenated blood from the right ventricle to the lungs, the exact mirror image of the pulmonary vein. The aorta, the carotids and the coronaries all carry oxygenated blood away from the heart.

• practice — not a real PYQ

  The pulmonary veins open into which chamber of the human heart?

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

  Answer(b) Left atrium — the four pulmonary veins return oxygenated blood from the lungs to the left atrium, which passes it through the bicuspid (mitral) valve to the left ventricle for pumping into the aorta. The right atrium receives deoxygenated blood from the venae cavae and the coronary sinus.