Every one of us has a unique heartbeat. A healthy heart beats without tiring – about a hundred thousand times a day and will beat some three billion times in a lifespan of 80 years. It is uniquely built for that muscular task, from the design of the four chambers (the upper two known as atriums and the lower two, ventricles), to the four valves that keep blood flowing in the right direction, to the cardiac muscle (myocardium) that makes up the thick middle-third of the heart and which is responsible for the heart’s vital contraction that is needed to pump blood through the circulatory system. If you’re poetic, you could say that there’s a music in every heartbeat, and you are not wrong. This “music” is what I want talk about today. A good place to start is study how the key parts of the heart are put together as shown in the following diagram.

Let the Music Begin

If you listen to your heart through a stethoscope, you’ll hear the double sound sometimes described as “lub dub”. The “lub” is the sound made by two large values (the mitral and tricuspid) closing at the same time during the active part of the beat, known as systole. This is when blood is squeezed out of the ventricles. The “dub” is the sound made by the other two valves (the pulmonary and aortic) closing up to prevent backflow while the ventricles refill. These soft percussive sounds have been compared to a soft tapping of the finger on a desk, though that description is somewhat limited because what you hear are subtle, intimate and primal sounds unique to every individual. Your “Lub dub” is never the same as mine.

All this music would not be there without the work performed by heart muscle cells called cardiomyocytes. Their primary function is to contract to generate the pressure needed to pump blood through the circulatory system. Indeed, one of the wonders of the heart is simply that it contracts strongly and efficiently as it does. Indeed, ourheart muscle cells are somehow able to twitch spontaneously and in synchrony like the musicians of an orchestra.

How does this happen?

The answer is that there is a “conductor” that coordinates the playing. This conductor is called the sinoatrial node. It consists of a few thousand specialized cells in the wall of the heart over the right atrium. To initiate a beat, the node contracts, and generates an electrical impulse, a signal that travels first to the atrial chambers, causing them to contract, then to a second node which delays it until the atria have fully contracted. Once they have done so, this second node sends the electrical signal to the ventricles, causing them to contract in turn.

The signal from the sinoatrial node is powerful enough to initiate the heart’s contraction because all its cells fire exactly at the same time, the synchrony that was mentioned earlier. This “music” goes on and on as long as we are alive because the heart muscle is rich in myoglobin, a source of stored oxygen for aerobic respiration, and glycogen (stored energy). It also has very many large mitochondria, which function as cell “batteries.” Together, they supply the energy that almost literally keeps the music of the heart going until the three billion beats are up. Then the music stops.

Note:
This post is adapted from Caspar Henderson, A New Map of Wonders, Granta, London, 2017.