The cardiac conduction system !

Concept
Exam priority: high

A wiring path carries each beat from the top of the heart to the bottom, with a short pause so the atria empty before the ventricles squeeze.

Updated 2026-11-01

Also called: SA node, AV node, His–Purkinje system

See it in 3D: Sinoatrial (SA) node · Atrioventricular (AV) node · Bundle of His · Bundle branches · Interventricular septum · Right coronary artery

What it is

The impulse starts in the SA node, spreads across both atria, is delayed in the AV node, then runs fast down the bundle of His, the left and right bundle branches and the Purkinje fibres. The delay lets the atria finish filling the ventricles; the fast distal system makes both ventricles contract almost together, from apex to base.

Key facts

SA node: subepicardial, at the junction of the superior vena cava and right atrium; usually supplied by the right coronary artery.
AV node: in the floor of the right atrium (triangle of Koch), near the coronary sinus opening; supplied by the right coronary artery in most people.
The left bundle branch splits into anterior and posterior fascicles; the right bundle runs in the moderator band.
Nodal tissue is slow because it has few gap junctions, small cells and a Ca²⁺-driven upstroke.

Electrocardiogram: a normal beat in lead II

HR 75 /minRR 0.80 sPR 0.16 sQRS 0.09 sQT 0.37 sQTc 0.41 s

Tap a wave (P, QRS, T, U), an interval (PR, ST, QT) or the RR.

ECG trace (lead II)
Small box: 0.04 s · 0.1 mV
Big box: 0.20 s · 0.5 mV
Selected part

Tap the PR interval and the QRS to see which part of the conduction system each one measures.

Action potential

Maximum diastolic potential −60 mVThreshold −40 mVPeak +10 mVCycle ≈ 700 ms (≈ 85 bpm)

Tap a phase (the pills above or its band) or a refractory period (ARP, RRP) at the base of the plot.

Mainly Na⁺
Mainly K⁺
Mainly Ca²⁺
Maximum diastolic potential
Threshold
ARP: absolute refractory period
RRP: relative refractory period

Illustrative model drawn by Durgical from typical values; each cell type has its own time scale and values are approximate, not for clinical decisions.

Conduction speeds and times (approximate)

Parameter	Value	Note
Atrial muscle	≈ 0.3–1 m/s
AV node	≈ 0.02–0.05 m/s
His–Purkinje system	≈ 2–4 m/s
Ventricular muscle	≈ 0.3–0.5 m/s
PR interval	0.12–0.20 s
QRS duration	< 0.12 s	usually 0.06–0.10 s

Approximate values for a healthy adult at rest; each laboratory sets its own ranges.

Clinical notes

AV block: first degree = long PR; second degree = some P waves not conducted; third degree = atria and ventricles beat independently.
Inferior myocardial infarction (right coronary artery) often causes AV-nodal block and bradycardia, frequently transient.
Chagas cardiomyopathy typically damages the right bundle branch and the left anterior fascicle.

Test yourself

Think of the answer, then tap the question to see it.

What is the physiological purpose of the AV-nodal delay?

It gives the atria time to contract and top up ventricular filling before the ventricles contract, and it limits how many fast atrial impulses reach the ventricles.

In atrial fibrillation with an atrial rate above 400/min, why is the ventricular rate much lower?

The AV node's slow conduction and long refractory period block most impulses, so only a fraction pass to the ventricles.

A patient has a QRS of 0.14 s with a normal PR. Where is the conduction problem most likely?

Below the His bundle, in a bundle branch (or the ventricle itself), because spread through the ventricles is slow while atrial and AV-nodal conduction are normal.

Why does an inferior myocardial infarction often slow AV conduction?

In most people the right coronary artery supplies the AV node, so its occlusion causes nodal ischemia and vagal reflexes that slow or block conduction.

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See it in 3D

The structures of Durgical's 3D anatomy where it happens.

In Pharmacology

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Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: 19.2 Cardiac Muscle and Electrical Activity (OpenStax (Rice University), 2022); Physiology, AV Junction (StatPearls (NCBI Bookshelf), 2025); Atrioventricular Block (StatPearls (NCBI Bookshelf), 2025); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 10). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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Representative model, for educational purposes only.