What are the events in the cardiac cycle?

The cardiac cycle is the series of events that leads to blood being pumped to the lungs and around the body.

The cycle begins with the SAN (sino-atrial node). This is the hearts 'pacemaker'. It sends out a nerve impulse which spreads over the atria and causes them to contract (atria systole), forcing blood into the ventricals. The signal is not able to spread to the ventricals as a layer of non-connective tissue blocks the signal at the base of the atria.

The signal from the SAN is recieved at the AVN (atrio-ventricular node) at the top of the ventricals. This node sends a signal down the perkyne (or purkinje) fibres which travel in the insulating Bundle of His through the inter-ventrica septum to the base of the ventricles.

At the base of the ventricals (apex) a wave of contraction passes up the ventricals causing them to contract pushing blood up and out of the ventricals.

These events can be tracked on an ECG trace.