During the cardiac cycle , there is a delay between the excitation of the atria and excitation of the ventricles. Explain how this is achieved and why this delay is essential.

Excitation from SAN (sino-atrial node) spreads across atria, causing them to contract first. The excitation is conducted to the AV (atrio-ventricular) node, which introduces a 0.12s delay. This refractory period allows the atria to fully contract, allowing the blood into the ventricles before the ventricles contract.