It is known as the contraction period that the heart suffers while pumping blood into the circulation.
The period of time that begins with the contraction of the atria and ends with the ventricular relaxation is known as the cardiac cycle.
The period of relaxation that occurs when the chambers fill with blood is calleddiastole .
Both the atria and the ventricles experience systole and diastole, and it is essential that these components are carefully regulated and coordinated to ensure that blood is efficiently pumped throughout the body.
Systole is the period of contraction of the ventricles of the heart and occurs between the first and second heart sounds of the cardiac cycle (the sequence of events in a single heart beat).
The systole causes the ejection of blood to the aorta and the pulmonary trunk.
With a duration generally of 0.3 to 0.4 seconds, the ventricular systole is introduced by a very short contraction period, followed by the ejection phase, during which 80 to 100 cc of blood leave each ventricle.
During systole, blood pressure reaches its maximum (systolic blood pressure), usually between 90 and 120 mm of mercury in humans.
In an electrocardiogram , the beginning of the ventricular systole is marked by deviations of the QRS complex, which are a set of waves that appear in the electrocardiogram, graphically representing the depolarization of the ventricles.
Atrial systole occurs towards the end of the ventricular diastole, completing the filling of the ventricles.
In an electrocardiogram, atrial systole is associated with the deflection of the P wave, this wave appears before the QRS complex, graphically representing atrial depolarization.
Pressures and flow
Fluids, whether gases or liquids, are materials that flow in accordance with pressure gradients, that is, they move from regions with higher pressure to regions with lower pressure.
Consequently, when the chambers of the heart are relaxed (diastole), blood will flow to the atria from the veins, which have a higher pressure.
As blood flows to the atria, the pressure will increase, so the blood will initially passively move from the atrium to the ventricles.
When the action potential causes the muscles of the atria to contract (atrial systole), the pressure inside the atria rises further, pumping blood into the ventricles.
During ventricular systole, the pressure increases in the ventricles, pumping blood to the pulmonary trunk from the right ventricle to the aorta from the left ventricle.
Phases of the cardiac cycle
At the beginning of the cardiac cycle, both the atria and the ventricles are relaxed (diastole).
Blood flows to the right atrium from the superior and inferior vena cava and the coronary sinus.
Blood flows to the left atrium from the four pulmonary veins.
The two atrioventricular valves, the tricuspid and mitral valves, are open, so blood flows unimpeded from the atria to the ventricles.
Approximately 70-80 percent of ventricular filling occurs by this method.
The two semilunar valves, the pulmonary and aortic valves, are closed, which prevents the backflow of blood to the right and left ventricles from the pulmonary trunk to the right and the aorta to the left.
The contraction of the atria follows depolarization, represented by the P wave of the electrocardiogram.
As the atrial muscles contract from the upper portion of the atria to the atrioventricular septum, the pressure increases within the atria and blood is pumped into the ventricles through the open atrioventricular valves (tricuspid and mitral or bicuspid).
Atrial contraction, also known as ” atrial kick “, contributes with the remaining 20 to 30 percent of filling.
Atrial systole ends before ventricular systole, as the atrial muscle returns to diastole.
The ventricular systole follows the depolarization of the ventricles and is represented by the QRS complex in the electrocardiogram.
It can conveniently be divided into two phases, which last a total of 270 ms.
At the end of atrial systole and just before atrial contraction, the ventricles contain approximately 130 ml of blood in an adult at rest in the standing position.
This volume is known as end-diastolic volume or preload.
Initially, as the muscles of the ventricle contract, the pressure of the blood inside the chamber increases, but it is not yet high enough to open the semilunar valves (pulmonary and aortic) and expel blood from the heart.
However, blood pressure rises rapidly above that of atria that are now relaxed and in diastole.
This increased pressure causes blood to flow to the atria, closing the tricuspid and mitral valves.
Since blood is not expelled from the ventricles at this early stage, the volume of blood within the chamber remains constant.
Consequently, this initial phase of ventricular systole is known as isovolumic contraction, also called isovolumetric contraction.
In the second phase of ventricular systole, the ventricular ejection phase, the contraction of the ventricular muscle has raised the pressure within the ventricle to the point that it is greater than the pressures in the pulmonary trunk and the aorta.
The blood is pumped from the heart, pushing the semilunar, pulmonary and aortic valves.
The pressure generated by the left ventricle will be appreciably greater than the pressure generated by the right ventricle, since the pressure in the aorta will be much greater.
However, both ventricles pump the same amount of blood.
This amount is known as stroke volume. The stroke volume will normally be in the range of 70 to 80 ml.
Since the ventricular systole started with a end-diastolic volume of approximately 130 ml of blood, this means that there are still 50 to 60 ml of blood remaining in the ventricle after contraction.
This volume of blood is known as the final stroke volume.
Ventricular or diastole relaxation follows the repolarization of the ventricles and is represented by the T wave of the electrocardiogram.
The T wave follows after a pause to the QRS complex, it is a slightly asymmetrical waveform.
It is also divided into two distinct phases and lasts approximately 430 ms.
During the early phase of ventricular diastole, as the ventricular muscle relaxes, the pressure on the remaining blood within the ventricle begins to decrease.
When the pressure inside the ventricles falls below the pressure in both the pulmonary trunk and the aorta, the blood flows back to the heart, producing the dicrotic notch, that is, a small depression, which is seen in the tracings of the heart. Blood pressure
The semilunar valves are closed to prevent reflux to the heart.
Because the atrioventricular valves remain closed at this point, there is no change in the volume of blood in the ventricle, so the early phase of the ventricular diastole is called the isovolumetric ventricular relaxation phase.
In the second phase of the ventricular diastole, called the late ventricular diastole, as the ventricular muscle relaxes, the pressure on the blood within the ventricles decreases further.
Finally, it falls below the pressure in the atria.
When this happens, blood flows from the atria to the ventricles, pushing out the tricuspid and mitral valves.
As the pressure falls within the ventricles, blood flows from the main veins to the relaxed atria and from there to the ventricles.
Both chambers are in diastole, the atrioventricular valves are open and the semilunar valves remain closed, completing the cardiac cycle.