Our heart is like a pump that never rests. The distribution of the dirty blood to the lungs and clean blood all through the body is organized by a system that produces an electrical current. Every second, small electrical currents are created in our hearts in order to start the contractions and make sure it is continuing to function. Every current starts from a particular place and gets distributed to the entire heart.
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.
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?
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.
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.
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?