If you put a stethoscope over the chest, you’ll usually hear something that sounds like lub dub, lub dub, lub dub, which repeats over and over again, with each cardiac cycle, or heartbeat. Now, the question is, where does this sound come from? Normally, blood is constantly moving through the four chambers of the heart- coming through the veins into the right atrium, going to the right ventricle, then shooting off via the pulmonary arteries to the lungs and coming back from the pulmonary veins into the left atrium and the left ventricle, to be pumped into the aorta. So, in every step, some valves have to open and others have to close. Valves are just “communicating doors” that, when open, allow blood to pass through, and when closed, hold blood within a chamber. So, in total, our heart has four valves- two atrioventricular valves, which separate the atria from the ventricles and are the mitral valve, on the left side, and the tricuspid valve, on the right side, and two semilunar valves, which separate the ventricles from the large arteries coming off of them and are the pulmonary valve, on the right side, and the aortic valve, on the left side. And when these valves are closing, just like a door slamming shut, they are going to make a sound that is transmitted in the direction of the blood flow. Now the heart is positioned in such a way that the sound of the closing of each of these valves is projected onto a small area on the chest wall. 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. Making our way down, between the fourth and fifth rib, next to the left lower border of the sternum, is where you can best hear the tricuspid valve closing. Finally, let’s move down to between the fifth and sixth rib, so in the left fifth intercostal space, near the midclavicular line. The midclavicular line is the imaginary line that gets drawn from the midpoint of the left clavicle, or the collarbone, straight down, and you can find where it intersects with the fifth intercostal space. That’s where the mitral valve closing is best heard. Now in reality, a lot of these things are happening at once, like a factory with lots of things happening in parallel. For example, the right and left atria are both full of blood, and that blood moves through the tricuspid and the mitral valve to get down into the ventricles. Initially, the blood flows passively into the ventricles, but near the end when there’s just a bit left, there’s an atrial contraction that gives the blood an extra hard push to help get it out. This part of the heartbeat, when blood is filling the relaxed ventricles is called diastole. Now once the ventricles have filled up, both of the atrioventricular valves snap shut, creating a long, loud sound that sounds a bit like “lub”. That’s the first heart sound, or S1. And because it’s basically the tricuspid and mitral valve closing, it’s best heard in the tricuspid valve and mitral valve area. So, at this point, the ventricles are full of a whole lot of blood and are ready to squeeze it out. And to do that, the aortic valve, on the left side, and the pulmonic valve, on the right side, quietly open up. Blood flows from the left ventricle into the aorta and from the right ventricle into the pulmonary arteries. This part of the heartbeat, when blood in the ventricles are contracted and pushing blood out is called systole. Eventually, the ventricles finish squeezing, so these two valves close down, making a short, sharp sound that sounds a bit like a “dub”. And this dub is called the second heart sound, or S2. This is heard loudest in the aortic valve and pulmonary valve area. During inspiration, though, if you listen carefully with a stethoscope, this S2 sound actually splits into two separate sounds. That’s because the diaphragm muscle lowers during inspiration, and that creates negative pressure in the chest to bring in air, and that negative pressure also brings a bit more venous blood back to the right atrium and right ventricle. It takes a little bit longer for the right ventricle to squeeze the extra blood into the pulmonary arteries and it takes a little bit longer for the pulmonary valve to close. So during inspiration, the closing of the pulmonary valve is heard slightly later than the aortic valve, and that’s called the physiologic splitting of the S2. Now after both the aortic and pulmonary valves have shut down, the atrioventricular valves open again, letting the cycle start all over again. All right, as a quick recap, heart sounds are generated as blood flows through the beating heart. There are two normal heart sounds- S1, caused by the atrioventricular valves closing, at the beginning of systole, and S2, caused by the aortic and pulmonary valves closing, at the beginning of diastole.