Question
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Typical lub-dub sounds heard in heartbeat are due to
A. Closing of bicuspid and tricuspid valves
B. Closing of semilunar valves
C. Closure of bicuspid-tricuspid valves followed by semilunar valves
D. Blood under pressure through aorta.

Answer
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Hint: Valves present in the heart prevent the backflow of the blood between the chambers. The closing of AV and semilunar valves at the beginning and end of the ventricular systole, respectively, produce heart sounds.

Complete answer:
To answer this question, we should know the heart valves and the sequence during the cardiac cycle.
We know that atrioventricular valves (AV valves) are present between the atria and ventricles on both sides of the heart. The AV valve between the right atria and right ventricle has three cusps and is called the tricuspid valve. On the other hand, the AV valve on the left side has two cusps and is also called the bicuspid or mitral valve.
Likewise, the pulmonary semilunar valve and aortic semilunar valve regulate the entry of blood from the right and left ventricles into the pulmonary trunk and the aorta respectively.
Now, Let’s discuss the cardiac cycle beginning with the atrial systole, that is the contraction of atria. The contraction of atria coincides with the diastole of the ventricles. It means that the ventricles are relaxed. Atrial contraction increases the blood pressure in the atria and pushes the blood into the two ventricles. At this time, both AV valves are open, and the semilunar valves are closed.
It is followed by the ventricular systole during which both ventricles are in systole while atria are in diastole. At this time, semilunar valves are open, and both the AV valves (bicuspid-tricuspid valves) are closed. The closing of the AV valves during the ventricular systole produces the first heart sound called "LUB". At this stage, the increased pressure in ventricles pushes the blood from the right and left ventricles into the pulmonary trunk and aorta respectively.
The ventricular systole is followed by combined diastole during which both atria and both ventricles are in diastole. At this time, both atrioventricular valves are open, and the semilunar valves are closed. Closing of the semilunar valves at this stage produces the second heart sound called "DUB".

So, the correct answer is C. Closure of bicuspid-tricuspid valves followed by semilunar valves.

Note: One should know the cardiac cycle for answering such questions. The sequence of the events that occur during the cardiac cycle is most important. When we talk about systole and diastole, it refers to the contraction and relaxation of the cardiac muscles present in the heart wall. Systole of one heart chamber increases the blood pressure in it while its relaxation increases the volume of the chamber and reduces the blood pressure inside.