Cardiac Cycle : Atrial Systole, Ventricular Systole & Cardiac Output

Table of Contents

The cardiac cycle is a sequence of events that takes place during one heartbeat, involving the alternating contractions and relaxations of the heart’s chambers. It consists of two main phases: systole, the contraction phase, and diastole, the relaxation phase. During each cardiac cycle, blood is pumped through the heart, ensuring that it circulates efficiently throughout the body.

Atrial Systole

The cardiac cycle commences with atrial systole, which lasts approximately 0.1 seconds.
During this phase, the atria contract, and the ventricles relax.
The depolarisation of the sinoatrial (SA) node leads to atrial depolarisation, marked by the P wave in the electrocardiogram (ECG).
This atrial depolarisation triggers atrial systole, during which the contracting atria exert pressure on the blood within them, forcing blood through the open atrioventricular (AV) valves into the ventricles.
Atrial systole contributes around 25 mL of blood to the existing blood volume in each ventricle (about 105 mL).
The end of atrial systole also marks the end of ventricular diastole, leaving each ventricle with approximately 130 mL of blood at the end of its relaxation period (diastole). This blood volume is known as the end-diastolic volume (EDV).

Ventricular Systole

Following atrial systole, the ventricles enter the contracting phase known as ventricular systole, which lasts around 0.3 seconds.
As ventricular depolarisation commences (marked by the QRS complex in the ECG), ventricular systole begins.
The rising pressure inside the ventricles pushes blood against the atrioventricular (AV) valves, forcing them to shut.
This results in a brief period of isovolumetric contraction when both the semilunar (SL) and AV valves are closed, and ventricular volume remains constant (isovolumic).
During continued ventricular contraction, pressure inside the chambers rises sharply, surpassing the pressure in the aorta (about 80 mmHg) and the pressure in the pulmonary trunk (about 20 mmHg).
This causes both SL valves to open, initiating the ejection of blood from the heart.
The period when the SL valves are open is called ventricular ejection, which lasts approximately 0.25 seconds.
The pressure in the left ventricle reaches about 120 mmHg, while the pressure in the right ventricle rises to about 25–30 mmHg.
The left ventricle ejects about 70 mL of blood into the aorta, and the right ventricle ejects the same volume of blood into the pulmonary trunk.
The remaining blood volume in each ventricle at the end of systole, approximately 60 mL, is called the end-systolic volume (ESV).

Relaxation Period

As ventricular systole ends, the heart enters the relaxation period, which lasts for about 0.4 seconds.
Both the atria and ventricles are relaxed during this phase. Ventricular repolarisation causes ventricular diastole.
As the ventricles relax, the pressure within the chambers drops, and blood in the aorta and pulmonary trunk starts to flow backward toward the regions of lower pressure in the ventricles.
This backflowing blood closes the semilunar valves, and the aortic valve closes at a pressure of about 100 mmHg. This closure produces the dicrotic wave on the aortic pressure curve.
During this interval of isovolumetric relaxation, ventricular blood volume remains constant as all four valves are closed.
As ventricular pressure falls below atrial pressure, the AV valves open, and ventricular filling begins.
The major part of ventricular filling occurs just after the AV valves open.
The blood that accumulates in the atria during ventricular systole rapidly rushes into the ventricles.
By the end of the relaxation period, the ventricles are about three-quarters full, and the P wave in the ECG marks the start of another cardiac cycle.

Heart Sounds

As the heart undergoes these complex changes during the cardiac cycle, it produces characteristic heart sounds that can be heard through a stethoscope. Each cardiac cycle gives rise to four heart sounds, but the first two sounds (S1 and S2) are the most prominent.

The first sound, often described as a “lubb” sound, is louder and longer than the second sound. It is caused by blood turbulence associated with the closure of the atrioventricular (AV) valves soon after ventricular systole begins.
The second sound, known as a “dupp” sound, is shorter and not as loud as the first and is due to blood turbulence associated with the closure of the semilunar (SL) valves at the beginning of ventricular diastole.

Cardiac Output

Cardiac output (CO) refers to the volume of blood ejected from the left (or right) ventricle into the aorta (or pulmonary trunk) each minute. It is determined by two key factors: stroke volume (SV), the volume of blood ejected by the ventricle during each contraction, and heart rate (HR), the number of heartbeats per minute. Mathematically, cardiac output can be calculated as follows:

CO = SV × HR

In a typical resting adult male, stroke volume averages around 70 mL per beat, and heart rate is approximately 75 beats per minute. Therefore, the average cardiac output is 70 mL/beat × 75 beats/min = 5250 mL/min = 5.25 L/min. This means that the entire blood volume of a typical adult male (about 5 liters) flows through the pulmonary and systemic circulations each minute.

Cardiac reserve represents the difference between a person’s maximum cardiac output and their cardiac output at rest. An average person usually has a cardiac reserve of four to five times their resting cardiac output. Endurance athletes may have a higher cardiac reserve, while individuals with severe heart disease may have limited cardiac reserve, restricting their ability to perform daily activities.

Regulation of Stroke Volume

The regulation of stroke volume is influenced by three factors: preload (stretch on the heart before contraction), contractility (forcefulness of contraction), and afterload (pressure the heart must overcome to eject blood). Increased preload and contractility lead to a larger stroke volume, while increased afterload decreases it. Balancing these factors is vital for efficient heart function and blood circulation.

Regulation of Heart Rate

Heart rate is regulated by the autonomic nervous system and hormones released by the adrenal medullae. The cardiovascular center in the medulla oblongata adjusts heart rate through the sympathetic and parasympathetic branches. Sympathetic stimulation increases heart rate during exercise and stress, while parasympathetic stimulation slows it during rest. Hormones like epinephrine and norepinephrine from the adrenal medullae enhance heart rate and contractility. Additionally, thyroid hormones, cations (K⁺, Na⁺, and Ca²⁺), age, gender, physical fitness, body temperature, and certain medical conditions can impact resting heart rate.

Summary

The Cardiac cycle involves alternating contractions and relaxations of the heart’s chambers, resulting in blood circulation. Heart sounds (S1 and S2) are produced during the cycle. Cardiac output is the amount of blood ejected from a ventricle per minute and is determined by stroke volume and heart rate. Stroke volume is regulated by preload, contractility, and afterload. Heart rate is controlled by the autonomic nervous system and hormones. Various factors can influence resting heart rate.

FAQs on Cardiac Cycle

What is the cardiac cycle?

The cardiac cycle is a sequence of events that occurs during one heartbeat, involving the alternating contractions and relaxations of the heart's chambers (atria and ventricles). It consists of two main phases: systole (contraction) and diastole (relaxation). The cycle ensures efficient blood circulation throughout the body.

What happens during atrial systole?

The atrial systole is the phase of the cardiac cycle where the atria contract while the ventricles relax. It lasts approximately 0.1 seconds. During this phase, the depolarisation of the sinoatrial (SA) node leads to atrial depolarization (marked by the P wave in an ECG). The contracting atria exert pressure on the blood within them, forcing blood through the open atrioventricular (AV) valves into the ventricles.

What is ventricular systole?

Ventricular systole is the phase of the cardiac cycle when the ventricles contract. It follows atrial systole and lasts around 0.3 seconds. As ventricular depolarisation starts (marked by the QRS complex in an ECG), ventricular systole begins. The rising pressure inside the ventricles forces the atrioventricular (AV) valves to shut, and the semilunar (SL) valves open to eject blood from the heart.

What are heart sounds?

Heart sounds are the characteristic sounds produced by the heart during the cardiac cycle. Each cycle gives rise to four heart sounds, but the first two (S1 and S2) are the most prominent. S1 is described as a lubb sound and occurs during ventricular systole, associated with the closure of the AV valves. S2 is a dupp sound, occurring during ventricular diastole, associated with the closure of the SL valves.

How is cardiac output calculated?

Cardiac output (CO) is the volume of blood ejected from the left (or right) ventricle into the aorta (or pulmonary trunk) each minute. It is calculated by multiplying stroke volume (SV), the volume of blood ejected by the ventricle during each contraction, with heart rate (HR), the number of heartbeats per minute: CO = SV × HR.

What regulates stroke volume?

Stroke volume is regulated by three factors: preload (stretch on the heart before contraction), contractility (forcefulness of contraction), and afterload (pressure the heart must overcome to eject blood). Increased preload and contractility lead to a larger stroke volume, while increased afterload decreases it.

How is heart rate regulated?

Heart rate is regulated by the autonomic nervous system and hormones. The cardiovascular center in the medulla oblongata adjusts heart rate through the sympathetic and parasympathetic branches. Sympathetic stimulation increases heart rate during exercise and stress, while parasympathetic stimulation slows it during rest. Hormones like epinephrine and norepinephrine enhance heart rate and contractility.

What factors can influence resting heart rate?

Resting heart rate can be influenced by various factors, including age, gender, physical fitness, body temperature, thyroid hormones, cations (K+, Na+, and Ca2+), and certain medical conditions. These factors can either increase or decrease resting heart rate.