Hemodynamics and pumping function
The myocardium possesses a specialized conduction system that generates excitation automatically. It is this system that ensures the synchronous operation of the right and left parts of the organ.
During circulation, blood passes through the heart twice. First, it enters the right chambers and is directed into the pulmonary circulation. After returning to the left chambers, blood is pumped into the systemic circulation.
The pumping function works like a batch pump. The chambers fill with blood, then the myocardium contracts, and blood is ejected under pressure into the vessels.
Main blood volumes
To understand cardiac function, a graph of left ventricular (LV) pressure versus volume is used. The graph of a normal cycle looks like a closed loop. It highlights key indicators:
- EDV (end-diastolic volume): The maximum amount of blood in the ventricle before its contraction begins. Normally about 130 ml.
- ESV (end-systolic volume): The volume of blood remaining in the chamber after contraction is complete. On average, 50 ml.
- SV (stroke volume): The difference between EDV and ESV. This is the portion of blood ejected in a single contraction. On the pressure-volume loop, SV represents the width of the loop.
Phases of the normal cardiac cycle
Left ventricular function is clearly divided into periods of contraction and relaxation, during which the state of the valve apparatus changes.
- Filling period (diastole). The aortic valve is closed, the mitral valve is open. Blood flows from the left atrium into the ventricle. LV volume increases from ESV to EDV, while pressure remains low (0–10 mm Hg). The phase ends with the closure of the mitral valve.
- Isovolumetric contraction phase (systole). All valves are closed (atrioventricular and semilunar). The ventricle contracts as a closed chamber, so its volume does not change (isometric contraction — cardiomyocyte length remains constant while tension increases). Pressure rapidly jumps from ~10 to ~80 mm Hg. When it exceeds the diastolic pressure in the aorta, the aortic valve opens.
- Ejection period (systole). The mitral valve is closed, the aortic valve is open. Blood rushes into the aorta. LV volume decreases. Pressure first reaches a maximum (systolic pressure, ~120 mm Hg) and then declines. When ventricular pressure drops below aortic pressure, the aortic valve snaps shut.
- Isovolumetric relaxation phase (diastole). All valves are closed again. The myocardium relaxes at a constant volume (equal to ESV). Cell tension drops, length does not change. Intraventricular pressure decreases sharply. As soon as it falls below the pressure in the left atrium, the mitral valve opens, and the cycle starts over.
Effect of preload and afterload on the cycle
The cycle graph can change depending on hemodynamic conditions.
Increased preload Occurs with enhanced venous return. The ventricle fills more fully, leading to an increase in EDV. In accordance with the Frank-Starling law (the greater the stretch of the muscle before systole, the more powerful its contraction), the stroke volume also increases. On the graph, this looks like a rightward widening of the loop. Peak pressure changes little.
Increased afterload Occurs when outflow resistance increases (aortic stenosis, arterial hypertension). The aortic valve is forced to open at a higher pressure. The ventricle has to develop much higher peak (systolic) pressure to push the blood through. If myocardial contractility does not increase, the ventricle cannot empty fully: ESV increases and stroke volume decreases. Graphically, the loop becomes narrower and shifts upward.
Briefly: answers to topical questions
What heart sounds occur in different phases of the cardiac cycle and what causes them?
During different phases of the cardiac cycle, the first and second heart sounds occur, as well as the pathological opening snap of the mitral valve.
- First sound (systolic) — occurs during the isometric contraction phase and the beginning of rapid ejection. Caused by the closure of atrioventricular valves (vibration of leaflets, chordae tendineae, and partially semilunar valves).
- Second sound (diastolic) — occurs during the isovolumetric relaxation phase of the ventricles. Caused by the snapping shut of the aortic and pulmonary valves.
- Mitral valve opening snap — occurs in diastole. Caused by the sudden tension of fused leaflets.
What is the duration of each phase of the cardiac cycle in seconds at a normal heart rate?
At a normal heart rate (75 bpm), the total cycle duration is 0.8 s. Sources indicate the duration only for individual phases (in milliseconds, which correspond to fractions of a second):
- Atrial systole — 0.11 s (right) and 0.10 s (left).
- Slow blood ejection — 0.13 s (for both ventricles).
- Protodiastole — 0.05 s (right ventricle) and 0.04 s (left).
- Slow filling — 0.16 s (right ventricle) and 0.17 s (left).
As heart rate increases, the duration of all phases decreases.
What is the pressure dynamics in the right ventricle and pulmonary artery during the cardiac cycle?
Pressure dynamics in the right ventricle and pulmonary trunk vary depending on the cycle phase.
- Diastole (filling) — at the end of the slow filling phase, pressure in the right ventricle rises to 3 mm Hg, and by the end of atrial systole to 5 mm Hg. During the protodiastolic phase, pressure in the pulmonary trunk continues to fall.
- Systole (ejection) — ejection pressure from the right ventricle into the pulmonary trunk reaches 25–30 mm Hg.
Additionally, systolic pressure in these structures increases during a forced inspiration.
It is easy to remember the state of the valves during isovolumetric phases: the word "iso" means "constant, equal". Volume can only remain constant in a completely hermetic chamber, which means absolutely ALL valves are closed at these moments.
Class and exam questions
What is the essence of the Frank-Starling law?
The law states: the more cardiomyocytes are stretched during ventricular filling (increasing EDV), the greater the force with which they subsequently contract, increasing stroke volume.
What happens to the length and tension of cardiomyocytes during isometric phases?
Muscle fiber length remains unchanged. During isometric contraction, their tension increases sharply, and during the isovolumetric relaxation phase, it drops.
How to calculate stroke volume?
Stroke volume (SV) is the difference between end-diastolic (EDV) and end-systolic (ESV) volumes.
What else to study on this topic
- Detailed analysis of the pressure-volume loop
- Physiological mechanisms of cardiac regulation
- Pathological hemodynamic changes in valve defects
- The Frank-Starling law at the cellular level
- Conduction system of the heart and automatism
Master the entire topic in the "Physiology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on the "Cardiac cycle" and 4,128 other subject pages
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 114 video lessons on the subject
Other topics in the "Cardiac Physiology" section
Free video lessons: PhysiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.