Cardiac Cycle: Phases, Heart Sounds and Explanation
Introduction
The heart continuously contracts and relaxes to pump blood throughout the body. These repeated events form a process known as the cardiac cycle.
A cardiac cycle includes all the events that occur during one complete heartbeat, from the beginning of one heartbeat to the beginning of the next.
It involves the contraction of the atria and ventricles, relaxation of the heart chambers and opening and closing of the heart valves.
For example, when the ventricles contract, blood is pumped towards the lungs and the rest of the body. When they relax, they fill with blood again.
[IMG 1 — Horizontal anatomical infographic showing the cardiac cycle in sequence: atrial systole, ventricular systole and complete cardiac diastole, with arrows indicating blood movement through the heart.]
1. What Is the Cardiac Cycle?
The cardiac cycle is the sequence of contraction (systole) and relaxation (diastole) of the atria and ventricles during one complete heartbeat.
It ensures that blood flows in the correct direction through the heart.
The cardiac cycle mainly consists of three broad events:
Atrial systole – contraction of the atria.
Ventricular systole – contraction of the ventricles.
Complete cardiac diastole – relaxation of both atria and ventricles.
2. Duration of the Cardiac Cycle
At a normal resting heart rate of approximately 75 beats per minute, one cardiac cycle takes about 0.8 seconds.
The approximate duration of its major events is:
Phase | Approximate duration |
|---|---|
Atrial systole | 0.1 seconds |
Ventricular systole | 0.3 seconds |
Complete cardiac diastole | 0.4 seconds |
Total cardiac cycle | 0.8 seconds |
Important: These durations are approximate values for a resting adult. They change with heart rate. When heart rate increases, the duration of the cardiac cycle decreases, particularly the relaxation period.
3. Phases of the Cardiac Cycle
A. Atrial Systole
Atrial systole is the contraction of both atria.
During this phase:
Blood flows from the atria into the ventricles.
The tricuspid and mitral valves remain open.
The pulmonary and aortic valves remain closed.
The ventricles receive the final amount of blood needed for filling.
Atrial contraction completes ventricular filling before ventricular systole begins.
[IMG 2 — Horizontal anatomical diagram showing both atria contracting, open tricuspid and mitral valves, closed semilunar valves and arrows showing blood entering the ventricles.]
B. Ventricular Systole
Ventricular systole is the contraction of the right and left ventricles. It is divided into two main stages.
1. Isovolumetric Contraction
At the beginning of ventricular contraction:
The ventricles start contracting.
The tricuspid and mitral valves close.
The pulmonary and aortic valves are still closed.
Ventricular pressure increases, but blood is not yet ejected.
The term isovolumetric means that the volume of blood inside the ventricles remains the same because all four valves are closed.
Closure of the atrioventricular valves produces the first heart sound (S1).
2. Ventricular Ejection
When ventricular pressure becomes greater than the pressure in the pulmonary artery and aorta:
The pulmonary valve opens.
The aortic valve opens.
The right ventricle pumps blood towards the lungs.
The left ventricle pumps blood towards the body.
Blood is now ejected from both ventricles into their respective arteries.
[IMG 3 — Horizontal anatomical diagram showing ventricular contraction and blood ejection: right ventricle to pulmonary artery and left ventricle to aorta, with atrioventricular valves closed and semilunar valves open.]
C. Complete Cardiac Diastole
Cardiac diastole is the relaxation phase of the heart. During complete cardiac diastole, both atria and ventricles are relaxed.
It includes the following events:
1. Isovolumetric Relaxation
The ventricles begin to relax.
The pulmonary and aortic valves close.
All four heart valves remain closed briefly.
Ventricular pressure decreases without a change in ventricular blood volume.
Closure of the aortic and pulmonary valves produces the second heart sound (S2).
2. Ventricular Filling
When ventricular pressure becomes lower than atrial pressure:
The tricuspid and mitral valves open.
Blood flows from the atria into the ventricles.
The ventricles begin filling again.
The cycle is ready to repeat.
Most ventricular filling occurs passively before the next atrial contraction.
[IMG 4 — Horizontal anatomical diagram showing relaxed ventricles, open mitral and tricuspid valves, closed aortic and pulmonary valves, and blood flowing from atria into ventricles.]
4. Blood Flow During the Cardiac Cycle
The cardiac cycle maintains the continuous movement of blood through the heart and lungs.
Complete pathway of blood flow:
<escape>Body tissues</escape>
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Deoxygenated blood enters the superior and inferior vena cava.
↓
Right atrium
↓
Tricuspid valve
↓
Right ventricle
↓
Pulmonary valve
↓
Pulmonary artery
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Lungs — blood releases carbon dioxide and receives oxygen.
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Pulmonary veins
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Left atrium
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Mitral valve
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Left ventricle
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Aortic valve
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Aorta
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Body tissues — oxygenated blood is delivered.
5. Heart Sounds During the Cardiac Cycle
Heart sounds are produced mainly by the closure and associated vibrations of the heart valves and surrounding structures.
Two main heart sounds are normally heard during the cardiac cycle.
A. First Heart Sound (S1)
The first heart sound is commonly described as "Lub."
It occurs when the mitral and tricuspid valves close.
It marks the beginning of ventricular systole.
It is associated with the beginning of ventricular contraction.
B. Second Heart Sound (S2)
The second heart sound is commonly described as "Dub."
It occurs when the aortic and pulmonary valves close.
It marks the beginning of ventricular diastole.
It is associated with the relaxation of the ventricles.
Easy way to remember:
S1 (Lub) – closure of mitral and tricuspid valves.
S2 (Dub) – closure of aortic and pulmonary valves.
[IMG 5 — Horizontal comparison infographic showing S1 (Lub) caused by closure of mitral and tricuspid valves and S2 (Dub) caused by closure of aortic and pulmonary valves.]
6. Relationship Between the Cardiac Cycle and ECG
An electrocardiogram (ECG) records the electrical activity of the heart. This electrical activity controls the timing of atrial and ventricular contractions.
The main ECG components are:
P wave: Represents atrial depolarization, which is followed by atrial contraction.
QRS complex: Represents ventricular depolarization, which is followed by ventricular contraction.
T wave: Represents ventricular repolarization, which is associated with ventricular relaxation.
The ECG records electrical events, whereas the cardiac cycle describes the mechanical events of contraction, relaxation and blood flow.
[IMG 6 — Horizontal educational ECG diagram showing P wave, QRS complex and T wave aligned with atrial systole, ventricular systole and ventricular relaxation.]
7. Importance of the Cardiac Cycle
The cardiac cycle is essential for maintaining normal blood circulation.
Its major functions include:
Maintaining continuous blood flow through the heart.
Pumping deoxygenated blood towards the lungs for oxygenation.
Delivering oxygenated blood to body tissues.
Maintaining adequate blood supply to vital organs.
Ensuring one-way movement of blood through the proper opening and closing of heart valves.
Abnormalities in the cardiac cycle, such as problems with valve function or heart rhythm, can affect the pumping efficiency of the heart.
Key Concepts
The cardiac cycle includes all events during one complete heartbeat.
Systole refers to contraction, while diastole refers to relaxation.
Atrial systole completes ventricular filling.
Ventricular systole pumps blood into the pulmonary artery and aorta.
Complete cardiac diastole allows the heart chambers to relax and fill.
S1 (Lub) occurs with closure of the mitral and tricuspid valves.
S2 (Dub) occurs with closure of the aortic and pulmonary valves.
The ECG shows the electrical events associated with the mechanical activity of the heart.