The Action Potential — Five Phases That Keep You Alive

I want to talk about something that happens inside every single heart cell, roughly once every second, for your entire life.

It is called the action potential. And it is, without question, one of the most beautifully engineered sequences in all of biology.

Five phases. Each one handed off to the next like a relay race.

Phase 0 — the door bursts open. Sodium floods in. The cell goes from resting to fully charged in less than a millisecond. This is the spike on your ECG. This is the QRS.

Phase 1 — a brief pause. Potassium steps out, sodium channels close. The cell catches its breath for just a moment.

Phase 2 — the plateau. Calcium comes in slowly, balancing potassium going out. This is the ST segment. This is where the ventricle actually squeezes — where the mechanical force of contraction happens. The cell is holding its charge so the muscle can do its work.

The ST segment is not a flat line. It is a held breath. The moment of force.

Phase 3 — repolarisation. Potassium rushes out. The cell resets. This is the T wave — the heart preparing for the next beat.

Phase 4 — rest. The pumps restore the balance. Sodium back out, potassium back in. Ready again.

Here is what gets me about all of this.

Every ECG you have ever seen is just the sum of billions of these five-phase cycles happening across the heart wall at slightly different times. The P wave, the QRS, the T wave — they are not abstract lines on a graph. They are the electrical signature of sodium and potassium and calcium moving through channels that evolution spent hundreds of millions of years perfecting.

When an ST segment elevates, it means something went wrong in phase 2. When the QT prolongs, it means phase 3 is running late. When a QRS widens, the relay race broke down somewhere in the bundle branches.

The ECG is not a test. It is a translation. Five phases, written in voltage, across twelve leads.

Once you see it that way, you never look at a tracing the same way again.