Give the sequence of blood circulation in mammalian heart.
(a) Left auricle
(b) Right auricle
(c) Left ventricle
(d) Right ventricle

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Correct — option (3), which reads (b), (d), (a), (c): right auricle, then right ventricle, then left auricle, then left ventricle. Follow one portion of blood round the circuit and the order writes itself. Blood that has already served the body comes back deoxygenated through the superior and inferior venae cavae and empties into the right auricle, the chamber the modern textbooks call the right atrium. When that chamber contracts the blood passes through the tricuspid valve into the right ventricle. The right ventricle then contracts and drives it out through the pulmonary semilunar valve into the pulmonary artery, which carries it to the lungs — the one artery in the body that carries deoxygenated blood. In the capillaries around the alveoli it gives up carbon dioxide and takes up oxygen, and returns through the pulmonary veins, the only veins carrying oxygenated blood, into the left auricle or left atrium. From there it passes through the bicuspid or mitral valve into the left ventricle, and the left ventricle drives it out through the aorta to the whole body, after which it returns once more to the right auricle and the circuit begins again. That is why the sequence runs right-side chambers first and then left-side chambers, rather than atrium to atrium and then ventricle to ventricle: the two sides of the mammalian heart are completely separated by the interatrial and interventricular septa, and there is no route from the right side to the left side inside the heart at all. The only way across is through the lungs. This is what is meant by double circulation — blood passes through the heart twice for every complete round of the body, once on the pulmonary circuit between the right ventricle and the left auricle, and once on the systemic circuit between the left ventricle and the right auricle. Because the mammalian heart is four-chambered with that complete separation, oxygenated and deoxygenated blood never mix, which is what allows the high metabolic rate of a warm-blooded animal. The left ventricle, which has to push blood through the whole systemic circuit rather than only through the lungs, has by far the thickest muscular wall of the four chambers, and that thickness is itself a favourite examination point. Note also the vocabulary the paper uses: 'auricle' is the older name for the atrium and is still standard in Indian school textbooks, so the labels in the stem are simply the four chambers of the heart.

Why the other options are wrong — (a) This reads right auricle, left auricle, right ventricle, left ventricle — the two atria first and then the two ventricles. It is the most tempting wrong order because it mirrors the way the chambers are drawn on a diagram, upper pair above lower pair, and because a candidate who half-remembers that the atria contract together and the ventricles contract together may read that as a sequence for a single portion of blood. It is not. The atria do contract together, but they are contracting on two different blood streams: the right atrium on blood returning from the body and the left on blood returning from the lungs. Nothing passes from the right atrium into the left. The interatrial septum is complete after birth — the foetal opening in it, the foramen ovale, closes at birth and remains as the depression called the fossa ovalis — so an order that steps sideways from one atrium to the other describes a route that does not exist.  (b) This is simply the four labels in the order the paper prints them: left auricle, right auricle, left ventricle, right ventricle. It reproduces the list rather than answering the question, and it fails twice over. It begins in the left auricle, which is where blood arrives from the lungs rather than from the body, so it begins in the middle of the circuit; and it then moves from the left auricle into the right auricle, a passage that does not exist in a heart with a complete interatrial septum. When a matching or sequencing question offers the option that restates the printed order, that option is worth suspecting on sight — printed order is how the examiner lays the items out, never an answer.  (d) This reads right ventricle, left ventricle, right auricle, left auricle — the exact reverse of the printed order, and wrong in the same two ways. It starts in a ventricle, when blood entering the heart always enters an atrium: the ventricles are the pumping chambers that expel blood into the great arteries, and no vein empties into either of them. It then moves from the right ventricle to the left ventricle, which is impossible in a mammal, since the interventricular septum is complete and separates the two ventricles entirely. That completeness is the very feature that distinguishes the mammalian and avian heart from the three-chambered heart of an amphibian or of most reptiles, in which some mixing of oxygenated and deoxygenated blood does occur.

Concept: The mammalian heart has four chambers: two thin-walled receiving chambers, the right and left auricles or atria, and two thick-walled pumping chambers, the right and left ventricles. Valves keep the flow one-way — the tricuspid between the right atrium and right ventricle, the bicuspid or mitral between the left atrium and left ventricle, and the semilunar valves at the mouths of the pulmonary artery and the aorta. The path of the blood is a figure of eight through the heart. Deoxygenated blood from the body enters the right atrium through the superior and inferior venae cavae, passes to the right ventricle, and is pumped through the pulmonary artery to the lungs; oxygenated blood returns by the pulmonary veins to the left atrium, passes to the left ventricle, and is pumped through the aorta to the body. This arrangement is called double circulation, because a single portion of blood passes through the heart twice in one complete round: once on the pulmonary circuit and once on the systemic circuit. The septa dividing the two sides are complete in mammals and birds, so oxygenated and deoxygenated blood never mix; amphibians and most reptiles have three-chambered hearts in which they do, and fishes have two chambers and a single circulation. The heartbeat is initiated by the sino-atrial node in the wall of the right atrium, the pacemaker, and conducted through the atrio-ventricular node and the bundle of His to the ventricles; the muscle of the heart is itself supplied by the coronary arteries, the first branches of the aorta.

State PCS 2021 Q63 — Science & Technology PYQ with Answer

State PCS 2021 Q63 — Science & Technology PYQ with Answer