Describe the electrical activity of the heart.

The heart is known as myogenic and can beat of its own accord, and this is tightly controlled to ensure the optimum blood flow through the heart to deliver oxygen to the rest of the body. 

Diastole refers to when the heart muscle is relaxed and systole refers to when the heart muscle is contracting. 

The pacemaker of the heart is the Sino-Atrial node (SAN) and this is found in the wall of the right ventricle. As blood pools here from the Vena Cava, depolarisation of the SAN causes contraction of both the atria to push blood into the ventricles. 

The electric signal from the SAN reaches the atrio-ventricular node (AVN) found in the septum between the two atria. This delays the electrical signal spreading across the ventriles to allow time for the ventricles to sufficiently fill with blood. Once ready, the AVN passes the signal down the Bundle of His which are located in the walls of the ventricle to cause coordinated and sequential contraction to expel blood out of the ventricles. 

Extrinsic factors such as the autonomic nervous system and adrenaline can affect the firing of the SAN to increase or decrease heart rate.