The only vein that carries oxygenated blood is :

Question

UnlockIAS · Accepted Answer

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.

Why the other options are wrong — (b) 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) 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) 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.

Concept: 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.

State PCS 2024 Q68 — Science & Technology PYQ with Answer

State PCS 2024 Q68 — Science & Technology PYQ with Answer