Humans can live relatively long lives; some people can even live a century, or more! A lot goes on in our bodies to keep us alive. But nothing is as tireless as our heart. Starting soon after the first month in our mother’s bellies up until our last seconds on this earth, our heart quietly ticks away. Our heart beats more than the number of seconds we live. How many times is that in 80 years? Ten million? One hundred million? Maybe a billion? The correct answer is closer to three billion (3,000,000,000) beats. These beats may look straightforward at first glance; however, the heart has a lot of precise mechanisms that occur during each and every beat. You would be surprised at how the tiniest of malfunctions can lead to severe consequences. Things can go very wrong in many different ways. But for most of us, they don’t! Keep reading, and I believe you will better appreciate your heart’s architecture and engineering, and marvel at the fact that it keeps beating for as long as it does.

Let us begin with the basics: what does our heart look like and how does it pump? The heart has four rooms inside of it, two on the top called the atria and two on the bottom called the ventricles. The atria take in the blood coming into the heart and empty it into the ventricles, which then pump blood out. The left and right sides of the heart are completely separated from each other. The right side receives blood from the body and pumps it to the lungs. The left side receives blood from the lungs and pumps it to the entire body. There are also valves found at the exits of each of the four rooms. The main function of these valves is to only allow blood to pass in one direction. This is important for many reasons. The heart is a very reactive organ in terms of remodeling. In other words, it changes its structure to compensate for changes in pressure or volume. A backward leak of blood would cause an increase in pressure and volume in the previous compartment. Over time, this leads to that compartment getting larger or thicker. In the short-term, these changes are crucial to preserve its function. However, in the long-term, this leads to lower efficiency of its pumping function, which can ultimately lead to heart failure. Every room, every wall, and every door are masterfully placed in our heart to form the perfect pump.

After learning about our heart’s intricate architecture, let’s look at how this fantastic pump works. Much like any other pump, our heart is electrically powered. Luckily though, it doesn’t need to be plugged into an outlet. Our heart generates its own electricity at the sinoatrial (SA) node, which is located at the very top of the heart. This node serves as both the generator and the pacemaker of our heart. It is what initiates a heartbeat. The electrical pulse it generates travels down through our heart and causes it to quickly contract. However, electricity is very fast. The electric pulse can reach every cell in the heart faster than a blink of an eye. This lightning-fast pulse would cause the entire heart to contract in one large beat. But as we all know, the heartbeat consists of two parts. This two-part beat is essential for the heart to pump blood. This is where heart’s second node, the atrioventricular (AV) node, comes into play. This node serves as a braking system that slows down the electricity passing from the atrial (top) circuit to the ventricular (bottom) circuit. “How long is this delay?”, you may ask. Well, just about one tenth of a second! This tiny delay is the reason we hear the heartbeat as two distinct sounds. It allows for the atria to pump all of its blood into the ventricles (the first sound) before the ventricles pump it into the body (the second sound). Otherwise, the ventricles would pump while half-empty, leaving the body to starve! As the saying goes: “Speed kills”!

Now we have a better understanding of the architecture and electrical engineering of the heart. So, how do doctors tell a healthy heart from an unhealthy one? Just like building inspectors, a doctor has specialized tools at their disposal to check the heart’s infrastructure. The three most popular tools are a good old-fashioned stethoscope, an electrocardiogram, and an echocardiogram.

Stethoscope

One of the hallmarks of the medical profession, the stethoscope has remained relevant even among modern-day medicine’s fancy gadgets. As you all know, it allows doctors to directly listen to the heart. The most common abnormalities that can be found with a good listen are arrhythmias and dysfunctional valves. An arrhythmia is when the heart doesn’t follow a regular rhythm. This usually occurs when there is an issue in the heart’s electrical circuit. In most cases, this can easily be heard directly through a stethoscope. Similarly, dysfunctional valves tend to cause more noise than usual, which are called “murmurs.” An abnormal valve may lead to a “click” when opening or a “whoosh” when blood passes through it.

Electrocardiogram

We previously talked about how electricity is a big part of the heart’s functions. Doctors can diagnose many heart problems just by looking at an electrocardiogram (ECG), which provides a window into the movement of electricity through the heart. Each bump and spike seen on an ECG strip represents the compartments of the heart contracting or relaxing. By looking at an ECG, doctors can tell if electricity is being conducted normally, and if the heart is pumping as it should. The most common problems that can be diagnosed using an ECG include heart attacks, arrhythmias, and other deadly rhythms that require an external electric shock (known as defibrillation).

Echocardiogram

So, we can hear the heart and monitor its electrical activity, but can we see it in action? Absolutely! An echocardiogram shows the heart pumping in real-time. It is great for a comprehensive assessment of the heart and can help identify issues that cannot be identified by a stethoscope or ECG.  This “video” of the heart allows doctors to see exactly how strong each part of the heart is pumping, how much blood it’s pumping, and whether there are any problems with the valves. It is great at diagnosing heart failure, which is when the heart pumps out less than a certain percentage of the blood it receives.

Finally, let’s talk about the infamous heart attack, the silent killer of millions every year. The heart is a generous organ: it delivers blood to every corner of the body. However, it also needs some of this life-giving blood! For this reason, it is fed by three large vessels that hug the heart that supply it with the blood it needs to keep pumping. A heart attack (or myocardial infarction) occurs when one of these vessels gets occluded.  Even though heart attacks are sudden, the disease process behind them usually take years to develop. The vessels feeding the heart slowly get narrower and narrower due to age, high blood cholesterol, or other various factors. Once one of them shuts off completely, that area of the heart stops. Since the heart is so active, a halt in the delivery of blood is extremely dangerous. Every minute counts! That is why patients are rushed to the hospital to have their vessels opened with medications and procedures. Once it’s too late, the muscles in that area of the heart die and the heart becomes permanently scarred. Luckily, the other two vessels are enough to keep the heart going! The heart, being as tireless as it is, will keep beating!

Now we are at the end of this article. While you read this, your heart ticked away a couple hundred more times. It delivered the energy your eyes and brain needed to see and understand this article. It’s deep in your chest and it always has your back. Take a minute to reflect on it, appreciate the work it does, and never forget to count it among your blessings!

The heart is composed of four compartments: two atriums and two ventricles. The blood that reaches the heart first accumulates in the atriums. From here, it is sent to the ventricles. Afterwards, it is redistributed to the body by the contractions of the ventricles. The harmony of this process depends on the electrical currents in our hearts.

How is the electrical current formed?

There is a particular region in the heart called the sinus node. The sinus node is strip of a muscle that is 15 mm in length, 3 mm in width, and 1 mm in thickness, and is located in the right atrium of the heart. The cells of this strip are responsible for producing electrical currents, and are created in a different fashion from the rest of the cells that are responsible for producing contractions. This is where the electrical currents in our hearts are periodically produced. Every cell in the body contains elements such as sodium, calcium, potassium and chlorine that are electrically charged. The elements which are electrically charged are called ions. These ions also exist in the extracellular environment. The intra cellular and extra cellular concentrations of these ions are different from each other. This situation causes a difference in the electrical potential between the interior and exterior of a cell. This difference is called a membrane potential. Periodically, the membrane potentials of the sinus cells show sudden jumps – meaning they suddenly increase and then suddenly decrease. Since the cells are in close contact with each other, such a jump in the membrane potential of one cell triggers a jump in the membrane potential of another cell. The electrical currency that enables the contraction of the heart is produced by this continuous triggering of cells. On average, 70 electrical currents per minute are produced in the sinus node. These currents start being produced while a person is in the womb of their mother and continues their whole lifetime. The heart of an embryo starts beating while it is only 22 days old. However, the height of the embryo at this point has not even reached 1 cm. Isn’t it an amazing force that creates the beating heart of such a small embryo and keeps it going a lifetime?

How is the electrical current distributed?

Another node called the atrio ventricular node was created in between the atriums and ventricles in our heart. While the current coming from the sinus node is spread to the whole of the atrium, it is by this node that the current is sent to particular fibers. The task of this node is to hang on to the current coming from the sinus node for a while. Why does the current need to be held on to? Because blood can only enter the ventricles while it is resting and by holding on to it, the contraction of the ventricles is disabled while the contraction of the atriums is taking place. By this process, the blood coming from the atrium can enter the ventricle. Therefore, the blood fills in the ventricles and can be distributed throughout the body. The blood circulation is enabled in a flawless manner by allowing the atriums to do their duty while the ventricles wait.

After passing through the atrio ventricular node, the electrical currency eventually goes through the purkinje fibers. These fibers surround the ventricles like a web and are composed of cells that can conduct electrical current in a very fast manner. Compared to the atrio ventricular node, the electrical current can be conducted 150 times faster in the purkinje fibers. Therefore, the current reaches every point of the ventricles in a very short period of time. Every muscle in the ventricles contracts in a time shorter than one tenth of a second.

The muscles in the ventricles rapidly contract, one by one, depending on when the current reaches them. The contraction starts at the end of the ventricles and carries on towards the main veins exiting the heart. By this orderly and harmonious contraction, the blood is pumped from the end of the heart towards the main veins exiting the heart to be distributed among the body. Because all the ventricle muscles are stimulated very fast, the contraction also happens very fast, resulting in a strong pumping effect. The design of this system is incredibly wise, right down to its most minute detail.

Movement in heart muscle potential

As all cells in our body, the cells in the heart also have a membrane potential. We had stated before that this membrane potential is the result of the difference in intra and extra cellular ion concentrations. The charges of these ions are different from each other. For example, sodium and potassium have plus one (+1) charges, calcium has a plus two (+2) charge, and chlorine has a negative one (-1) charge. The resting potential of a cell is negative. This means that there are more negative ions within the cell when compared to its environment. Sodium, calcium, and potassium ions are mobile through the membrane. While sodium and calcium have a higher concentration outside the cell, potassium has a higher intra cellular concentration compared to its environment. There are channels created on the cell membrane that allow ions to pass through the membrane. The sudden increase in the membrane potential that was explained before causes a sudden rush of sodium ions inside the cell. This is such a rapid movement that it is concluded in a tenth of a second. Right after the entrance of the sodium ions, calcium ions also enter. Because these ions are positively charged, the membrane potential becomes positive.

With the entering of calcium ions into the cell, calcium ions are also released from the storages within the cell. By triggering the protein necessary for these contractions, the calcium ions become a means for the contraction of the heart muscles. Meanwhile, the potassium channels open and these ions within the cell pass to the extra cellular environment. The loss of positive ions results in the membrane potential being negative again. Therefore, the sudden jump in membrane potential that is the basis for the electrical current is created.

However, at this point there are extra amounts sodium and calcium within the cell and extra amounts of potassium outside the cell. The concentrations need to be returned to their original values for the next jump in the membrane to be possible. This task is given to a protein called the sodium-potassium pump that pumps out sodium from the cell and pumps in potassium. If this pump had not been created, the ion balance in any of the cells within the body would be impossible to re-establish. As a result, the life of the cells would come to an end. However, because of the remarkable intricacy of our cells, life is made possible for us.

Afterwards, some amount of the calcium ions are pumped out of the cell with a similar pump, while the rest are stored within the cell. The decrease in the concentration of calcium relaxes the muscle. Now the heart muscle has gone into relaxation and therefore is ready for the next contraction.

If the movement of the ions becomes unbalanced, the rhythm of our heart is disturbed. The unbalance in the ion movements or blockage in heart veins can be reasons for heart rhythm disorders. Even small heredity-based defects in the ions pumps affect the movement of these ions and can cause heart rhythm disorders. This situation shows that nothing is created by coincidence.

Movement in the sinus node

The jump in the membrane potential of a heart cell depends on the membrane potential jump of the previous cell. Through the gaps in between the cells that are in contact with each other, the positive ions that exit a cell reach the membrane of the cell next to it and trigger the opening of its ion pumps. As a result, the membrane potential of that cell starts changing. At this point, you may have this question: how does the electrical current start in one end of the sinus node that is not previously triggered by any cell?

This concept is explained by the ion transfer mechanism of the node cells being different than the muscle cells. Before explaining this, it should be noted that even while resting, a mechanism for allowing an ion exchange of the cell with its surrounding has been created. In the node cells, this exchange while at rest has been created in a way that the sodium and calcium exchange is larger and the potassium exchange is lower compared to the muscle cells during resting conditions. Therefore, the membrane potential of the node cells is less negative and slowly increases with time. As a result of this slow but steady increase, after a while it reaches a threshold. When it reaches it, the calcium channels in the membrane suddenly open and there is a rush of calcium ions into the cell. Thus, the jump in the membrane potential is created independently from another cell.

As it can be observed, even a single contraction of our heart depends on a very detailed, delicate, and complex system. Moreover, this system is repeated over a hundred thousand times within one day. After reflecting on this, how can we claim this system runs by coincidence or chance?