Step by Step
SA
SA node — the pacemaker
Located in the right atrium near the superior vena cava, the sinoatrial (SA) node fires spontaneously at 60-100 beats per minute, setting the overall heart rate. Its signal spreads across both atria, causing them to contract.
AV
AV node — the intentional delay
The atrioventricular (AV) node sits at the junction between the atria and ventricles. It introduces a brief, deliberate delay (about 0.1 seconds) — this is the only electrical pathway connecting the atria to the ventricles, and the delay allows the atria to finish emptying into the ventricles before the ventricles begin contracting.
His/Branch
Bundle of His and bundle branches
The signal travels through the Bundle of His, down through the interventricular septum, then splits into left and right bundle branches that travel down each side of the septum.
Purk
Purkinje fibers — bottom-up contraction
Purkinje fibers spread the signal rapidly through the ventricular walls, starting from the apex (bottom) of the heart and moving upward — this bottom-up contraction pattern efficiently ejects blood upward into the great vessels.
Applied Walkthrough
1
The SA node fires spontaneously, initiating an electrical signal that spreads across both atria, causing them to contract and push blood into the ventricles.
2
The signal reaches the AV node, where it's deliberately delayed by about 0.1 seconds — giving the atria just enough extra time to finish fully emptying their contents into the ventricles before the ventricles start contracting.
3
The signal then passes through the Bundle of His and splits into the left and right bundle branches, traveling down through the interventricular septum.
4
Finally, Purkinje fibers spread the signal through the ventricular walls from the apex upward, causing the ventricles to contract from the bottom up — efficiently squeezing blood upward and out through the great vessels, exactly the direction needed for ejection.
Exam Application
Exams test whether you can correctly sequence the conduction pathway (SA → AV → Bundle of His → bundle branches → Purkinje fibers), and whether you understand why the AV node's delay and the Purkinje fibers' bottom-up spread are both functionally necessary.
⚠ Common Trap
The most common trap is not understanding why the AV delay matters — without it, the ventricles would start contracting before the atria finished emptying, resulting in inefficient, poorly-timed pumping.
✓ Quick Self-Check
1. What is the SA node, and what is its role?
The heart's natural pacemaker, located in the right atrium, which sets the heart rate by firing spontaneously at 60-100 bpm.
Tap to reveal / hide
2. What is the function of the AV node's delay?
To allow the atria to finish emptying into the ventricles before the ventricles begin contracting.
Tap to reveal / hide
3. What is the only electrical pathway between the atria and ventricles?
The AV node.
Tap to reveal / hide
4. Why do Purkinje fibers spread the signal from the apex upward?
So the ventricles contract from the bottom up, efficiently ejecting blood upward into the great vessels.
Tap to reveal / hide
5. What does the QRS complex on an ECG represent?
Ventricular depolarization.
Tap to reveal / hide