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Cardiac Cycle

cyclus cardiacus

For medical students2 min readUpdated 2026-10-10

The cardiac cycle is the rhythmic and strictly sequential activity of the right and left sides of the heart. It includes impulse generation, propagation, and alternating contractions (systole) and relaxations (diastole) of the atria and ventricles, resulting in the ejection of blood into the vascular bed.

SynchronyThe right and left sides of the heart contract and relax simultaneously
Pulmonary circulationBlood from the right side of the heart is ejected into the pulmonary circulation
Systemic circulationBlood from the left side of the heart enters the systemic circulation
Cycle outcomeEjection of stroke volume (SV) into blood vessels

Hemodynamics and Pumping Function

The myocardium features a specialized conduction system that generates electrical impulses automatically. This system ensures the synchronized action of both sides 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 acts as a batch pump. The chambers fill with blood, the myocardium contracts, and blood is forcefully ejected into the vessels.

Key Blood Volumes

To understand cardiac function, clinicians use a pressure-volume relationship graph for the left ventricle (LV). A normal cycle graph appears as a closed loop containing key parameters:

Phases of the Normal Cardiac Cycle

Left ventricular function is clearly divided into contraction and relaxation periods, during which the valvular apparatus changes position.

  1. Filling Period (Diastole). The aortic valve is closed, and 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 mmHg). This phase ends with the closure of the mitral valve.
  2. 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 spikes rapidly from ~10 to ~80 mmHg. When it exceeds diastolic pressure in the aorta, the aortic valve opens.
  3. Ejection Period (Systole). The mitral valve is closed, and the aortic valve is open. Blood rushes into the aorta. LV volume drops. Pressure first reaches a maximum (systolic pressure, ~120 mmHg) and then declines. When ventricular pressure falls below aortic pressure, the aortic valve snaps shut.
  4. Isovolumetric Relaxation Phase (Diastole). All valves are closed again. The myocardium relaxes at a constant volume (equal to ESV). Cellular tension drops while length remains unchanged. Intraventricular pressure decreases sharply. As soon as it drops below left atrial pressure, the mitral valve opens, and the cycle begins anew.

Effects of Preload and Afterload on the Cycle

The cardiac cycle graph can change depending on hemodynamic conditions.

Increased Preload Occurs when venous return increases. The ventricle fills more fully, leading to an increased EDV. According to the Frank-Starling law (the greater the stretch of the muscle before systole, the stronger the subsequent contraction), the stroke volume also increases. On the graph, this appears as a rightward expansion of the loop. Peak pressure changes very little.

Increased Afterload Occurs when resistance to ejection increases (aortic stenosis, arterial hypertension). The aortic valve must open at a higher pressure. The ventricle must generate significantly 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.

Mnemonic

Remembering valve states during isovolumetric phases is simple: the prefix "iso" means "constant, equal." Volume can only remain constant in a completely sealed chamber, which means absolutely ALL valves are closed at these moments.

Frequently asked questions

What heart sounds occur during different phases of the cardiac cycle, and what causes them?

The 1st and 2nd heart sounds, as well as the pathological opening snap of the mitral valve, occur during different phases of the cardiac cycle.

  • First heart sound (S1, systolic) — occurs during isovolumetric contraction and the beginning of rapid ejection. Caused by the closure of atrioventricular valves (vibration of leaflets, chordae tendineae, and partly semilunar valves).
  • Second heart sound (S2, diastolic) — occurs during isovolumetric relaxation of the ventricles. Caused by the snapping shut of the aortic and pulmonary valves.
  • Opening snap of the mitral valve — occurs in diastole. Caused by the abrupt 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 duration of the cycle is 0.8 s. Source data lists durations for individual phases (in milliseconds, corresponding to fractions of a second):

  • Atrial systole — 0.11 s (right) and 0.10 s (left).
  • Slow ventricular 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 mmHg, and at the end of atrial systole to 5 mmHg. During protodiastole, pressure in the pulmonary trunk continues to fall.
  • Systole (ejection) — ejection pressure from the right ventricle into the pulmonary trunk reaches 25–30 mmHg.

Additionally, systolic pressure in these structures increases during forced inspiration.

What is the core principle of the Frank-Starling law?

The law states: the more cardiomyocytes are stretched during ventricular filling with blood (increased EDV), the greater the force with which they subsequently contract, increasing stroke volume.

What happens to cardiomyocyte length and tension during isovolumetric phases?

Muscle fiber length remains unchanged. During isovolumetric contraction, their tension increases sharply, while during isovolumetric relaxation, it drops.

How is stroke volume calculated?

Stroke volume (SV) is the difference between end-diastolic volume (EDV) and end-systolic volume (ESV).

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