Difficulty: Intermediate | Prerequisites: Basic cardiac anatomy (four chambers, valves, pulmonary vs systemic circuits). Review Chapter 12 if you are unsure about the physical layout of the heart.
This material covers how the heart generates and conducts its own electrical signals, how those signals appear on an ECG tracing, and how the mechanical events of the cardiac cycle (valve openings, pressure changes, volume changes) follow from that electrical activity. It sits at the intersection of electrophysiology and haemodynamics, and nearly every exam question on cardiac function depends on understanding the sequence: electrical event first, mechanical event second. If you know the conduction pathway and can map each ECG wave to what the heart is physically doing at that moment, the clinical scenarios become straightforward.
The SA node fires first and sets the pace. The signal travels through the AV node (which deliberately delays it so the atria finish contracting), then down the Bundle of His and Purkinje fibres to trigger ventricular contraction. On an ECG, the P wave is atrial depolarisation, the QRS complex is ventricular depolarisation, and the T wave is ventricular repolarisation. The cardiac cycle alternates between phases where all valves are closed (isovolumetric contraction and relaxation) and phases where blood is moving (filling or ejection), and the two heart sounds (S1, S2) mark valve closures.
SA node (sinoatrial node)
The heart's primary pacemaker, located in the right atrium. It spontaneously depolarises at the fastest intrinsic rate (roughly 70-80 bpm), which is why it normally sets the rhythm. Think of it as the conductor of the orchestra: everyone else could play on their own, but the SA node keeps them in time.
AV node (atrioventricular node)
The only electrical connection between the atria and the ventricles. It deliberately slows conduction (the AV nodal delay) so the atria have time to finish emptying before the ventricles contract. If the SA node fails, the AV node can take over as pacemaker, though at a slower intrinsic rate (roughly 40-60 bpm).
Bundle of His and Purkinje fibres
The fast-conducting pathways that carry the electrical signal from the AV node down through the interventricular septum and out to the ventricular myocardium. The Purkinje fibres ensure the ventricles depolarise from apex to base, producing an efficient, coordinated squeeze.
P wave
The ECG deflection representing atrial depolarisation. In simple terms, this is the electrical signal spreading across both atria just before they contract.
QRS complex
The ECG deflection representing ventricular depolarisation. It is larger than the P wave because the ventricular muscle mass is much greater. Atrial repolarisation happens during this time but is hidden within the QRS.
T wave
The ECG deflection representing ventricular repolarisation, the electrical "resetting" of the ventricles. In simple terms, this is the ventricles recovering and preparing for the next beat.
PR interval
The time from the start of the P wave to the start of the QRS complex. It captures the AV nodal delay. A prolonged PR interval means conduction through the AV node is abnormally slow.
Isovolumetric contraction
The brief phase after AV valve closure and before semilunar valve opening, during which the ventricles are contracting but no blood is leaving (all four valves are shut). Ventricular pressure rises steeply.
Isovolumetric relaxation
The brief phase after semilunar valve closure and before AV valve opening, during which the ventricles are relaxing but no blood is entering. Ventricular pressure drops steeply.
S1 (first heart sound)
Produced by closure of the AV valves (mitral and tricuspid) at the start of ventricular systole.
S2 (second heart sound)
Produced by closure of the semilunar valves (aortic and pulmonary) at the start of ventricular diastole.
The SA node depolarises spontaneously and sets the heart rate. It has the fastest intrinsic rate, so it "wins" control of the rhythm.
The impulse spreads across both atria (producing the P wave), then converges on the AV node.
The AV node introduces a deliberate delay (approximately 0.1 seconds). This is the single most important timing mechanism in the heart: it ensures the atria finish contracting and emptying blood into the ventricles before the ventricles fire.
From the AV node, the signal travels rapidly down the Bundle of His, splits into left and right bundle branches, and fans out through the Purkinje fibres to the ventricular myocardium.
If the SA node fails entirely, the AV node becomes the backup pacemaker (at roughly 40-60 bpm). If the AV node also fails, the Purkinje fibres can generate a very slow escape rhythm (roughly 20-40 bpm).
P wave: atrial depolarisation. Every normal heartbeat starts with a P wave.
PR interval: the time from P wave onset to QRS onset. Captures the AV nodal delay. Normal is roughly 0.12-0.20 seconds. A prolonged PR interval means something is slowing conduction through the AV node.
QRS complex: ventricular depolarisation. The large amplitude reflects the large ventricular muscle mass. Atrial repolarisation is buried inside it and not visible.
T wave: ventricular repolarisation. The ventricles electrically reset before the next cycle.
QT interval: from QRS onset to T wave end, capturing total ventricular electrical activity (depolarisation plus repolarisation).
ST segment: the flat segment between the QRS and T wave. It represents the plateau phase of the ventricular action potential, when the ventricles are fully depolarised.
Ventricular filling (diastole): AV valves are open, semilunar valves are closed. Blood flows passively from atria into ventricles. Atrial systole (the atrial "kick") tops off filling at the end, contributing roughly 20% of total ventricular filling at rest.
Isovolumetric contraction: AV valves snap shut (producing S1), semilunar valves not yet open. All four valves are closed. Ventricular pressure rises rapidly but volume does not change.
Ventricular ejection: ventricular pressure exceeds arterial pressure, semilunar valves open, blood is ejected into the aorta and pulmonary artery.
Isovolumetric relaxation: semilunar valves snap shut (producing S2), AV valves not yet open. All four valves are closed again. Ventricular pressure falls rapidly but volume does not change.
The cycle then repeats with ventricular filling.
S1 marks the beginning of systole (AV valve closure).
S2 marks the beginning of diastole (semilunar valve closure).
During ventricular ejection, only the semilunar valves (aortic and pulmonary) are open.
During ventricular filling, only the AV valves (mitral and tricuspid) are open.
Heart rate from ECG: HR = 300 / (number of large boxes between R-R peaks)
Normal PR interval: 0.12 to 0.20 seconds (3-5 small boxes)
Normal QRS duration: less than 0.12 seconds (less than 3 small boxes)
Intrinsic pacemaker rates: SA node ~70-80 bpm; AV node ~40-60 bpm; Purkinje fibres ~20-40 bpm
Valve event sequence (one cycle): AV valves close (S1) → isovolumetric contraction → semilunar valves open → ejection → semilunar valves close (S2) → isovolumetric relaxation → AV valves open → filling
The PR interval is the reason clinicians monitor ECGs for heart block: a progressively lengthening PR interval (first-degree or second-degree AV block) tells you the AV node is struggling to conduct, and can flag the need for a pacemaker before the patient becomes symptomatic. The AV nodal delay itself is why atrial fibrillation, despite chaotic atrial activity, does not always cause dangerously fast ventricular rates: the AV node acts as a gatekeeper, filtering out many of the chaotic impulses.
Students often think the AV node is the primary pacemaker. It is the backup. The SA node normally drives the rhythm because its intrinsic rate is faster.
Students often confuse the T wave with atrial repolarisation. The T wave is ventricular repolarisation. Atrial repolarisation occurs during the QRS complex and is not separately visible.
Students sometimes assume "isovolumetric" means nothing is happening. Pressure is changing dramatically during these phases; it is volume that stays constant because all valves are closed.
Students often mix up which valve closure produces which heart sound. S1 = AV valves (start of systole). S2 = semilunar valves (start of diastole).
⚠️ The conduction pathway sequence (SA node → AV node → Bundle of His → Purkinje fibres) is tested almost every year. Know the order and what happens when each component fails.
⚠️ Matching ECG waves to electrical events is a favourite multiple-choice format: P wave = atrial depolarisation, QRS = ventricular depolarisation, T wave = ventricular repolarisation.
⚠️ The cardiac cycle phase sequence and which valves are open or closed during each phase appear in scenario questions. "What happens immediately after AV valve closure?" = isovolumetric contraction.
⚠️ A prolonged PR interval on ECG = delayed AV conduction. This is a standard clinical application question.
⚠️ Expect a question on what happens at high heart rates: filling time decreases (diastole shortens), which can reduce stroke volume.
True or false: The AV node normally has a faster intrinsic rate than the SA node. (False. The SA node is fastest, which is why it sets the rhythm.)
Fill in the blank: The ECG wave that represents ventricular repolarisation is the ____. (T wave)
True or false: During isovolumetric contraction, the semilunar valves are open. (False. All four valves are closed.)
Fill in the blank: Closure of the AV valves produces the ____ heart sound. (S1, first)
True or false: A prolonged PR interval suggests delayed conduction through the Bundle of His. (False. It primarily reflects delayed conduction through the AV node.)
Q: Which structure normally initiates each heartbeat?
A: The SA node. It has the highest intrinsic depolarisation rate and therefore sets the rhythm for the entire heart.
Q: If the SA node fails completely, which structure is most likely to become the new pacemaker?
A: The AV node, because it has the next-fastest intrinsic rate (roughly 40-60 bpm).
Q: What is the primary purpose of the AV nodal delay?
A: To allow the atria enough time to finish contracting and emptying blood into the ventricles before ventricular contraction begins.
Q: Which ECG component corresponds to ventricular depolarisation?
A: The QRS complex.
Q: Which event immediately follows closure of the AV valves?
A: Isovolumetric contraction. The ventricles are contracting with all valves closed, building pressure.
Q: Which valves are open during ventricular ejection?
A: The semilunar valves (aortic and pulmonary). The AV valves are closed.
Q: A patient has delayed conduction through the AV node. Which ECG interval is prolonged?
A: The PR interval, because it captures the time from atrial depolarisation through the AV nodal delay to the onset of ventricular depolarisation.
Q: What does the T wave on the ECG represent?
A: Ventricular repolarisation.
Q: A patient has a heart rate of 130 bpm. Assuming ventricular filling time decreases substantially, which variable is most likely to decrease first?
A: Stroke volume. Shorter filling time means less blood enters the ventricles (lower EDV), so each beat ejects less blood.
Q: Closure of the semilunar valves produces which heart sound?
A: S2 (the second heart sound).
This connects directly to cardiac output and stroke volume (Part 2 of these notes), because the electrical events covered here are the trigger for the mechanical pumping that determines how much blood the heart ejects per beat. It also connects to blood pressure regulation (Part 3), because the autonomic nervous system (sympathetic and parasympathetic) modulates the SA node firing rate and AV conduction speed to adjust heart rate as part of the baroreceptor reflex. Understanding the cardiac cycle phases is essential for interpreting pressure-volume loops, which may appear in more advanced coursework.
SA node, sinoatrial node, AV node, atrioventricular node, Bundle of His, Purkinje fibres, Purkinje fibers, cardiac conduction system, pacemaker cells, autorhythmicity, ECG, EKG, electrocardiogram, P wave, QRS complex, T wave, PR interval, QT interval, ST segment, atrial depolarisation, ventricular depolarisation, ventricular repolarisation, cardiac cycle, systole, diastole, isovolumetric contraction, isovolumetric relaxation, ventricular ejection, ventricular filling, S1 heart sound, S2 heart sound, AV valve closure, semilunar valve closure, mitral valve, tricuspid valve, aortic valve, pulmonary valve, APK2105, UF anatomy and physiology, Chapter 13, Chapter 14