The Cardiac Cycle

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Summary

  • The sequence of one heart beat ending to the beginning of another is known as the cardiac cycle
  • The heart consists of two pumps that work together where both sides of the heart relax and contract with one another
  • Systole is the repetition in contractions of the heart chambers
  • Diastole is the relaxation of the heart chambers
  • Blood moves through the circulatory system from regions of high pressure to low pressure
  • There are three stages to the cardiac cycle; atrial systole, ventricular systole and diastole
  • Atrial systole is where the atria within the heart contract and tops up the ventricles that are already filled with blood.
  • Ventricular systole is where the ventricles chambers contract and forces the blood up and out of the heart and into the arteries
  • Diastole is where all four chambers in the heart relax and re-fill with blood
  • SAN causes the atria to contract and the AVN causes a delay before the ventricles contract

Stages of the cardiac cycle

All the events that come and play within the heart come together to become the cardiac cycle. The cardiac cycle is a series of contractions that ensure blood is flowing in the correct direction

The cycle is broken down into three stages:
• Cardiac Diastole – the entire heart is relaxed
• Atrial Systole, also known as Ventricular Diastole
• Ventricular Systole, also known as Atrial Diastole

Cardiac Diastole
In cardiac diastole the entire heart is relaxed. Both the atria and ventricles are relaxed and the blood enters at low pressure through the veins, the pulmonary vein and the vena cava into the atria. As the blood flows in the atria the blood pressure begins to increase causing the AV valves to become opened allowing for the blood to enter the ventricles.

Atrial Systole
This is where the atria contracts when they are around 50% empty. This ensures all the blood is emptied from the atria and enters the ventricles. This causes the pressure within the ventricles to slightly increase shutting the AV valves, to prevent the backflow of blood back to the atria.

Ventricular Systole
The next stage is ventricular systole, where the ventricles contract from the bottom of the heart also known as the apex of the heart and upwards. Now the pressure further increases in the ventricles above the pressure within the arteries (pulmonary arteries and aorta). Due to the pressure change, blood is able to flow out through the semilunar valves and allows for the blood to leave.

Figure 1: Three phases of the cardiac cycle; During (a) cardiac diastole, the heart muscle is relaxed and blood flows into the heart. During (b) atrial systole, the atria contract, pushing blood into the ventricles. During (c) atrial diastole, the ventricles contract, forcing blood out of the heart.

Actions of the valves aand pressure changes

Pressure changes – The pressure graph

Mammals have a double circulatory system; blood is kept within the blood vessels allowing the pressure within them to be maintained and regulated. Figure 2 illustrates the pressure changes that occur within the heart during one cardiac cycle.

  • Pressure changes in the left side of the heart
  • The atrial pressure shown in Figure 2 (yellow line) has the least changes in the pressure. The pressure is always relatively low due to the thin walls of the atrium and therefore, cannot create a force to increase the pressure. The atria fill up with blood that leads to a slight increase in blood however, there is drop again when the left AV valve opens allowing some of the blood to enter the ventricles.
  • The ventricular pressure (green line) begins low at first, but then undergoes a huge pressure change during ventricular systole since the ventricles fill with blood as the atria contracts. This shuts the left AV valve and the pressure rises dramatically due to the ventricles thick muscular walls. As the pressure arises the aorta blood is passed through the semilunar valves and enters the aorta. Pressure then falls as the ventricles empty and the walls relax.
  • The pressure in the aorta remain high to begin (shown with the red line)
  • It is important to remember the key time when the valves open and when they close; the AV valve closes when the ventricular pressure exceeds the atrial pressure, indicated on Figure 2. The AV valve will only open again when the ventricular pressure is below the atrial pressure.
  • Looking above on the top part of the graph shows when the semilunar valve opens and closes. The semilunar valve will open when the ventricular pressure exceeds above the aortic pressure and the semilunar valve will close when the ventricles pressure would be below the pressure within the aortic artery.
Figure 2: A typical illustration of the pressure graph, the x axis represents time with the recoding of a heartbeat. The y-axis shows the pressure within the heart
  • The sounds of a heartbeat “lub” and “dub” are made by the sounds of the valves snapping shut (below).

Coordination and regulation of the cardiac cycle

For the heart to function effectively, there must be a fine control and balance of events that take place during the cardiac cycle described above.
Heart tissue is myogenic which means it would initiate its own contraction allowing the heart to contract without needing to be connected to the body. If cardiac muscle was left to contract by itself this could bring some problems to the heart. Since the atria contract faster than the ventricles this could lead to fibrillation hence there is a need for a way to control the cardiac cycle.
In the heart there are two nodes that are responsible to keep the cardiac cycle running correctly. A heart beat starts off at the region of tissue called the sinoatrial node (SAN) which is located above the right atrium. The SAN acts as the hearts pacemaker and ensures the heart is beating at a constant regular rate. This is achieved by the SAN giving out regular electrical signals spread throughout the heart and spreads throughout the atrial muscles that causes the atria to contract (atrial systole). This is what starts the contraction of the atria.

The second node is the known as the atrioventricular node (AVN) which is located near the AV valve. The role of the AVN is to pass the electrical signal to the middle of the heart also known as the septum. There is also a delay in the electrical pulse at the AVN (between the atria and ventricles contracting) which allows the atria to fully empty before the ventricles control. The electrical pulse next passes the signal into extremely insulated fibres called the Bundle of His. The insulation of these fibres ensures the electrical signal cannot escape. The Bundle of His carries the electrical signal to the apex, bottom of the heart. At the bottom of the heart the Bundle of His splits into two. The non-insulated fibres called Purkinje fibres spread up the walls of the ventricles and cause the ventricles to contract from bottom of the heart to cause the blood to be forced out from the ventricles, out of the semilunar valves and towards the aorta and pulmonary artery.

Figure 4: Control of the cardiac cycle

Electrocardiograms (ECG)

An electrocardiogram (ECG) is a medical device that measures the electrical pulse of the heart. Since the heart undergoes a series of electrical changes related to the waves of excitation, sensors that are connected to a monitor can detect these electrical signals. The ECG produces a trace of electrical activity shown in Figure 6.
Doctors can use the ECG trace to examine the patients electrical activity within the heart and understand better about the status of the patients health.

A normal healthy ECG trace shows what is known as the “PQRST shape” (Figure 5). Wave P is the stage that corresponds to atrial systole, excitation and contraction of the atria. Ventricular systole forms the “QRS” complex and the start of ventricular diastole corresponds to the “T” wave.

Figure 5: The start is diastole where both the atria and ventricles are relaxed. The P wave shows the depolarisation of the atria and is followed by atrial systole (contraction). Atrial systole continues to the end of the QRS complex and the point where the atria relax. The QRS complex shows the relaxation of the ventricles and is followed by ventricular systole (contraction). The T wave represents the a further depolarization (re- polarization) and marks the start of ventricular relaxation.

Below in Figure 6 shows the ECG from a healthy patient, patient suffering from a heart attack and one patient suffering from fibrillation. Fibrillation, already mentioned above is where the atria and ventricles contract and relax in an irregular manner causing strenuous stress to the heart that leads to death if not treated immediately (Figure 6c).

Figure 6: Three different ECG traces; a) Normal ECG, b) Patient suffered from heart attack and c) fibrillation ECG trace

References

[1]. https://openstax.org/books/biology-2e/pages/40-3-mammalian-heart-and-blood-vessels
[2]. https://commons.wikimedia.org/wiki/File:Wiggers_Diagram_2.svg#/media/File:Wiggers_Diagram_2.svg
[3]. https://openstax.org/books/biology-2e/pages/40-3-mammalian-heart-and-blood-vessels
[4]. https://en.wikipedia.org/wiki/Electrocardiography