Biology – The heart | e-Consult

The cardiac cycle is initiated by the sinoatrial node (SAN), often referred to as the heart's natural pacemaker. The SAN spontaneously generates electrical impulses, initiating atrial contraction. These impulses then spread throughout the atria, causing them to contract and effectively pump blood into the ventricles.

The electrical impulse then reaches the atrioventricular node (AVN). The AVN acts as a gatekeeper, delaying the impulse slightly. This delay is crucial because it allows the atria to fully contract and empty their contents into the ventricles before ventricular contraction begins. The AVN then transmits the impulse down the bundle of His.

The Purkinje tissue is a network of fibres that rapidly conducts the electrical impulse throughout the ventricles. This rapid conduction ensures that the ventricles contract in a coordinated manner, effectively pumping blood out to the pulmonary artery and aorta. The Purkinje fibres allow for a rapid and widespread depolarization of the ventricular muscle cells, leading to a powerful and efficient contraction.

Sequence of events:

1. SAN firing: Electrical impulse generated.
2. Atrial contraction: Impulse spreads through atria, causing contraction.
3. AVN delay: Impulse is delayed at the AVN.
4. Ventricular contraction: Impulse travels down the bundle of His and Purkinje fibres, causing ventricular contraction.