Cardiac Cycle Phases: Systole and Diastole Step-by-Step Diagram
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The Cardiac Cycle
The cardiac cycle has 2 main phases:
Systole
Diastole
Systole and diastole are defined by whether the heart is depolarized (contracting) or repolarized (relaxed).
This lecture will walk you through one cycle of the heart.
We will use a step-by-step diagram that will be sure to have you understanding the cardiac cycle in minutes!
But if that wasn’t enough…..
We will also review the order of the blood flow through the heart step-by-step.
As we walk through the cardiac cycle, we will also review the conduction system of the heart including the SA node, AV node, bundle of His, right and left bundle branches, and Purkinje fibers.
So what does this mean for you?!
In just this one lecture you will learn how the cardiac cycle, blood flow through the heart, and the cardiac conduction system all come together!
Make sure to also check how the cardiac cycle applies to the different parts of an ECG/EKG waveform!
The “EKGs Made Easy” lecture explains how the P wave, PR segment, QRS complex, ST segment, T wave, and TP segment are formed!
Anatomy of the Heart
Before we get started, let’s briefly review the anatomy of the heart.
If you want more information on the anatomy of the heart, make sure to check out the step-by-step guide below!
Heart Anatomy: Labeled Diagram, Structures, Function, and Blood Flow
It is filled with memory tricks to remember the main structures of the heart!
Cardiac Chambers
There are 4 chambers of the heart:
Right Atrium
Right Ventricle
Left Atrium
Left Ventricle
The atria are located in the upper (superior) part of the heart, and the ventricles are located in the lower (inferior) part of the heart.
In other words, the atria are positioned above the ventricles.
Great Vessels
There are 5 great vessels connected to the heart:
Superior Vena Cava
Inferior Vena Cava
Main Pulmonary Artery (Pulmonary Trunk)
Pulmonary Veins
Aorta
The superior and inferior vena cava connect to the right atrium, and they are the major veins that carry deoxygenated venous blood from the rest of the body back to the heart (right atrium).
The main pulmonary artery (pulmonary trunk) is the blood vessel that emerges from the right ventricle and delivers deoxygenated blood from the right ventricle to the lungs.
The pulmonary veins deliver oxygenated blood from the pulmonary circulation back to the heart (left atrium).
The aorta is the main blood vessel that emerges from the left ventricle and delivers oxygenated blood from the left ventricle to the rest of the body.
Heart Valves
There are 4 main valves in the heart:
Tricuspid Valve
Pulmonic Valve
Mitral Valve
Aortic Valve
The tricuspid and mitral valves are located between the atria and ventricles.
Specifically, the tricuspid valve is positioned between the right atrium and right ventricle, and the mitral valve is located between the left atrium and left ventricle.
The pulmonic and aortic valves are located between the ventricles and great vessels.
Specifically, the pulmonic valve is positioned between the right ventricle and pulmonary trunk, and the aortic valve is located between the left ventricle and aorta.
SVC = Superior Vena Cava; IVC = Inferior Vena Cava; RA = Right Atrium; RV = Right Ventricle; LA = Left Atrium; LV = Left Ventricle; PA = Main Pulmonary Artery/Trunk
Tricuspid Valve = Located between the RA and RV; Mitral Valve = Located between the LA and LV; Pulmonic Valve = Located between the RV and PA; Aortic Valve = Located between the LV and Aorta
Blood Flow through the Heart
Now that we understand the anatomy of the heart, let’s review the blood flow through the heart.
If you want more information on the blood flow through the heart, make sure to check out the step-by-step guide below!
Blood Flow Through the Heart: A 12 Step Diagram
You will learn simple memory tricks to remember it all!
Here is the order of the blood flow through the heart:
SVC/IVC
Right Atrium
Tricuspid Valve
Right Ventricle
Pulmonic Valve (Pulmonary Valve)
Main Pulmonary Artery (Pulmonary Trunk)
Lungs
Pulmonary Veins
Left Atrium
Mitral Valve
Left Ventricle
Aortic Valve
Aorta
Deoxygenated venous blood from the rest of the body (systemic circulation) travels to the right atrium via the superior vena cava (SVC) and inferior vena cava (IVC).
Blood will then flow from the right atrium, through the tricuspid valve, and into the right ventricle.
From the right ventricle, blood will travel through the pulmonic valve, and into the pulmonary circulation (lungs) via the pulmonary trunk and arteries.
The deoxygenated blood becomes oxygenated as it passes through the pulmonary circulation and lungs.
The oxygenated blood will then travel from the lungs to the left atrium through the pulmonary veins.
Blood will then flow from the left atrium, through the mitral valve, and into the left ventricle.
Lastly, the oxygenated blood will flow from the left ventricle, through the aortic valve, and to the rest of the body via the aorta and its branches.
Blood Flow Through the Heart: Inferior vena cava and superior vena cava, right atrium, tricuspid valve, right ventricle, pulmonic valve, pulmonary trunk, lungs, pulmonary veins, left atrium, mitral valve, left ventricle, aortic valve, aorta, rest of the body
Diastole vs Systole
Now that we understand the anatomy and blood flow of the heart, let’s discuss the cardiac cycle.
There are 2 main phases to the cardiac cycle:
Diastole
Systole
Diastole is defined as the phase in which the heart, particularly the ventricles, is at rest.
The relaxed heart allows for blood to fill the cardiac chambers.
Systole is defined as the phase in which the heart, particularly the ventricles, is contracting.
The contraction allows for blood to pump into the pulmonary and systemic circulation via the main pulmonary artery and aorta respectively.
Let’s walk through one cardiac cycle beginning with diastole.
Diastole: The phase in which the heart, particularly the ventricles, are at rest allowing the cardiac chambers to fill with blood.
Systole: The phase in which the heart, particularly the ventricles, are contracting and pumping blood forward into the pulmonary artery and aorta.
Step 1: Early Diastole
As defined above, diastole is the cardiac phase in which the heart is at rest, especially the ventricles.
During early diastole, the ventricles are in a low pressure state because they are relaxed and not contracting.
The low pressure state of the heart makes it easier for blood to flow into the heart, and the atria begin to fill with blood as a result.
The deoxygenated blood from the systemic circulation enters the right atrium via the superior vena cava and inferior vena cava.
The oxygenated blood from the pulmonary circulation enters the left atrium via the pulmonary veins.
Image: The ventricles are in a low pressure (P) state during early diastole because they are relaxed and not contracting. The low pressure augments blood flow into the heart.
Image: Deoxygenated blood from the body enters the right atrium via the SVC and IVC. Oxygenated blood from the pulmonary vasculature (lungs) enters the left atrium via the pulmonary veins.
As the right and left atria fill with blood, the atrial pressures begin to increase.
Remember we said the ventricles are in a low pressure state during early diastole.
Therefore, the atrial pressure will eventually surpass the ventricular pressure, and this will cause the tricuspid and mitral valves to open.
Once the right atrial pressure is greater than the right ventricular pressure, the tricuspid valve will open.
Once the left atrial pressure is greater than the left ventricular pressure, the mitral valve will open.
When the tricuspid and mitral valves open, blood will begin to flow from the atria to the ventricles due to differences in pressure, as well as a suction mechanism.
Up to this point in the cardiac cycle, all of the blood flow has been passive from the differences in pressure.
There has been no cardiac conduction or contraction taking place.
Image: As blood fills the right and left atria, the atrial pressure (P) increases relative to the ventricular pressure, and the tricuspid and mitral valves open allowing for blood to flow from the atria to the ventricles.
Step 2: Late Diastole
As the right and left ventricle fill with blood, the ventricular pressure begins to increase.
As the ventricular pressure approaches the atrial pressure, it becomes more difficult for blood to flow passively from the atria to the ventricles (because there is less of a pressure difference).
Therefore, the atria must increase their pressure in order to facilitate further blood flow from the atria to the ventricles.
The atria increase their pressure by contracting.
This brings us to the conduction system of the heart.
For an easy step-by-step guide of the conduction system of the heart, make sure to check out the EZmed conduction system lecture!
In a normal functioning heart, the conduction system begins at the SA node.
The SA node is located in the back of the right atrium near the entry point of the superior vena cava.
The SA node is the primary pacemaker of the heart.
The SA node is made up of pacemaker/nodal cells that have the ability to generate their own spontaneous action potentials.
For a simple review of pacemaker cells and their action potentials, make sure to check out the EZmed lecture: Cardiac Action Potentials Made Easy!
Once the SA node generates a spontaneous action potential, the action potential will then travel though the right atrium via the internodal pathways and to the left atrium via Bachmann’s bundle.
As the action potential travels through the atria, the atria depolarize and contract.
Atrial contraction will increase atrial pressure, which will allow for more blood to flow from the atria to the ventricles.
The process of atrial depolarization and contraction occurs during late diastole.
Image: As the ventricles fill with blood, the ventricular pressure (P) increases. Therefore, the atria must increase their pressure to allow for blood to continue to flow from the atria to the ventricles. The atria increase their pressure by contracting.
Image: The SA node is the primary pacemaker of the cardiac conduction system and sends an action potential through the right and left atria, leading to atrial depolarization and contraction.
AV Node
Once the action potential generated by the SA node travels through the atria, it will then converge onto the AV node.
The AV node is made up of pacemaker cells that have the ability to generate their own action potentials (just like the SA node).
However, the rate in which the AV node generates spontaneous action potentials is slower than that of the SA node, which is why the SA node is the primary pacemaker in a normal functioning heart.
In other words, the action potential from the SA node arrives at the AV node before the AV node is able to generate its own action potential.
The AV node is located at the base of the right atrium near the interventricular septum.
Since the AV node is located between the atria and ventricles, the AV node act as the gatekeeper that delivers the action potential from the atria to the ventricles.
An important function of the AV node is to slow down the conduction velocity of the action potential.
The reason for this is to allow time for the atria to contract before sending the electrical impulse to the ventricles, which will depolarize and contract the ventricles.
Otherwise the atria and ventricles would be contracting at the same time and it would be difficult for blood to flow through the heart.
Image: The action potential generated by the SA node will travel to the AV node, where the conduction velocity is slowed to allow time for the atria to contract before depolarizing and contracting the ventricles.
Step 3: Early Systole
The action potential will then travel from the AV node, through the bundle of His, through the right and left bundle branches, and finally through the Purkinje fibers.
This part of the conduction system is primarily located in the interventricular septum and ventricular myocardium.
Therefore, as the action potential travels through the bundle of His, right and left bundle branches, and Purkinje fibers, it will depolarize the ventricular contractile myocytes (muscle cells).
The right bundle branch mainly depolarizes the right ventricle, and the left bundle branch mainly depolarizes the left ventricle.
Ventricular depolarization will result in ventricular contraction.
The heart is now in systole, in which the ventricles are contracting to pump blood forward into the pulmonary trunk and aorta.
Ventricular contraction will increase ventricular pressure, which will eventually surpass the atrial pressure.
Moreover, the atria repolarize (relax) as the ventricles depolarize, thereby making the pressure difference between the atria and ventricles even greater.
When the right ventricular pressure surpasses the right atrial pressure, the tricuspid valve will close.
When the left ventricular pressure surpasses the left atrial pressure, the mitral valve will close.
Once the right ventricular pressure is greater than the pulmonary trunk pressure, then the pulmonic valve will open.
Once the left ventricular pressure is greater than the aorta pressure, then the aortic valve will open.
As the pulmonic and aortic valves open, blood will flow out of the right and left ventricles respectively.
Image: The action potential travels from the AV node, through the bundle of His, followed by the right and left bundle branches, followed by the Purkinje fibers.
The ventricles depolarize and contract, leading to increased ventricular pressure.
The tricuspid and mitral valves close.
The pulmonic and aortic valves open, leading to blood flow from the right and left ventricles to the pulmonary trunk and aorta respectively.
Step 4: Late Systole to Early Diastole
As blood exits the ventricles, the ventricular pressure will start to decrease again.
Moreover, the ventricles will repolarize and relax which will further decrease the ventricular pressure.
Once the right ventricular pressure is less than the pulmonary trunk pressure, the pulmonic valve will close.
Once the left ventricular pressure is less than the aortic pressure, the aortic valve will close.
Blood will no longer exit the right and left ventricles, and the heart will be in a relaxed state again.
This brings us back to early diastole in which the heart is relaxed and filling with blood (See step 1 again).
Image: The ventricles repolarize and relax again, thereby decreasing ventricular pressure.
The pulmonic and aortic valves close, and the heart is back in early diastole again.
Summary
Hopefully this lecture provided you with a clear step-by-step explanation of the cardiac cycle, blood flow through the heart, the cardiac conduction system, and overall heart physiology.
Leave a comment down below if you found the content useful!
Remember the order in which blood flows through the heart is the following:
Inferior vena cava and superior vena cava, right atrium, tricuspid valve, right ventricle, pulmonic valve, pulmonary trunk/arteries, lungs, pulmonary veins, left atrium, mitral valve, left ventricle, aortic valve, and aorta.
Blood flows through the heart from differences in pressure created by the volume of blood in the heart chambers, depolarization/contraction, and repolarization/relaxation.
The cardiac conduction system is responsible for depolarizing the heart and involves the SA node, Bachmann’s bundle, internodal pathways, AV node, bundle of His, right and left bundle branches, and Purkinje fibers.
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