BIOL 2402 Ch 18 Lecture Notes – Cardiac cycle
Chapter 18: Cardiac Cycle- Lecture Notes
A cardiac cycle consists of all the events that take place during a heartbeat. A single heart beat is made of a contraction
phase called systole and a relaxation phase called diastole. Thus, a cardiac cycle consists of atrial and ventricular systole
and diastole. During each cardiac cycle, the atria and ventricles contract and relax in succession forcing blood from areas of
high pressure to areas of low pressure. As pressure changes occur valves open and close leading to ejection of blood. The
cardiac cycle (see Figure below) shows the relationship between ECG, heart sounds, volume of blood in the ventricle and
pressures within the atria, ventricles and aorta. Although the figure shows the relationship for the left side of the heart, the
same events occur on the right side but the pressures are significantly lower. As we will study later, the volume of blood
ejected from the left and right ventricle however remains the same. With a heartbeat of 75 beats per minute, each cardiac
cycle lasts for 0.8 seconds, which means that one systole and diastole of atria and ventricles occurs in just 0.8 seconds!
The cardiac cycle is divided into three phases –
1. Atrial systole Atpiaescontractpump intoventricals
blood whicharerelaxedDepolarization wave
2. Ventricular systole ventricalscontractto
off
pump bloodoutatriumsrelaxed
3. Relaxation period distoreAtriumVentriclesareRelaxedwhile theotheriscontractingwallowflow
of citotheatrium1ventricle
blood
Study each of these phases carefully using the figure below. The labels on the figure match with numbers in the text below.
Atrial systole
During atrial systole phase, which lasts for 0.1 seconds, atria are contracting while the ventricles are relaxing (diastole).
1. Depolarization of the SA node causes atrial depolarization, marked by the P wave in the ECG.
2. Atrial depolarization causes atrial systole. As the atria contract, atrial pressure rises forcing blood through the open
AV valves into the ventricles.
3. The atria eject about 302s ml blood into the ventricles. The ventricles already contain about 105 ml blood (see last
step) thus filling the ventricles with a total volume of 130 ml. This volume of blood in the ventricles at the end of
ventricular diastole is called end-diastolic volume (EDV). 130mL
4. The QRS complex in the ECG marks the onset of ventricular depolarization. followed ventricularrepolarization
by
During this phase AV valves are open allowing for blood to fill the ventricles. At the same time semilunar valves are closed.
Ventricular systole
During ventricular systole phase, which lasts for 0.3 seconds, ventricles are contracting while atria are relaxing (diastole).
bc ofQRScomplex pressureruse bicuspiddoseSIWb
5. Ventricular depolarization leads to ventricular systole. As ventricular pressure rises, pushing blood against the AV
valve (bicuspid on the left side) thus closing the valve (produces the first heart sound S1 or lub sound). Thus, for
this brief of time (0.05 seconds), the ventricles are contracting with all valves shut. This period is called
Isovolumetric contraction (Iso = same, Volumetric = volume). Thus during this phase the ventricles are contracting
but as all valves are shut there is no change in the volume of the ventricles.
6. As the pressure within the ventricles rises, it exceeds the aortic pressure of 80 mm Hg (Pulmonary artery pressure
= 20 mm Hg) leading to an opening of the semilunar valves and ejection of blood into the aorta. This phase of
ventricular ejection lasts for about 0.25 seconds. The pressure in the left ventricles rises further to 120 mm of Hg
1
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Chapter 18: Cardiac Cycle- Lecture Notes
A cardiac cycle consists of all the events that take place during a heartbeat. A single heart beat is made of a contraction
phase called systole and a relaxation phase called diastole. Thus, a cardiac cycle consists of atrial and ventricular systole
and diastole. During each cardiac cycle, the atria and ventricles contract and relax in succession forcing blood from areas of
high pressure to areas of low pressure. As pressure changes occur valves open and close leading to ejection of blood. The
cardiac cycle (see Figure below) shows the relationship between ECG, heart sounds, volume of blood in the ventricle and
pressures within the atria, ventricles and aorta. Although the figure shows the relationship for the left side of the heart, the
same events occur on the right side but the pressures are significantly lower. As we will study later, the volume of blood
ejected from the left and right ventricle however remains the same. With a heartbeat of 75 beats per minute, each cardiac
cycle lasts for 0.8 seconds, which means that one systole and diastole of atria and ventricles occurs in just 0.8 seconds!
The cardiac cycle is divided into three phases –
1. Atrial systole Atpiaescontractpump intoventricals
blood whicharerelaxedDepolarization wave
2. Ventricular systole ventricalscontractto
off
pump bloodoutatriumsrelaxed
3. Relaxation period distoreAtriumVentriclesareRelaxedwhile theotheriscontractingwallowflow
of citotheatrium1ventricle
blood
Study each of these phases carefully using the figure below. The labels on the figure match with numbers in the text below.
Atrial systole
During atrial systole phase, which lasts for 0.1 seconds, atria are contracting while the ventricles are relaxing (diastole).
1. Depolarization of the SA node causes atrial depolarization, marked by the P wave in the ECG.
2. Atrial depolarization causes atrial systole. As the atria contract, atrial pressure rises forcing blood through the open
AV valves into the ventricles.
3. The atria eject about 302s ml blood into the ventricles. The ventricles already contain about 105 ml blood (see last
step) thus filling the ventricles with a total volume of 130 ml. This volume of blood in the ventricles at the end of
ventricular diastole is called end-diastolic volume (EDV). 130mL
4. The QRS complex in the ECG marks the onset of ventricular depolarization. followed ventricularrepolarization
by
During this phase AV valves are open allowing for blood to fill the ventricles. At the same time semilunar valves are closed.
Ventricular systole
During ventricular systole phase, which lasts for 0.3 seconds, ventricles are contracting while atria are relaxing (diastole).
bc ofQRScomplex pressureruse bicuspiddoseSIWb
5. Ventricular depolarization leads to ventricular systole. As ventricular pressure rises, pushing blood against the AV
valve (bicuspid on the left side) thus closing the valve (produces the first heart sound S1 or lub sound). Thus, for
this brief of time (0.05 seconds), the ventricles are contracting with all valves shut. This period is called
Isovolumetric contraction (Iso = same, Volumetric = volume). Thus during this phase the ventricles are contracting
but as all valves are shut there is no change in the volume of the ventricles.
6. As the pressure within the ventricles rises, it exceeds the aortic pressure of 80 mm Hg (Pulmonary artery pressure
= 20 mm Hg) leading to an opening of the semilunar valves and ejection of blood into the aorta. This phase of
ventricular ejection lasts for about 0.25 seconds. The pressure in the left ventricles rises further to 120 mm of Hg
1
This study source was downloaded by 100000900412927 from CourseHero.com on 01-07-2026 14:13:27 GMT -06:00
https://www.coursehero.com/file/50980291/Exam-2-Chapter-18-Cardiac-Cycle-lecture-notespdf/
