⚡ Cardiovascular System Lesson

SA → AV → Bundle → Purkinje: the conduction pathway

Every heartbeat starts as an electrical signal, not a mechanical one. This pathway is the exact route that signal follows through the heart, in order.

Pacemaker

Delay

His

Only link

L/R spread

Purk.

Ventricles

📖 Full Breakdown

The five stops of the cardiac electrical signal

This exact sequence corresponds directly to the waves you'll see on an ECG in the next lesson.

SA node

The natural pacemaker

Located in the right atrium. Fires spontaneously at 60–100 bpm, initiating every normal heartbeat. This is what the P wave on an ECG represents.

AV node

The gatekeeper

Delays the signal by about 0.1 second before passing it to the ventricles — this pause allows the atria to finish contracting and fully fill the ventricles first. This delay is the PR interval on ECG.

Bundle of His

The only bridge

The sole electrical connection between the atria and ventricles — all signal must pass through here, since the fibrous skeleton of the heart otherwise electrically insulates atria from ventricles.

Bundle branches

Left and right

Carry the signal down either side of the interventricular septum toward each ventricle.

Purkinje fibers

Rapid ventricular spread

Distribute the signal rapidly throughout the ventricular myocardium, producing a coordinated, near-simultaneous contraction. This produces the QRS complex on ECG.

🩺 Clinical / Exam Application

A patient's heart rate drops to 45 bpm with a wide, unusual QRS pattern on the monitor. If the SA node fails, the AV node can take over as a backup pacemaker, but only at 40–60 bpm — slower than normal and without the AV node's usual delay function intact. If the AV node also fails, the ventricles themselves can generate an even slower, less reliable rhythm around 20–40 bpm. Knowing this back-up hierarchy tells clinicians how urgently a pacemaker is needed based on which level of the conduction system has failed.

⚠️ Exam Alert

A frequently tested detail: if you damage the Bundle of His itself (not just the AV node), you get complete heart block because it is the ONLY electrical pathway between atria and ventricles — there is no alternate route for the signal to take.

🚧 Common Trap

Students often think the AV node is just a "relay station" with no independent function. In fact its 0.1-second delay is functionally essential — without it, the ventricles would contract before finishing filling from the atria, reducing cardiac output on every single beat.

✅ Quick Check

If a patient's SA node fails completely, what backup pacemaker takes over, and at approximately what rate?

📝 Exam Prep

Common Exam Questions

❓ What is the normal pacemaker of the heart and where is it located?

✅ The SA (sinoatrial) node — located in the right atrium. It fires at 60–100 bpm and initiates every normal heartbeat.

❓ Why does the AV node delay the signal before it reaches the ventricles?

✅ The 0.1-second delay allows the atria to finish contracting and completely fill the ventricles before ventricular contraction begins, maximizing the blood volume pumped on each beat.