Lesson Explainer: The Regulation of a Heartbeat Biology

In this explainer, we will learn how to describe the electrical regulation of a heartbeat and recall what is meant by systole and diastole.

As we know, the heart is responsible for pumping blood to the various organs of the body. We perceive the pumping action of our heart as the heartbeat. The normal rate of the heartbeat for a healthy adult is around 70 beats per minute. During an average lifetime, the human heart beats around 2.5 billion times!

You can feel your own heartbeat in your chest or by checking your pulse in your wrist. You might have noticed that your heart beats faster when you exercise or when you are nervous or excited. Your heart rate also lowers during the night when you are asleep, and it gradually increases after you wake up. This is because your heart rate changes according to your body’s physical state (i.e., whether you are exercising or at rest) and psychological state (i.e., whether you are calm or excited).

The pumping action of the heart involves cycles of contractions and relaxations of the chambers of the heart. These contractions and relaxations are regulated by electrical signals. Specialized tissues in the heart generate and conduct these electrical signals at regular intervals, causing the heart to pump.

In Figure 1, you can see the different chambers and blood vessels of the heart. Let’s take a look at these structures and relate their functions to the sequence of events in the heartbeat.

To begin with, all the chambers of the heart are in a relaxed state, as shown in Figure 2. This allows the blood being delivered to the heart to enter the atria on both sides, delivered by the superior vena cava, the inferior vena cava, and the pulmonary vein.

In the upper-right corner of the right atrium of the heart, there is a structure made of specialized cardiac muscle cells and nerve cells, called the sinoatrial node (SAN). The sinoatrial node is embedded in the right atrial wall, near the connection between the right atrium and the large veins, as shown in Figure 3. This node generates electrical signals without the need for any external stimulus and can generate these signals at a rate of around 70 per minute.

Once the right atrium is filled with blood, the sinoatrial node generates an electrical signal. This causes a wave of electrical activity to spread through the right and left atria, causing them to contract, as depicted in Figure 3. The valve guarding the opening from the atria to the ventricles opens, and the contraction pushes blood into the ventricles on either side, which remain relaxed.

The signal from the sinoatrial node reaches another node, called the atrioventricular node (AVN), found in the lower-left corner of the right atrium near the septum, which separates the atria and ventricles. The electrical signal generated by the sinoatrial node is received by the atrioventricular node, as shown in Figure 3.

The electrical signal continues through a bundle of specialized muscle fibers called the bundle of His, or the atrioventricular bundle, as displayed in Figure 4. This structure carries the electrical signal toward the ventricles. Here, at the interventricular septum, the bundle of His branches into a left and a right bundle, carrying the electrical impulse toward the base of the ventricles.

These bundles branch into finer specialized muscle fibers that travel upward along the outer muscular walls of the two ventricles. These fibers, which can conduct electrical signals, are called Purkinje fibers or Purkyne fibers. Through these fibers, the electrical impulses reach the walls of the ventricles, as shown in Figure 5. This causes the simultaneous contraction of the ventricles on either side, enabling them to pump blood out of the heart through the blood vessels to the organs of the body.

Example 1: Identifying the Electrical Impulse from the Sinoatrial Node

The diagrams provided show a simplified outline of the cardiac cycle. The purple arrows indicate the movement of an electrical impulse, and the purple shading indicates when an area is contracting.

Which diagram best demonstrates the initial movement of the electrical impulse from the sinoatrial node?

Answer

The pumping action of the heart, and the rate at which it pumps, is regulated by an electrical conduction system. Specialized structures in the heart, made of cardiac muscle cells and nerve cells, generate and conduct electrical signals, causing the different chambers of the heart to contract as the signal reaches them.

In the upper-right corner of the right atrium, there is a structure called the sinoatrial node (SAN). This node is capable of generating electrical signals without the need for any external stimulus.

The signal generated by the sinoatrial node spreads through the left and right atria, causing them to contract. This contraction pushes blood through the valves and into the ventricles on both sides, which remain relaxed, so that they can be filled with blood.

Let’s examine the figures. We know that the signal from the sinoatrial node comes from the upper-right corner of the right atrium and causes the contraction of the two atria, the upper two chambers of the heart.

Figure 2 shows an apt representation of this signal. The purple arrows, indicating the direction of movement of the electrical impulse, originate from the sinoatrial node in the upper-right corner of the heart. The purple shading, indicating the contraction of an area, spreads across the two atria.

Figure 2, therefore, best demonstrates the initial movement of the electrical impulse from the sinoatrial node.

The sinoatrial node is called the pacemaker of the heart, since it generates its own electrical signals and regulates the rate at which the heart beats. It is also for this reason that the heart is described as being myogenic—this means that the electrical impulse originates from muscular tissue.

Let’s quickly recap what we have learnt about the electrical conduction system in the heart. Figure 6 represents the stages of electrical conduction in blue and the corresponding contractions and relaxations of the chambers of the heart in orange.

Example 2: The Passage of Electrical Impulse in the Cardiac Cycle

Starting with an electrical impulse being generated in the sinoatrial node, which of the following outlines the passage of electrical impulses in the cardiac cycle in the correct order?

1. SAN bundle of His Purkinje fibers AVN
2. SAN Purkinje fibers AVN bundle of His
3. SAN AVN Purkinje fibers bundle of His
4. SAN bundle of His AVN Purkinje fibers
5. SAN AVN bundle of His Purkinje fibers

Answer

The cardiac cycle is regulated by a system of electrical conduction in the heart. Specialized structures, made of cardiac muscle cells and nerve cells, are responsible for generating and conducting electrical signals in the heart, causing the chambers to contract as the signal reaches them. The order of passage of the electrical signal is represented in the figure below.

The electrical impulse is first generated by the sinoatrial node (SAN), located in the upper-right corner of the right atrium. This node regulates the rate at which the heart beats and is therefore called the pacemaker of the heart.

From the SAN, the wave of electrical activity spreads through the two atria of the heart, causing them to contract. It reaches another node, called the atrioventricular node (AVN), located in the lower-left corner of the right atrium. The AVN is located close to the interventricular septum, which is the muscular wall that separates the atria from the ventricles.

The AVN receives the signal from the SAN and passes it to the bundle of His, which is a bundle of specialized muscle fibers along the interventricular septum. This bundle conducts the electrical impulse from the atria to the ventricles. The bundle of His branches into a left and a right bundle, down the interventricular septum, and toward the base of the two ventricles.

From here, the left and right bundles branch into finer specialized muscle fibers that travel upward along the outer muscular walls of the ventricles. These fibers are called Purkinje fibers and can conduct electrical signals. The electrical signal conducted through the Purkinje fibers causes the two ventricles to contract, pumping blood out of the heart.

The correct order of passage of the electrical impulses in the cardiac cycle is therefore SAN AVN bundle of His Purkinje fibers.

Now that we have covered the mechanisms of electrical conduction in the heart, there is one important question that still needs to be answered—what causes the beating sound that we hear our heart making?

The beating of the heart is often described as making a “lubb-dupp” sound. The two sounds are clearly distinguishable—the “lubb” sound is a long, low-pitched sound, whereas the “dupp” sound is shorter and higher-pitched. As we have learnt, the two ventricles go through cycles of contractions and relaxations as the blood moves within the heart and to the rest of the body. This cycle of contractions and relaxations is called the cardiac cycle, and it has two distinct phases: systole and diastole.

The period of contraction of the ventricles during the heartbeat is called systole. When the ventricles contract, the valves guarding the opening from the atria to the ventricles close, to prevent blood from moving back toward the atria. The closing of these valves, which are called the atrioventricular valves, causes the longer, lower-pitched “lubb” sound.

As the ventricles contract, blood is pumped out of the heart, through the pulmonary artery and the aorta. Once the systole, or ventricular contraction, is complete, the four chambers of the heart all relax and the atrioventricular valves open. This period is called diastole.

As the ventricles relax, the valves in the heart guarding the opening from the ventricles to the pulmonary artery and the aorta close. The closing of these valves causes the “dupp” sound, which is shorter and higher-pitched.

Blood flowing through the vessels of the circulatory system is in a constant state of motion. The heart, as we know, pumps blood around the body, exerting a force on the blood. This force enables the blood to remain in motion throughout the circulatory system. The blood, in turn, exerts pressure on the walls of the blood vessels that carry it. This pressure, called the blood pressure, is an important measure of the proper functioning of the heart and blood vessels.

During the systolic phase of the cardiac cycle, when the ventricles contract and blood is pumped out of the heart, the blood pressure is at its highest. This is called the systolic blood pressure. In a healthy adult, the systolic pressure is usually around 120 mmHg.

On the other hand, during the diastolic phase, the ventricles relax. This means that the blood pressure is at its lowest during the diastolic phase of the cardiac cycle, in between beats. In a healthy adult, the diastolic blood pressure is usually around 80 mmHg.

The normal blood pressure of a healthy adult is therefore considered to be 120/80 mmHg. Since arteries are the vessels carrying blood away from the heart, blood is pushed through arteries with greater force. Arteries also have thicker, more muscular walls than veins. This means that arteries have higher blood pressure than veins that, in contrast, carry blood flowing with less force. Since veins have thinner muscular walls, the skeletal muscles around the veins play an important role in the movement of blood through veins. When the skeletal muscles around a vein contract, this helps to contract the vein itself, pushing the blood along its path back to the heart.

It is also interesting to note that the blood pressure decreases in vessels that are further away from the heart. When measuring blood pressure, the reading is taken from the arms since the vessels here are closer to the heart and are therefore a more accurate measure of the force with which the heart is pumping blood.

Blood pressure is measured using an instrument called a sphygmomanometer, which consists of an inflatable cuff that wraps around the arm and a column of mercury that is used to read the pressure of the blood flow. The cuff is inflated, tightening around the arm until the blood flow to the lower arm is completely restricted. A stethoscope is then positioned at the elbow, over the blood vessels, as the air is slowly and steadily released from the cuff.

As the cuff loosens, blood begins to flow into the lower arm again. This blood is at the highest possible pressure, or the systolic blood pressure. At this point, a tapping sound can be heard through the stethoscope, and the pressure shown on the mercury column is noted as the systolic blood pressure. Over the next few seconds, the blood flow returns to normal, eventually reaching the lowest pressure, or the diastolic pressure. At this point, the tapping sound stops, and the pressure shown on the mercury column is noted as the diastolic blood pressure.

There are also digital or electronic sphygmomanometers in which the pressure is recorded by a device rather than being manually observed from a column of mercury. Digital sphygmomanometers are, however, less accurate and reliable than mercuric ones.

Measurement of blood pressure is a useful way of detecting possible problems in the heart or blood vessels of a patient. Increased blood pressure is called hypertension and may be caused by several different factors. For example, the narrowing of the arteries caused by the accumulation of plaque formed by cholesterol and fats along the walls of the arteries or increased blood volume caused by water retention as a result of high quantities of sodium.

Let’s review everything we have learned about the regulation of a heartbeat.