Lena Torres
The human heart makes a lub-dub sound, caused by the closing of the heart's valves. The first heart sound, or S1, forms the lub and is composed of the mitral and tricuspid valves closing. The second heart sound, or S2, forms the dub and is composed of the aortic and pulmonic valves closing. The third heart sound, or S3, is rarely heard and is also called a protodiastolic gallop. The fourth heart sound, or S4, is usually abnormal.
| Characteristics | Values |
|---|---|
| First heart sound | S1, lub, composed of M1 (mitral valve closure) and T1 (tricuspid valve closure) |
| Second heart sound | S2, dub, composed of A2 (aortic valve closure) and P2 (pulmonary valve closure) |
| Third heart sound | S3, rarely audible, benign in youth, some trained athletes, and pregnant women |
| Fourth heart sound | S4, almost always abnormal, caused by ventricular hypertrophy or myocardial ischemia |
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What You'll Learn
Heart valves closing
The human heart has four valves: the aortic, mitral, pulmonary, and tricuspid valves. These valves control blood flow by opening and closing to let blood flow from one area of the heart to another. As they open and close, they make two sounds that create the sound of a heartbeat.
The first heart sound, or S1, forms the "lub" of "lub-dub" and is composed of components M1 (mitral valve closure) and T1 (tricuspid valve closure). Normally, M1 precedes T1 slightly. It is caused by the closure of the atrioventricular valves at the beginning of ventricular contraction or systole. When the ventricles begin to contract, so do the papillary muscles in each ventricle. The papillary muscles are attached to the cusps or leaflets of the tricuspid and mitral valves via chordae tendineae (heart strings). When the papillary muscles contract, the chordae tendineae become tense and thereby prevent the backflow of blood into the lower-pressure environment of the atria. The chordae tendineae act a bit like the strings on a parachute, and allow the leaflets of the valve to balloon up into the atria slightly, but not so much as to evert the cusp edges and allow backflow of blood.
The second heart sound, or S2, forms the "dub" of "lub-dub" and is composed of components A2 (aortic valve closure) and P2 (pulmonary valve closure). The S2 sound results from reverberation within the blood associated with the sudden block of flow reversal. Splitting of S2, also known as physiological split, normally occurs during inhalation because the decrease in intrathoracic pressure increases the time needed for pulmonary pressure to exceed that of the right ventricular pressure. A widely split S2 can be associated with several different cardiovascular conditions, and the split is sometimes wide and variable, while other times it is wide and fixed.
The third heart sound, or S3, is rarely heard and is also called a protodiastolic gallop, ventricular gallop, or informally the "Kentucky gallop". It occurs at the beginning of diastole after S2 and is lower in pitch than S1 or S2 as it is not of valvular origin. The third heart sound is benign in youth, some trained athletes, and sometimes in pregnancy, but if it re-emerges later in life, it may signal cardiac problems, such as a failing left ventricle as in dilated congestive heart failure (CHF).
The location of the heart sound can help determine its cause. For example, the S1 heart sound—consisting of mitral and tricuspid valve closure—is best heard at the tricuspid (left lower sternal border) and mitral (cardiac apex) listening posts. If you place a stethoscope between the second and third rib, known as the right second intercostal space, just next to the upper border of the sternum, you’ll hear the aortic valve closing. Then, if you place a stethoscope in the left second intercostal space, at the left upper sternal border, you can hear the pulmonary valve closing.
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Blood flow through the four chambers
The human heart has four chambers, which can be thought of as rooms in a house. Blood flows through these chambers, delivering oxygen and nutrients to all organs and tissues and removing waste. The heart valves, or "doors", open and close to manage blood flow and keep it moving in the proper direction.
Oxygen-poor blood from the body enters the heart through two large veins called the superior and inferior vena cava. The blood enters the heart's right atrium and is pumped to the right ventricle, which in turn pumps the blood to the lungs through the pulmonary artery. The lungs add oxygen to the blood, and the oxygen-rich blood returns to the heart through the pulmonary veins, entering the left atrium. From the left atrium, blood flows into the left ventricle. The left ventricle generates the high pressure needed to pump the blood to the whole body through the aorta, a large artery that carries blood away from the heart.
The tricuspid valve connects the right atrium and right ventricle, the pulmonary valve connects the right ventricle and pulmonary artery, the mitral valve connects the left atrium and left ventricle, and the aortic valve connects the left ventricle and aorta. The valves open to allow blood to flow out of a chamber and close to allow the chamber to refill with blood. When these valves close, they make a sound that is transmitted in the direction of the blood flow. The sound of the closing of each of these valves can be heard by placing a stethoscope on the chest in specific areas. The first heart sound (S1) is the closure of the atrioventricular (mitral and tricuspid) valves, and the second heart sound (S2) is the closure of the semilunar (aortic and pulmonary) valves.
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Atrioventricular valves
The human heart has four valves: two atrioventricular valves and two semilunar valves. The atrioventricular valves, also known as the mitral and tricuspid valves, separate the atria from the ventricles. The mitral valve is on the left side of the heart, while the tricuspid valve is on the right.
The atrioventricular valves open to allow blood to flow from the atria to the ventricles and close to prevent blood from flowing backward from the ventricles to the atria. When the valves close, they create a sound that is transmitted in the direction of blood flow. The closing of the mitral and tricuspid valves produces the first heart sound, known as S1. This sound is best heard between the fourth and fifth rib, next to the left lower border of the sternum, which is where the tricuspid valve closing can be heard most clearly.
The mitral valve has two leaflets, while the tricuspid valve has three. The mitral valve allows blood to flow from the left atrium to the left ventricle, while the tricuspid valve allows blood to flow from the right atrium to the right ventricle. These valves are essential for ensuring blood flows in the correct direction and preventing backflow.
The closure of the atrioventricular valves is normally a single sound, as the mitral and tricuspid valves close almost simultaneously at the beginning of systole. The sound is long and loud, often described as sounding like "lub", which is the first part of the "lub-dub" sound associated with the heartbeat.
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Semilunar valves
The human heart has four valves, two of which are semilunar valves. These valves are pocket-like, half-moon-shaped structures located between the ventricles and the two major arteries. The aortic valve is present between the left ventricle and the aorta, and the pulmonary valve is present between the right ventricle and the pulmonary artery.
The semilunar valves are crucial for maintaining unidirectional blood flow. They permit blood to flow from the ventricles into the aorta and pulmonary artery, while also preventing the backflow of blood from these arteries into the ventricles. The opening and closing of the semilunar valves are influenced by pressure gradients within the heart. When the pressure in the ventricles exceeds that in the arteries, the valves open, allowing blood to flow forward. Conversely, when the arterial pressure surpasses ventricular pressure, the valves close, preventing backflow.
The closure of the semilunar valves, particularly the aortic valve, contributes to the familiar "lub dub" sound of the heartbeat. This sound is best heard through a stethoscope placed in specific locations on the chest wall. The closure of the aortic valve is audible in the right second intercostal space, while the pulmonary valve's closure can be heard in the left second intercostal space.
The semilunar valves are susceptible to various conditions that can impair their function. One common issue is stenosis, or the narrowing of the valves, which can be caused by scarring from rheumatic fever or calcium buildup with aging. Stenosis hinders the valves' ability to open fully, leading to obstruction of blood flow and increased cardiac workload. Another issue is regurgitation, which occurs when the valves cannot close completely, resulting in blood flowing back into the ventricles. Regurgitation can be caused by several factors, including rheumatic fever, inflammation, stenosis, and congenital heart disease.
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Heart murmurs
A typical heartbeat makes two sounds, often described as "lub-dup" or "lubb-dupp", when the heart valves are closing. A heart murmur is an extra sound, such as a whooshing or swishing, made by rapid, choppy (turbulent) blood flow through the heart. Heart murmurs can be present at birth (congenital) or develop later in life (acquired). They are graded by intensity (loudness) from 1 to 6. A grade 1 murmur is faint and can be heard only with a stethoscope.
Most heart murmurs are harmless and are known as innocent murmurs. They do not cause any symptoms or problems and do not need treatment. Innocent murmurs are common in children and are caused by blood circulating through the heart’s chambers and valves, or through blood vessels near the heart. They are often caused by changes in blood flow, such as during excitement or fear, which can make the murmurs louder or softer. Innocent murmurs may disappear and then reappear, and they may continue for life without causing serious health problems.
However, some heart murmurs may be a sign of a serious heart condition. Non-innocent (or abnormal) heart murmurs are often caused by defective heart valves. For example, a stenotic heart valve has a smaller-than-normal opening and can’t open completely. Or a valve may also be unable to close completely, leading to regurgitation, which is blood leaking backward through the valve when it should be closed. Certain congenital defects and other conditions such as pregnancy, fever, anaemia or thyrotoxicosis (a condition caused by an overactive thyroid gland) can also cause murmurs. Tests such as an electrocardiogram (ECG) or echocardiogram (echo) can be used to check for heart and heart valve abnormalities.
In children, worrisome murmurs are usually due to a problem with the heart's structure that is present at birth (congenital heart defect). Congenital causes of worrisome heart murmurs include holes in the heart, such as atrial septal defect and ventricular septal defect, and cardiac shunts, which cause irregular blood flow between the heart chambers or blood vessels.
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Frequently asked questions
The normal heart sounds are often described as "lub" and "dub", which occur in sequence with each heartbeat.
The first heart sound, or S1, is caused by the closure of the mitral and tricuspid valves at the beginning of ventricular contraction.
The second heart sound, or S2, is caused by the closure of the aortic and pulmonic valves.
In addition to the first and second heart sounds, there are two rarer extra heart sounds, called S3 and S4, which form gallop rhythms. S3 is rarely heard and is considered benign in youth and some trained athletes. S4 is almost always abnormal.
