Third Heart Sound
Third Heart Sound, or S3, is the low-frequency heart sound that occurs just after S2 during early diastole as the ventricles fill rapidly. In Anatomy and Physiology II, it shows up in cardiac cycle and heart sound analysis.
What is Third Heart Sound?
Third Heart Sound, or S3, is the extra heart sound you hear just after S2 during early diastole in Anatomy and Physiology II. It comes from rapid ventricular filling, when blood moves into the ventricles and the ventricle walls or valve structures vibrate as flow suddenly slows down.
That timing matters. S1 marks the start of systole when the AV valves close, and S2 marks the end of systole when the semilunar valves close. S3 happens right after S2, during the filling phase, so it is part of the diastolic side of the cardiac cycle rather than the pumping phase.
A common way to describe it is as a gallop rhythm. If you listen carefully, the heartbeat can sound like lub-dub-ta, with the extra third sound making the rhythm feel more rapid or swinging. Because S3 is low pitched, it is usually easier to hear with the bell of a stethoscope than with the diaphragm.
Where you listen matters too. S3 is often heard best at the apex of the heart, especially when the person is in the left lateral decubitus position. That position brings the left ventricle closer to the chest wall, which makes low-frequency sounds easier to pick up.
S3 is not always abnormal. In children, young adults, and trained athletes, it can be a normal finding because their ventricles fill quickly and efficiently. In older adults, though, S3 can point to volume overload, reduced ventricular compliance, or heart failure, which means the ventricle is not handling incoming blood as smoothly as it should.
Why Third Heart Sound matters in Anatomy and Physiology II
Third Heart Sound shows up right where Anatomy and Physiology II starts tying structure to function in the cardiovascular system. Once you know when S3 happens, you can connect it to the diastolic filling phase instead of memorizing it as a random extra noise.
It also gives you a clue about ventricular performance. A normal heart usually fills quietly, so an S3 in an older adult can suggest the ventricle is dealing with extra volume or pumping less effectively. That makes the sound useful for connecting anatomy, mechanics, and disease.
In class, S3 often appears in heart sound identification, lab practicals, or case questions where you have to match a sound to the phase of the cardiac cycle. If you can place S3 after S2 and during rapid filling, you can usually reason your way through the question even if you forget the exact wording.
Keep studying Anatomy and Physiology II Unit 1
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open one-pagerHow Third Heart Sound connects across the course
Cardiac Cycle
S3 makes sense only if you can place it in the cardiac cycle. It occurs during early diastole, after ventricular contraction has ended and before the next systole begins. That timing links the sound to filling, pressure changes, and the return of blood into the ventricles.
Heart Sounds
S3 is one of the classic heart sounds discussed alongside S1 and S2, but it is not heard in every healthy person. It is an extra sound, so identifying it depends on recognizing both the normal sounds and the pattern created when a third sound is present.
Diastole
The third heart sound belongs to diastole, not systole. More specifically, it happens during the rapid filling phase, when blood rushes into the ventricles and then slows suddenly. If you mix up diastole and systole, you will place the sound in the wrong part of the cycle.
Valvular Insufficiency
Valvular insufficiency can change blood flow patterns and contribute to abnormal heart sounds. While S3 is not the same thing as a valve leak, both can show up when the heart is not moving blood normally, so a lab or case question may ask you to compare the sound with a broader cardiac problem.
Is Third Heart Sound on the Anatomy and Physiology II exam?
A quiz item may ask you to identify S3 on a heart sound recording, pick the phase of the cardiac cycle it belongs to, or explain why it can be normal in a young athlete but concerning in an older adult. You might also get a case study describing a low-pitched extra sound right after S2 and need to connect it to rapid ventricular filling. In a lab practical, the task is usually to match the sound, location, and patient position, especially apex and left lateral decubitus. When you answer, name the timing first, then the mechanism, then the clinical meaning if the question asks for it.
Third Heart Sound vs Heart Sounds
S3 is part of the broader group of heart sounds, but it is not the normal first or second sound you hear in every heartbeat. S1 and S2 are expected sounds from valve closure, while S3 is an extra low-pitched sound that may be normal in some people or signal a problem in others.
Key things to remember about Third Heart Sound
Third Heart Sound, or S3, is a low-pitched sound heard just after S2 during rapid ventricular filling in early diastole.
It is often described as a gallop rhythm because it adds an extra beat-like sound to the normal heartbeat pattern.
S3 can be normal in children, young adults, and athletes, but in older adults it can suggest volume overload or heart failure.
You usually hear it best at the apex with the patient in the left lateral decubitus position using the bell of the stethoscope.
If you can place S3 after S2 and connect it to filling rather than contraction, you have the core idea.
Frequently asked questions about Third Heart Sound
What is Third Heart Sound in Anatomy and Physiology II?
Third Heart Sound, or S3, is a low-frequency sound that happens just after S2 when the ventricles fill rapidly in early diastole. In Anatomy and Physiology II, it is used to connect heart sounds with the phases of the cardiac cycle. It can be normal in younger people, but in older adults it may point to ventricular dysfunction or fluid overload.
Why does S3 happen after S2?
S3 comes after S2 because S2 marks the end of systole and the start of diastole. Once the semilunar valves close, the ventricles relax and fill quickly, and that sudden filling can create the low-pitched sound. If you place it before S2, you are mixing it up with systolic events.
How is Third Heart Sound different from S1 and S2?
S1 and S2 are the normal closing sounds of heart valves, so they happen in every heartbeat. S3 is an extra sound that appears during early diastole and is linked to ventricular filling, not valve closure. That is why S3 can be a normal finding in some people or a sign of disease in others.
Where do you listen for Third Heart Sound?
S3 is usually best heard at the apex of the heart, especially when the patient is in the left lateral decubitus position. That position brings the left ventricle closer to the chest wall and makes low-frequency sounds easier to hear. The bell of the stethoscope is usually more useful than the diaphragm for this sound.