Describe how a heartbeat is initiated and coordinated?

The sinoatrial node (SAN) sends a wave of electrical activity across the atria causing atrial contraction. The non-conducting tissue stops the immediate contraction of the ventricles. The atrioventricular node (AVN) delays the impulse whilst the blood leaves the atria and fills the ventricles. After the ventricle fills, the AVN sends a wave of electrical activity down the Bundle of His which causes the ventricles to contract from the base up.