Transport in animals: (g) how heart action is initiated and coordinated

The idea

Heart action is initiated and coordinated by the heart's own conduction system, which generates and spreads electrical activity so contraction is rhythmic and coordinated. The sinoatrial node (SAN) in the right atrial wall acts as the pacemaker, spontaneously generating impulses that spread across the atria, causing atrial contraction. A layer of non-conducting tissue between atria and ventricles prevents impulses passing directly, so the atrioventricular node (AVN) receives the impulse and introduces a delay, allowing the atria to finish emptying before the ventricles contract. The impulse then travels down the bundle of His and Purkyne fibres to the ventricular apex and up the ventricle walls, causing coordinated ventricular contraction from the apex upwards.

What earns the marks

• States that the sinoatrial node (SAN) in the right atrial wall initiates each heartbeat and acts as the pacemaker, spontaneously generating electrical impulses.
• Explains that impulses from the SAN spread across the atria, causing the atria to contract.
• Explains that non-conducting tissue between the atria and ventricles prevents impulses passing directly, so the impulse is relayed via the atrioventricular node (AVN).
• Explains that the AVN introduces a delay so the atria finish emptying before the ventricles contract.
• Describes the impulse travelling from the AVN down the bundle of His and through Purkyne fibres to the ventricular walls, causing coordinated ventricular contraction.
• Explains that Purkyne fibres carry the impulse to the ventricular apex so the ventricles contract from the apex upwards, aiding efficient ejection of blood.

Common mistakes and how to put them right

• Naming the atrioventricular node as the pacemaker.

  Put it right by stating that the sinoatrial node initiates the heartbeat and acts as the pacemaker.

• Thinking the AVN delay is a fault or unnecessary.

  Put it right by explaining that the delay allows the atria to empty before ventricular contraction.

• Believing impulses pass directly from atria to ventricles through muscle.

  Put it right by explaining that non-conducting tissue separates them and the impulse must travel via the AVN and conduction fibres.

• Thinking Purkyne fibres generate the impulse.

  Put it right by stating that they conduct the impulse from the AVN to the ventricular walls.

Exam tips

• Name each structure in order (SAN, AVN, bundle of His, Purkyne fibres) and give its function, since questions often test the role of each component.
• Always explain the purpose of the AVN delay and the non-conducting tissue, as these are frequently examined reasons rather than mere facts.
• Link the direction of impulse spread through the ventricles to why contraction from the apex upwards aids ejection of blood.