Cardiac cycle
Definitions & Key takeaways
The cardiac cycle is a repeating process by which the heart pumps blood into circulation. It consists of two phases: the diastole (relaxation) phase and the systole (contraction) phase.
During diastole, major events include isovolumetric ventricular relaxation and ventricular filling, which enables the heart to relax and ventricles to get filled with blood. During systole, the main events are isovolumetric ventricular contraction and systolic ejection, meaning the heart contracts and pumps the blood out of the ventricles.
Introduction0:00–0:47
A cardiac cycle is the sequence of mechanical and electrical events that occurs with every heartbeat. Now, the heart is shaped like a cone and it contains two upper chambers, called atria; and two lower chambers, called ventricles.
Now, the left atrium receives oxygenated blood from the lungs via the pulmonary veins; while the right atrium receives deoxygenated blood from all of our organs and tissues via the superior and inferior vena cava.
From the atria, the blood flows into the lower chambers of the heart: the left ventricle, which pumps oxygenated blood to all our organs and tissues via the aorta; and the right ventricle, which pumps the deoxygenated blood back to the lungs via the pulmonary arteries.Alright, so each heartbeat consists of two phases: systole, which is when the heart contracts and pumps blood out of the ventricles; and diastole, which is when the heart relaxes and ventricles fill with blood.
Systole and diastole0:47–1:00
Now, the cardiac cycle graph is used to express events during one cardiac cycle. Along the y-axis are aortic pressure, left atrial pressure, and left ventricular pressure, heart sounds, ventricular volume, right atrial pressure curve, and ECG; while along the x-axis is time.
Cardiac cycle graph1:00–2:34
But, before we continue, here’s something to keep in mind: since there are no valves separating the right atrium from the superior vena cava and the jugular veins, the jugular venous pulse will follow the same pressure changes as the ones that arise in the right atrium.
In other words, an increase in the atrial pressure will result in an increased jugular venous pulse, and vice versa. Therefore, below the right atrial pressure curve let’s write JVP for jugular venous pulse.
And right above the graph, we’ll write the seven phases of the cardiac cycle. The first phase is the atrial contraction, which lasts about 0.1 seconds.
Now, the first phase of the cardiac cycle, atrial contraction, starts with the firing of the sinoatrial, or SA node, which sends an electrical signal that propagates outward through the walls of the heart and depolarizes the atria.
Atrial contraction2:34–3:39
On the ECG, this corresponds to the P wave. Now, atrial depolarization is followed by the contraction of the right and left atrium, and as the atria contract, the pressure within the left atrium increases.
Now, as the atria pumps blood into the ventricle, the ventricular volume increases and therefore the ventricular pressure slightly increases.
At the same time, as the right atrium contracts, an increase in the right atrial pressure will be noted as the a wave on the right atrial pressure curve.
In some conditions, such as ventricular hypertrophy, during this phase, the fourth heart sound (S4) can be heard. This sound is caused by vibration of the stiffened ventricular wall as the blood is pushed from the atria into the ventricles.
The next phase of the cardiac cycle is the isovolumetric contraction. On the ECG, this phase begins with the appearance of the QRS complex, which represents ventricular depolarization.
Isovolumetric contraction3:39–4:49
Ventricular depolarization is followed by ventricular contraction. But, prior to ventricular contraction, when the pressure within the ventricle exceeds the atrial pressure, atrioventricular valves close, producing the first heart sound (S1).
At this point, both the atrioventricular and aortic and pulmonary valves are closed, so the blood volume within the ventricles remain the same - hence the term “isovolumetric”.
But the ventricles contract, so ventricular pressure increases rapidly. However, in the left ventricle, the pressure doesn’t exceed aortic pressure, so the aortic valve is still closed.
At the same time, there’s a slight increase in the left and right atrial pressure because the increasing ventricular pressure makes the atrioventricular valves bulge into the atria.
This increase in the atrial pressure is registered as the C wave, on both left and right atrial pressure curves.By the end of isovolumetric contraction, the pressure within the ventricles becomes higher than the pressure within the aorta and pulmonary arteries, so the aortic and pulmonary valves open.
Rapid ventricular ejection4:49–6:12
This event marks the start of the next phase - rapid ventricular ejection. This phase is called rapid ventricular ejection due to a sudden ejection of a large amount of blood from the ventricles.
Now, as the left ventricle ejects the blood, the pressure from the left ventricle is equally transmitted to the aorta. In other words, both, ventricular and aortic pressures reach their maximum.
At the same time, the volume of blood within the left ventricle decreases sharply. On the ECG, this phase matches the ST segment - the flat section of the ECG between the end of QRS complex and the beginning of the T wave, which represents the period between ventricular depolarization and ventricular repolarization.
Now, as the ventricles eject blood, the ventricular pressure decreases and the atrioventricular valves return to their neutral position.
This makes the pressure within the left and right atrium to decrease, which is noted as the X descent on the left and right atrial pressure curves.
Reduced ventricular ejection6:12–7:02
On the ECG, this phase is marked by the beginning of the T wave, which stands for ventricular repolarization. During this phase, the blood outflow isn’t caused by ventricular contraction; instead, it occurs due to inertial energy of the blood.
Therefore, on the graph, you can see that ventricular pressure starts to decrease. In addition, the volume within the ventricle gradually continues to fall, because the blood is still moving out of the ventricle but at a slower rate.
At the same time, aortic pressure also starts to fall since there’s less blood leaving the ventricle. On the other hand, the pressure within the atria starts to increase, because they continue to collect blood that will be used during the next cardiac cycle.
At this point, ventricular diastole begins. On the ECG, the start of this phase is marked by the end of the T wave.
During this phase, ventricular pressure continues to fall since ventricles are relaxed. Also, since the aortic pressure is now higher than the left ventricular pressure, blood starts to flow backward toward the heart.
Ventricular diastole7:02–8:20
This backflow of blood results in a small dip in aortic pressure graph, which is called the dicrotic notch. Once the left ventricular pressure falls sufficiently, the aortic valve closes in order to prevent the backflow of blood, and this is the second heart sound (S2).It’s important to note that the aortic valve closes before the pulmonic valve.
At this point, atrioventricular valves and aortic valve and pulmonary valve are closed. In other words, there’s no blood entering or leaving the ventricles, thereby the volume within the ventricles remains the same - thus, the name isovolumetric ventricular relaxation.
The “v” wave is still going up, since atria are still filling with blood. From this point on, there are no ECG changes until the next cardiac cycle starts.
The volume of blood within the ventricles remains the same until the ventricular pressure is lower than the atrial pressure.Once this happens, the sixth phase, or rapid ventricular filling, begins.
When the atrial pressure exceeds the ventricular pressure, the two atrioventricular valves open, and the two ventricles start to fill rapidly with blood from the atria.The opening of atrioventricular valves leads to a rapid fall in atrial pressure, which is noted as the “y” descent.
Rapid ventricular filling8:20–8:58
Once opened, rapid filling of the ventricle produces the third heart sound, which can normally be heard in children; while in adults, the presence of the third heart sound probably indicates that there’s volume overload or ventricular dilation.
The last and the longest phase of the cardiac cycle is reduced ventricular filling, also called diastasis. During this phase and the previous phase, the ventricles get about 90% of the blood, while the other 10% they receive during the first phase of the next cardiac cycle, or - atrial contraction.
Reduced ventricular filling8:58–9:32
In other words, we can say that 90% of ventricular filling occurs before atrial contraction, thus it’s called passive ventricular filling.
Finally, during this phase, aortic pressure continues to fall.Alright, as a quick recap. The cardiac cycle refers to the sequence of events that repeat during each heartbeat and the cardiac cycle graph is used to express them.
Review9:32–10:33
Isovolumetric ventricular contraction, rapid ventricular ejection, and reduced ventricular ejection are the phases of ventricular systole, which is the phase of the heartbeat when the heart contracts and pumps the blood out of the ventricles.
On the other hand, isovolumetric ventricular relaxation, rapid ventricular filling, and reduced ventricular filling are the phases of ventricular diastole, which is the phase of the heartbeat when the heart relaxes and ventricles fill with blood.
out of the ventricles on the other hand isovolumetric ventricular, relaxation rapid ventricular filling and reduce ventricular filling are the phases of ventricular diastole which is the phase of a heartbeat when the heart relaxes and ventricles fill with blood.
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