How does the wave of conduction pass through the heart?

Firstly, the 'wave of excitation' is produced by the sinoatrial node (SAN) in the top of the right atrium - this is also known as the pacemaker, as this is what controls the rate of contractions. This wave passes through both atria, causing the muscle in the walls to contract. The wall between the atria and ventricles has a high resistance, so the electrical current can only pass to the ventricles through one specialised part of the septum called the atrioventricular node (AVN) - this passes on the wave of conduction to the ventricles after about 0.1s, causing a delay. This delay means that the ventricles do not contract at the same time as the atria - this is important as they need to fill with blood from the atria first.

The AVN then passes the wave on to the Purkinje fibres in the interventricular septum (between the ventricles), which then splits into the right and left bundle branches. The excitation spreads from the apex of the heart up through the ventricular walls, ensuring blood is pumped upwards out of the ventricles into the aorta (left) and pulmonary arteries (right). There is a short period of rest, before the SAN initiates the next contraction.