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The lub-dub of the human heart is magical. Its steady rhythm is like a personal soundtrack that keeps tempo with the fluctuations of our daily lives; increasing with stressors like a great workout session or pressing deadlines and easing into a more relaxed back beat as calm settles in before sleep or during mindful meditation.

The lub-dub can be heard and seen – as tracings from an electrocardiogram or EKG. This device uses a series of electrodes that are attached to the chest, arms and legs that measure and monitor the electrical activity of the heart. The tracing or visual display shows a distinctive waveform that corresponds to the electrical changes taking place inside the heart as it works to pump blood throughout the body. Each wave is marked by a letter and measurements between the letters show the strength and timing of the electrical signals that are necessary to keep that lub-dub humming like a metronome.


Each beat of the heart begins with an electrical signal from the SA node (sinoatrial node) located in the right atrium. The signal spreads to the left atrium and triggers both to contract and push blood into the ventricles. The signal moves to the AV node (atrioventricular node) and slows for the slightest instant as the ventricles fill with blood. This instant is recorded in the wave between the P and Q marks on the diagram.

From here (Q mark) the signal moves down the center (septum) of the heart between the ventricles and spreads quickly across both causing them to contract. The left ventricle triggers first (R mark) and then the right (S mark). Oxygen-rich blood heads out to the rest of the body through the aorta. The right ventricle sends oxygen-poor blood to the lungs through the pulmonary valve.

The ventricles relax (S-T wave) and the process starts all over again.

Want to learn more about how the heart works and the human body’s amazing circulatory system? Watch the Khan Academy video. Knowledge is power!

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