Describe how the SAN and AVN control the cardiac cycle.

The student should think about this in steps starting with:

1. SAN initiates the heartbeat: The sinoatrial node (SAN) initates heart contraction by sending electrical impulses across the atria. These are initiated spontaneously because the myocytes (heart cells) are myogenic.

2. Contraction of atria: This causes atrial contraction.

3. Delay at AVN: These impulses reach the atrio-ventricular node (AVN) which then delays the impulse any further to allow the atria to contract and push the blood in the atria through to the ventricles, emptying the atria.

4. AVN then passes further electrical impulses to base of heart: The AVN then sends the electrical impulses down the Purkinje fibres of the heart to its apex.

5. Ventricular contraction: Ventricular contraction is initated from the base of the heart upwards to empty the blood from the ventricles into the aorta and pulmonary artery. 

Words that an be exchanged: waves of electrical activity/ impulses.

If possible try and write out the full names of the acronyms: SAN and AVN.