Cardiac cycle
Cardiac Cycle
Events, duration, pressure changes and ventricular volume changes during the cardiac cycle.
1 Definition
2 Events of Cardiac Cycle
Each heartbeat consists of two major periods, systole and diastole.
During systole, heart contracts and pumps the blood through arteries.
During diastole, heart relaxes and the heart is filled with blood.
All the changes are repeated during every heartbeat, in a cyclic manner.
Classification of Events
Events of cardiac cycle are classified into two divisions:
- Atrial events which constitute atrial systole and atrial diastole.
- Ventricular events which constitute ventricular systole and ventricular diastole.
3 Divisions and Duration of Events of Cardiac Cycle
When heart beats at the normal rate of 72/minute, the duration of each cardiac cycle is about 0.8 second.
Atrial Events
Atrial events are divided into two divisions:
| Division | Duration |
|---|---|
| Atrial systole | 0.11 second |
| Atrial diastole | 0.69 second (about 0.7 second) |
Ventricular Events
Ventricular events are divided into two divisions:
| Division | Duration |
|---|---|
| Ventricular systole | 0.27 second (about 0.3 second) |
| Ventricular diastole | 0.53 second (about 0.5 second) |
Subdivisions of Ventricular Systole
| Subdivision | Duration |
|---|---|
| Isometric contraction period | 0.05 second |
| Ejection period | 0.22 second |
Subdivisions of Ventricular Diastole
| Subdivision | Duration |
|---|---|
| Protodiastole | 0.04 second |
| Isometric relaxation period | 0.08 second |
| Rapid filling phase | 0.11 second |
| Slow filling phase | 0.19 second |
| Last rapid filling phase (atrial systole) | 0.11 second |
Atrial diastole is not a separate phase. It coincides with the whole of ventricular systole and earlier part of ventricular diastole.
4 Description of Atrial Events
Atrial Systole
Atrial systole is also known as second or last rapid filling phase or presystole.
It is considered as the last phase of ventricular diastole.
Its duration is 0.11 second.
During atrial systole, only a small amount, i.e. 10% of blood is forced from atria into ventricles.
Atrial systole is not essential for the maintenance of circulation. Many persons with atrial fibrillation survive for years, without suffering from circulatory insufficiency.
However, such persons feel difficult to cope up with physical stress like exercise.
Pressure and Volume Changes
During atrial systole, intra-atrial pressure increases.
Intraventricular pressure and ventricular volume also increase, but slightly.
Fourth Heart Sound
Contraction of atrial musculature causes the production of fourth heart sound during atrial systole.
Atrial Diastole
After atrial systole, the atrial diastole starts.
Atrial diastole lasts for about 0.7 second (accurate duration is 0.69 second).
This long atrial diastole is necessary because this is the period during which atrial filling takes place.
Right atrium receives deoxygenated blood from all over the body through superior and inferior vena cavae.
Left atrium receives oxygenated blood from lungs through pulmonary veins.
Atrial Events vs Ventricular Events
During 0.7 second period of atrial diastole, first 0.3 second (0.27 second accurately) coincides with ventricular systole.
So, the heart relaxes as a whole for about 0.4 second.
5 Description of Ventricular Events
Isometric Contraction Period
Isometric contraction is a type of muscular contraction characterized by increase in length of muscle fibers without any change in the length of muscle fibers.
Isometric contraction of ventricular muscle is also called isovolumetric contraction.
Isometric contraction period in cardiac cycle is the first phase of ventricular systole.
Immediately after atrial systole, atrioventricular valves are closed due to increase in ventricular pressure.
Semilunar valves are already closed.
Now, the ventricles contract as closed cavities in such a way that there is no change in the volume of ventricular chambers or in length of muscle fibers.
Only the tension increases in ventricular musculature.
Because of increased tension in ventricular musculature during isometric contraction, the pressure increases sharply inside ventricles.
First Heart Sound
Closure of atrioventricular valves at the beginning of isometric contraction produces first heart sound.
Significance of Isometric Contraction
During isometric contraction period, ventricular pressure increases greatly.
This pressure increases above the pressure in aorta and pulmonary artery.
Semilunar valves open.
The rise in pressure in ventricle caused by isometric contraction is responsible for opening of semilunar valves.
Ejection Period
Due to opening of semilunar valves and isotonic contraction of ventricles, the blood is ejected out of both ventricles.
Hence, this period is called ejection period.
Ejection period is of two stages:
- Rapid ejection period: Immediately after opening of semilunar valves, a large amount of blood is rapidly ejected from both ventricles. Duration is 0.13 second.
- Slow ejection period: During this stage, blood is ejected slowly with much less force. Duration is 0.09 second.
End-Systolic Volume
Ventricles are not emptied at the end of ejection period and some amount of blood remains in each ventricle.
Amount of blood remaining in ventricles at the end of ejection period, i.e. at the end of systole, is called end-systolic volume.
Ejection Fraction
Ejection fraction means the fraction (or portion) of end-diastolic volume which is ejected out by each ventricle per beat.
From 130 to 150 mL of end-diastolic volume, 70 mL is ejected out from each ventricle (stroke volume).
Protodiastole
This is the first stage of ventricular diastole hence called protodiastole.
During protodiastole, the pressure in ventricles drops due to ejection of blood.
At the end of this period, intraventricular pressure becomes less than the pressure in aorta and pulmonary artery.
This causes closure of semilunar valves.
Atrioventricular valves are already closed.
No other change occurs in the heart during this period.
Thus, protodiastole indicates only end of systole and beginning of diastole.
Second Heart Sound
Closure of semilunar valves during this phase produces second heart sound.
Isometric Relaxation Period
Isometric relaxation is a type of muscular relaxation characterized by decrease in tension without any change in the length of muscle fibers.
Isometric relaxation of ventricular muscle is also called isovolumetric relaxation.
During isometric relaxation period, once again all the valves of heart are closed.
Both the ventricles relax as closed cavities, without any change in volume or length of the muscle fiber.
But intraventricular pressure decreases during this period.
Significance of Isometric Relaxation
During isometric relaxation period, the ventricular pressure decreases greatly.
When the ventricular pressure becomes less than the pressure in atria, the atrioventricular valves open.
Thus, the fall in pressure in ventricles caused by isometric relaxation is responsible for the opening of atrioventricular valves, resulting in filling of ventricles.
Rapid Filling Period
When AV valves are opened, there is a sudden rush of blood from atria into ventricles.
So, this period is called the first rapid filling period.
Filling during this period occurs without atrial systole.
About 70% of filling takes place during this period.
Third Heart Sound
Rushing of blood into ventricles during this phase produces third heart sound.
Slow Filling Period
After a sudden rush of blood, the ventricular filling becomes slow.
Now, it is called the slow filling.
It is also called diastasis.
Filling during this phase also occurs without atrial systole.
About 20% of filling occurs in this phase.
Last Rapid Filling Period
Filling becomes once again rapid because of atrial systole.
After slow filling period, the atria contract and push a small amount of blood into the ventricles.
About 10% of ventricular blood takes place during this period.
Flow of additional amount of blood into ventricle due to atrial systole is called atrial kick.
End-Diastolic Volume
End-diastolic volume is the amount of blood remaining in each ventricle at the end of diastole.
6 Pressure Changes During Cardiac Cycle
Pressure inside atria and ventricles of heart varies during different phases of each cardiac cycle.
Pressure changes in cardiac chambers have some significance.
Intra-Atrial Pressure Changes During Cardiac Cycle
Significance of Intra-Atrial Pressure
Pressure in atria is called the intra-atrial pressure.
Intra-atrial pressure is responsible for opening of atrioventricular valves and ventricular filling.
It is also a major factor for development of venous pulse.
Pressure Changes During Cardiac Cycle
During atrial diastole, the pressure in atrial falls down and reaches 0 mm Hg because of atrial filling.
During atrial systole, pressure rises sharply up to 5 mm Hg in right atrium and 7 mm Hg in left atrium.
Maximum and minimum pressures in the left and right atria are given below.
| Area | Maximum Pressure | Minimum Pressure |
|---|---|---|
| Left atrium | 7 to 8 mm Hg | 0 to 2 mm Hg |
| Right atrium | 5 to 6 mm Hg | 0 to 2 mm Hg |
| Left ventricle | 120 mm Hg | 5 mm Hg |
| Right ventricle | 25 mm Hg | 2 to 3 mm Hg |
| Systemic aorta | 120 mm Hg | 80 mm Hg |
| Pulmonary artery | 25 mm Hg | 7 to 8 mm Hg |
Intra-Ventricular Pressure Changes During Cardiac Cycle
Significance of Intraventricular Pressure
Intraventricular pressure is the pressure developed inside ventricles of heart.
It is essential for circulation of blood, because the flow of blood through systemic and pulmonary circulation depends upon pressure at which blood is pumped out of ventricles.
Pressure Changes During Cardiac Cycle
Pressure in ventricles reaches maximum during ejection period and reaches minimum during final phase of ventricular diastole.
Maximum and minimum pressures in the left and right ventricles are given in the pressure values above.
Aortic Pressure Changes During Cardiac Cycle
Significance of Aortic Pressure
Aortic pressure is the pressure developed in aorta.
It is necessary to maintain the blood flow through circulatory system.
Pressure Changes During Cardiac Cycle
Pressure in systemic aorta is always higher than that of pulmonary artery.
It is because the higher pressure in left ventricle than in right ventricle.
Maximum and minimum pressures in aorta are given in the pressure values above.
Minimum pressure in systemic aorta is much greater than the minimum pressure in left ventricle.
It is due to the presence of elastic tissues in the aorta which enable the aorta to recoil and maintain the minimum pressure at a higher level.
During ejection period of cardiac cycle, pressure in aorta increases and reaches the peak.
During diastole, it reduces gradually and reaches the minimum level.
At the time of closure of semilunar valves, an incisura occurs due to back flow of some blood towards the ventricles.
7 Ventricular Volume Changes During Cardiac Cycle
Significance of Volume of Blood in Ventricles
Volume of blood in the left ventricle is an important factor to maintain cardiac output and blood circulation.
Volume of blood in right ventricle is responsible for flow of blood into pulmonary circulation.
Volume of Blood in Right and Left Ventricles
End-Diastolic Volume and End-Systolic Volume
Amount of blood is the same in both right and left ventricles.
Maximum volume of blood in each ventricle after filling (end-diastolic volume) is 130 to 150 mL.
Minimum volume of blood left in the ventricles at the end of ejection period (end of systolic volume) is 60 to 80 mL.