← Laboratorium Biology · Human physiology

Circulatory system · cardiac cycle

The heart — a pump that never rests

Two pumps in one muscle: the right one pushes blood to the lungs, the left one to the whole body. Watch a spark from the sinoatrial node travel through the heart, the valves open and close in time with the ECG, and each beat pump out a portion of blood.

This page is for education only and is not medical advice. The model shows a simplified picture of a healthy adult heart. For health questions, see a doctor.

    Click a part in the drawing or a name above, and I'll highlight it and describe it.

    ECG — monitor

    Click a wave on the trace to name it. Sweep speed 25 mm/s, grid 0.04 s.

    The P wave — the atria are excited; the QRS complex — the ventricles are excited; the T wave — the ventricles return to rest (repolarization).

    The Wiggers diagram

    One cycle of the left side of the heart. Drag across the chart to scrub through time.

    Two circulations

    PULMONARY CIRCULATION lungs blood drops CO₂, picks up O₂ RA RV LA LV body organs brain, muscles, kidneys, skin… SYSTEMIC CIRCULATION pulmonary trunkpulmonary veinsaortavenae cavae

    How does it work?

    The heart is a muscle with four chambers. The right side (atrium and ventricle) receives blood from the venae cavae and pumps it through the pulmonary trunk to the lungs. The left side takes oxygenated blood from the pulmonary veins and drives it into the aorta. Both ventricles eject the same amount of blood with each beat, but the left one works against about 5 times higher pressure (about 120 mmHg versus about 25 mmHg), which is why its wall is much thicker.

    The valves work like check valves: they open and close on their own whenever the pressure difference across them changes. The closing of the atrioventricular valves (mitral and tricuspid) is heard as the first heart sound, S1, and the closing of the semilunar valves (aortic and pulmonary) as the second sound, S2.

    Phases of the cycle

    1. Atrial systole — tops up the ventricles with a final portion of blood (about 20% at rest).
    2. Isovolumetric contraction — all valves are closed; pressure in the ventricle rises while its volume stays the same.
    3. Ejection — pressure in the ventricle exceeds pressure in the aorta, and the aortic valve opens.
    4. Isovolumetric relaxation — the ventricle relaxes with its valves closed.
    5. Filling — the mitral valve opens and blood flows in, fast at first, then more slowly (diastasis).

    The electrical spark

    Every beat starts in the sinoatrial node. The wave of excitation spreads across the atria (the P wave), slows down by about 0.1 s in the atrioventricular node, and then races through the ventricles via the bundle of His, its branches and the Purkinje fibers (the QRS complex). The T wave is the ventricles returning to rest. The muscle contracts right after the electrical excitation.

    Formulas

    CO = HR · SVcardiac output = heart rate × stroke volume; e.g. 70/min · 71 mL ≈ 5.0 L/min
    SV = EDV − ESV    EF = SVEDV · 100%EDV — ventricular volume at the end of diastole, ESV — at the end of systole; in a healthy person EF is usually 55–70%
    T = 60 sHR    tcirc ≈ VbloodCOthe length of one cycle; the time it takes all the blood (about 5 L) to pass through the heart

    Simplifications: the cross-section is schematic — in reality the pulmonary trunk wraps around the aorta, and the pulmonary veins enter the left atrium from behind. The pressure and volume curves are a smoothed model of a healthy adult. The way stroke volume depends on heart rate matches the heart speeding up naturally (sleep → rest → exercise, when contraction strength and venous return also increase). If the heart were sped up artificially, without exercise, stroke volume would fall. Ventricular systole is counted from S1 to S2.

    Keyboard shortcuts

    • Space pause / resume
    • ↑ ↓ heart rate ±1 (with Shift ±5)
    • 1 2 3 sleep / rest / exercise
    • ← → step by 10 ms (pauses)
    • Z animation speed
    • M heart sounds, Esc mute
    • N labels, K conduction

    See also

    Neuron — the nerve impulseThe cell

    Did you know…?

    • At 70 beats per minute, the heart beats about 100,000 times a day, and over 80 years of life about 3 billion times — without ever taking a longer break.
    • A transplanted heart has no nerve connections, yet it still beats: its own sinoatrial node sets the rhythm. Without the braking influence of the vagus nerve it runs faster — about 90–110 beats per minute.
    • Heart sounds are not the noise of the muscle contracting but vibrations of blood and tissue as the valves snap shut. The second sound is sometimes “split,” because the pulmonary valve closes a moment after the aortic one — especially when you breathe in.

    Challenges

    • Cardiac output of 10 L/minSet the heart rate so the heart pumps 10.0 liters of blood per minute.
    • Stop at the first soundPause the animation exactly when the mitral valve closes (S1). Slowing it down or stepping with ←/→ helps.
    • Find the P waveClick the P wave on the ECG monitor — you can hide the wave labels first.
    • Systole equals diastoleAt rest, diastole lasts longer than systole. Find the heart rate at which both last the same (±5 ms).