The two-stroke engine in your chest
How a fist-sized muscle times every beat with its own electrical system.
At rest your heart beats about 75 times a minute, and every beat is the same carefully timed sequence: squeeze the top chambers, wait, squeeze the bottom ones, relax, refill. No conscious thought is involved. The heart even makes its own spark.
Scroll slowly through the middle of this story. For a few steps, your scrolling becomes the heart’s clock, slowing a single 0.8-second beat down so you can follow it.
Two pumps in one
This is a schematic slice through the heart, seen from the front. That puts the heart’s right side on your left. It is really two pumps side by side: four chambers, four one-way valves, and a wall of muscle between them.
The right side: to the lungs
Used blood, low in oxygen, returns from the body into the right atrium, then drops into the right ventricle, which pushes it a short way to the lungs. That trip only needs a peak pressure of about 25 mmHg.
The left side: to everything else
Fresh blood from the lungs fills the left atrium and left ventricle. The left ventricle has to drive blood around the whole body, peaking at about 120 mmHg. That’s why its wall is so much thicker.
0.00 s · The spark
Each beat starts in a small patch of cells in the right atrium: the sinoatrial (SA) node, the heart’s built-in pacemaker. Its signal spreads across both atria, and they squeeze the last blood down into the ventricles.
0.13 s · The pause
The signal reaches the atrioventricular (AV) node and is deliberately held back for about a tenth of a second. The delay gives the ventricles time to finish filling before they fire.
0.17 s · The wave, and “lub”
Released, the signal races down the septum and around the ventricles through fast fibres (the bundle of His and the Purkinje fibres). The ventricles squeeze. Pressure slams the valves between atria and ventricles shut. That’s the first heart sound: “lub”.
0.24 s · Ejection
Once ventricle pressure beats the pressure in the arteries, the exit valves swing open. Blood surges out to the lungs and the body.
0.48 s · “Dub”, and refill
The ventricles relax. Blood tries to fall back and snaps the exit valves shut: the second sound, “dub”. The inlet valves reopen and the heart passively refills, ready for the next spark at 0.8 s.
Reading an ECG
The trace on a hospital monitor records this electrical sequence. The small P wave is the atria firing, the tall spike (QRS) is the ventricles firing, and the T wave is the ventricles electrically resetting.
Every minute, every day
Each beat moves about 70 mL from each ventricle. That’s around 5 litres a minute at rest, roughly all of your blood, and about 100,000 beats a day.
Change the pace
When you exercise your heart beats faster, but the beat doesn’t just play at a higher speed. The squeeze shortens a little, and almost all of the time saved comes out of the rest and refill phase. Drag the slider to see how fast it shrinks.
Model: the squeeze scales with √(beat length), after Bazett’s relation. In real hearts the volume per beat changes too, so treat the output figure as illustrative.
Go deeperWhy the AV pause matters
The atria and ventricles are electrically insulated from each other by a ring of fibrous tissue. The AV node is normally the only way through, and it conducts slowly on purpose. Without that delay the atria and ventricles would squeeze at almost the same moment, and the ventricles would start pushing before they were full.
On an ECG this pause appears as the flat stretch between the P wave and the QRS complex (part of the PR interval).
Go deeperWhat the numbers assume
Timings are for a resting adult at about 75 beats per minute: a 0.8 s cycle with about 0.1 s of atrial squeeze, 0.3 s of ventricular squeeze and 0.4 s with every chamber relaxed. Volumes: about 130 mL in a ventricle when full, 60 mL left after the squeeze, so about 70 mL is pumped (an ejection fraction of around 55%).
70 mL × 72 beats ≈ 5 L/min. 70 beats/min × 1,440 min ≈ 100,800 beats a day. The diagram is schematic: real chambers are twisted around each other, and the great vessels cross in three dimensions.