Question

What does the sound lub dub means in cardiac cycle?

Answer 1

Cardiac cycle is a series of mechanical and electrical events taking place in the heart during one heartbeat or contraction and relaxation or one heartbeat. Our heart beats 72 times per minute normally which can be auscultated.

Complete answer:
Fluids flow according to pressure gradients that's why they move from the regions with higher pressure to regions with lower pressure. When the heart chambers are in a relaxed state, blood will flow into the atria, which are at higher pressure. As blood flows in, the pressure will increase, so the action potential triggers the muscles of the atria to contract, the pressure within the atria increases further, pumping blood into the ventricles. The pressure rises in the ventricles, which pumps blood into the pulmonary trunk from the right ventricle and in the aorta from the left. Phases of the Cardiac Cycle-
At the beginning, both the atria and ventricles are in diastole. Blood will flow into the right atria from the venae cavae. The atrioventricular valves - the tricuspid and mitral valves both open, so that blood flows from the atria and ventricles. The two semilunar valves are closed at this moment preventing backflow of blood into the ventricles.
Depolarization causes contraction of the atria which is represented by the P wave and causes blood pumping into the ventricles through the open tricuspid and bicuspid valves. Depolarization of the ventricles is recorded as the QRS complex on ECG. The muscles of the ventricles contract which increases the pressure of the blood, but still does not increase enough to open the semilunar valves. However, blood pressure quickly increases above that of the atria. This causes blood to flow back to the atria, closing of the tricuspid and mitral valves occurs which produces the sound lubb. The blood in the chamber remains constant. Now, the contraction of the ventricular muscle raises the pressure in the ventricle which is greater than the pressures of the pulmonary trunk and the aorta. Blood is ejected from the heart opening the semilunar valves. Repolarization causes ventricular relaxation which is represented by the T wave of the ECG. The ventricular muscle relaxes, pressure begins to fall, blood flows back to the heart. To prevent this, semilunar valves close and produce the sound dub.

Note:
Lub dub sounds are due to closure of valves and they can be heard by using a stethoscope. The process of hearing heartbeats by stethoscope is known as auscultation. Any abnormality in the beats forms a major basis of diagnosis.