Cycle chronostructure and phase criteria
The overall work of the ventricles is divided into 9 sequential phases. The starting point of the entire cardiac cycle is considered to be the ventricular presystole, which temporally coincides with atrial systole and lasts 100 ms.
The heart's transition from one phase to another is accompanied by intermediate events. Phase transitions are always characterized by changes in three key hemodynamic parameters:
- Blood pressure inside the heart chambers.
- Blood volume within the cavities.
- Position of the valve apparatus (opening or closure).
Ventricular systole itself is broadly divided into two periods: tension and ejection.
Myocardial tension period
This period prepares the heart chambers for the subsequent ejection of blood. Intraventricular pressure begins to rise rapidly. The period includes two phases:
- Phase of asynchronous contraction. Lasts 50 ms in the left ventricle and 70 ms in the right. Contraction of myocardial fibers begins unevenly due to the varying arrival time of excitation. The impulse sweeps through the tissues sequentially: first the areas near the bundle of His terminals, then the papillary muscles, the interventricular septum, and the apex. Contracting areas stretch neighboring zones, so the overall length of the ventricles and their volume do not change. Atrial pressure drops, and the leaflets of the atrioventricular valves passively float upward.
- Phase of isometric (isovolemic) contraction. Lasts 30 ms. Excitation synchronously encompasses all cardiomyocytes. It begins when the backflow of blood snaps the atrioventricular valves shut. The semilunar valves are still closed because the pressure is not yet sufficient to open them. The ventricle becomes completely sealed, pressure rises sharply inside it, while volume remains unchanged.
Note: The first heart sound ($S_1$) (systolic) is formed precisely during this period. It is caused by the vibration of the snapped shut valve leaflets, as well as the tension of the chordae tendineae and the muscular wall.
Blood ejection period
When intraventricular pressure begins to exceed the diastolic pressure in the vessels (aorta and pulmonary trunk), the blood flow opens the semilunar valves. The atrioventricular valves remain tightly closed. The ejection process begins, which is divided into two phases:
- Phase of rapid ejection. Takes 120 ms for the left ventricle and 110 ms for the right. The main volume of blood is ejected during this time. Since the influx of blood from the heart into the vessels significantly exceeds its outflow into the capillaries, pressure in the main arteries rises sharply: in the pulmonary trunk from 15 to 30 mm Hg, and in the aorta from 80 to 120 mm Hg. The phase ends at the moment of peak pressure in these vessels.
- Phase of slow ejection. Starts when ventricular pressure reaches its maximum and lasts 130 ms for both halves of the heart. The outflow of blood from the aorta and pulmonary trunk to the periphery begins to exceed its inflow from the heart, causing vascular pressure to gradually decrease.
Throughout both ejection phases, the pressure gradient is maintained: intraventricular pressure strictly exceeds the pressure in the great vessels. This is a mandatory physical condition for the continuation of blood ejection.
Briefly: answers to topical questions
To what values in mm Hg does the pressure in the left and right ventricles rise at the end of the tension period?
At the end of the tension period, intraventricular pressure rises to the values necessary to open the semilunar valves. In the left ventricle, pressure reaches 75–85 mm Hg, as it must exceed aortic pressure. In the right ventricle, pressure rises to 15–20 mm Hg, which is slightly higher than the pressure in the pulmonary trunk. Immediately upon reaching these values, the semilunar valves open, the tension period ends, and the period of blood ejection into the great vessels begins.
What is the total duration of ventricular systole in a human at rest?
The total duration of ventricular systole is 0.33 s (or 330 ms). This time is made up of sequentially occurring contraction phases:
- Asynchronous contraction — 50 ms.
- Isometric contraction — 30 ms.
- Rapid blood ejection — 120 ms.
- Slow blood ejection — 130 ms.
The first two phases form the tension period, and the subsequent two form the ejection period. Ventricular systole begins after atrial systole and ends with the transition of the myocardium to relaxation.
What volume of blood (in milliliters) is normally ejected by the ventricle during the ejection period?
During the ejection period, the ventricle normally ejects 65 ml of blood. This indicator represents the stroke volume (ejection volume) that the heart chamber pumps out in a single contraction. Exactly the same volume of blood is ejected from the left ventricle into the aorta and from the right ventricle into the pulmonary trunk. The stroke volume is calculated as the difference between the end-diastolic volume, equal to 125 ml, and the end-systolic volume, equal to 60 ml, which remains in the chamber after systole.
The acronym PSPV will help remember the sequence of contraction of myocardial areas during the asynchronous contraction phase: Bundle (of His) → Papillary muscles → Septum → Apex.
Class and exam questions
Why does ventricular volume not change during isometric contraction?
During this phase, the atrioventricular and semilunar valves are completely closed. The heart chamber turns into a closed, sealed cavity. Muscles contract, increasing pressure, but blood cannot leave the ventricle, so the volume remains constant.
What is the formation of the first heart sound associated with?
The first (systolic) sound is formed during the tension period. The main cause of its appearance is the vibration of tightly closing atrioventricular valves, as well as the strong tension of the chordae tendineae and the contracting myocardium.
What serves as a signal for the end of the slow ejection phase?
The slow ejection phase ends at the moment when ventricular cardiomyocytes cease active contraction and begin transitioning to the relaxation process (diastole).
How do the semilunar valves open?
They open solely due to the pressure gradient: as soon as the rising pressure in the ventricles becomes higher than the diastolic pressure in the aorta and pulmonary trunk, the blood flow mechanically pushes the valve leaflets apart.
What else to study on this topic
- Protodiastole and isometric relaxation phase
- Phases of rapid and slow blood filling
- Hemodynamics and the role of atrial systole (presystole)
- Physiological mechanisms of the formation of the first and second heart sounds
- Blood pressure dynamics during different phases of the cardiac cycle
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