The roles of the sinoatrial node (SAN), atrioventricular node (AVN) and…

The idea

The SAN in the right atrium depolarises spontaneously, setting the heart rate. The wave of depolarisation spreads over both atria, causing contraction. Blocked by a non-conducting collagen layer, the wave reaches the ventricles via the atrioventricular node (AVN). The AVN delays the wave for approximately 0.1 seconds, allowing atria to empty and ventricles to fill. The AVN passes the wave into the bundle of His, which divides into Purkyne tissue spreading up the ventricle walls from the apex. This causes ventricles to contract from the bottom upwards, forcing blood out. Separately, ions pass between cardiac cells via gap junctions.

What earns the marks

• The SAN initiates the wave of depolarisation, causing the atria to contract.
• The AVN delays the wave, allowing the atria to empty and ventricles to fill before ventricular systole.
• The bundle of His conducts the wave down the septum to the apex of the heart.
• Purkyne tissue carries the wave up the ventricle walls, causing contraction from the apex upwards.
• Gap junctions allow ions to move between cardiac muscle cells, distinct from the main conducting tissue.

Common mistakes and how to put them right

• Using vague terms like 'signals' or 'messages' instead of 'impulses' or 'waves of depolarisation'.

• Writing that the AVN speeds the impulse up, when its crucial function is to introduce a delay.

• Saying the ventricles contract from the top downwards, which would trap blood at the apex.

Exam tips

• Learn the pathway as distinct steps: SAN, atria, AVN delay, bundle of His, and Purkyne tissue.
• Always explain the purpose of the AVN delay by explicitly mentioning atrial emptying and ventricular filling.