In which phase of the cardiac cycle do the heart sounds S1 and S2 occur, and what specific events correspond to these sounds?
Added by David B.
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The cardiac cycle consists of two main phases: diastole (when the heart relaxes and fills with blood) and systole (when the heart contracts and pumps blood). Show more…
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Explain the events associated with S1 and S2 heart sounds, indicating where each of these sounds is best heard.
Adi S.
Kaela P.
isovolumetric contraction closed rises S1 isovolumetric relaxation drops depolarize ventricular filling S2 ventricular ejection The first phase of the cardiac cycle is . This is when a lot of blood is in the atria, which causes atrial pressure to be higher than the ventricles. The atrioventricular valves open, and blood moves from the atria to the ventricles. At the end of this phase, the atria contract to maintain pressure and blood flow into the ventricles. As blood leaves the atria, the pressure in the atria . As blood enters the ventricles, the pressure in the ventricles Towards the end of this phase, the pressure in the ventricles will exceed the pressure in the atria. At this point, the heart enters the second phase of the cardiac cycle: . At this point, the AV valves shut, and we hear . The atria repolarize and the ventricles (due to the QRS complex of the ECG), causing ventricular contraction. Although contracting, it is not yet moving blood into the arteries. All 4 heart valves are . In the 3rd phase of the cardiac cycle, called , the pressure in the ventricles finally exceeds the pressure in the arteries, and blood is ejected from the ventricles. At the end of this cycle, the T wave occurs. The last phase of the cardiac cycle is called , and has tension in the ventricle walls declining. This reduces the pressure in the ventricles. Blood wants to flow backwards from the arteries but the semilunar valves shut, creating The net effect of once cycle of the cardiac cycle is the movement of roughly 70ml of blood. This entire process takes about 0.8s for a typical person at rest (with a resting heart rate of roughly 70bpm). An athlete's heart would have a higher stroke volume due to intensive cardiac conditioning. This causes an athlete's heart to be able to beat less frequently at rest (with a resting heart rate of 50 or even 40 bpm).
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