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Physiology · Cardiovascular

The cardiac cycle !

  • Mechanism
  • Exam priority: high

One heartbeat is a fixed sequence: the heart fills, squeezes with closed valves, ejects, relaxes, and fills again.

Updated 2026-11-01

Also called: Wiggers diagram, Systole and diastole

See it in 3D: Left ventricle · Left atrium · Mitral anterior leaflet · Aortic right coronary cusp · Ascending aorta

What it is

The cardiac cycle is the sequence of pressure and volume changes in one beat. Valves open and close only because of pressure differences between chambers and arteries. At 75/min one cycle lasts 0.8 s, about one third systole and two thirds diastole; faster rates shorten diastole most.

How it works, step by step

  1. Atrial contraction (after the P wave) pushes a last volume into the ventricle: end-diastolic volume ≈ 120 mL.
  2. Ventricular depolarization (QRS) starts contraction; ventricular pressure exceeds atrial pressure and the mitral valve closes (S1).
  3. With both valves closed, pressure rises steeply at constant volume (isovolumetric contraction).
  4. When LV pressure exceeds aortic pressure (≈ 80 mmHg) the aortic valve opens and blood is ejected; pressure peaks ≈ 120 mmHg.
  5. As the ventricle repolarizes (T wave) and relaxes, its pressure falls below aortic pressure and the aortic valve closes (S2).
  6. Isovolumetric relaxation: pressure drops fast at constant volume (end-systolic volume ≈ 50 mL).
  7. When LV pressure falls below left atrial pressure the mitral valve opens and rapid filling begins, then slows (diastasis).

Cardiac cycle (Wiggers diagram)

Cardiac cycle (Wiggers diagram) Pressure (mmHg)040801200–25 mmHg detail: atrium and ventricle01020LV volume (mL)50120ECGHeart sounds0200400600800Time (ms)AortaLVdicrotic notchacvLALVEDV 120ESV 50PQRSTS1S2AOACMCMO1234567

HR 75/minCycle 800 msSystole ≈ 300 msSV 70 mLEF 58%

Tap a number (phase), a dot (valve) or a sound.

1 Atrial systole · 2 Isovolumetric contraction · 3 Rapid ejection · 4 Reduced ejection · 5 Isovolumetric relaxation · 6 Rapid filling · 7 Diastasis (slow filling).

MC mitral closure · AO aortic opening · AC aortic closure · MO mitral opening.

  • Aorta
  • Left ventricle (LV)
  • Left atrium (LA)
  • LV volume
  • ECG
  • S1 and S2
  • Ventricular systole
  • Ventricular diastole
  • Valve events (MC, AO, AC, MO)
Tap a phase number to see which valves are open and which ECG wave and heart sound go with it.

Left-ventricular pressure–volume loop

Left-ventricular pressure–volume loop: Normal, EDV 120 mL, ESV 50 mL 05010015020025004080120160LV volume (mL)LV pressure (mmHg)ESPVREDPVREaFillingEjectionIsovol. contr.Isovol. relax.MOMCAOAC

EDV 120 mLESV 50 mLSV 70 mLEF 58%ESP 105 mmHg

Normal

Normal loop: EDV about 120 mL, ESV about 50 mL, SV about 70 mL and EF about 60%. The loop's width is the stroke volume and its area is the stroke work.

Tap a corner of the loop (MC, AO, AC, MO) to see the valve event.

  • Current loop
  • Normal loop
  • ESPVR (contractility, Ees)
  • EDPVR (passive filling)
  • Ea (arterial afterload)
  • Stroke volume (SV)
Illustrative model drawn by Durgical (time-varying elastance: ESPVR, EDPVR and Ea); values are approximate and not for clinical decisions.

Reference values (adult left heart at rest)

ParameterValueNote
End-diastolic volume≈ 110–130 mL
End-systolic volume≈ 40–60 mL
Stroke volume≈ 70 mL
Ejection fraction≈ 55–70%
LV pressure (systolic / end-diastolic)≈ 120 / 5–12 mmHg
Aortic pressure≈ 120/80 mmHg
Cycle length at 75/min0.8 ssystole ≈ 0.3 s, diastole ≈ 0.5 s

Approximate values for a healthy adult at rest; each laboratory sets its own ranges.

Clinical notes

  • Aortic stenosis: a large pressure gradient between LV and aorta during ejection, a harsh systolic murmur and LV hypertrophy.
  • Atrial fibrillation removes the atrial kick, which matters most when the ventricle is stiff (hypertrophy, heart failure with preserved EF).

Test yourself

Think of the answer, then tap the question to see it.

During which phases of the cardiac cycle are all four valves closed?

Isovolumetric contraction (between mitral closure and aortic opening) and isovolumetric relaxation (between aortic closure and mitral opening).

Why does a heart rate of 160/min reduce stroke volume in a patient with a stiff left ventricle?

Faster rates shorten diastole more than systole; a stiff ventricle needs time and the atrial kick to fill, so end-diastolic volume and stroke volume fall.

What event produces the dicrotic notch on the aortic pressure tracing?

Closure of the aortic valve at the end of ejection: a brief backflow is stopped by the closing cusps and the elastic aorta recoils.

The ECG shows a QRS complex. Which mechanical event follows it most closely?

The start of ventricular contraction and closure of the mitral and tricuspid valves (S1), beginning isovolumetric contraction.

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See it in 3D

The structures of Durgical's 3D anatomy where it happens.

← All of physiology

Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: 19.3 Cardiac Cycle (OpenStax (Rice University), 2022); Aortic Stenosis (StatPearls (NCBI Bookshelf), 2025); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 9). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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Representative model, for educational purposes only.