What do the SA node and AV node do?

Cardiac muscles has intrinsic rhythmicity. That mean the muscle fibre contract rhythmically. The different types of cardiac muscle fibres contract at at different rhythm. SA node contract at the rate of 72 beats / minute. The other atrial muscle fibres contract at the rate of 60 beats / minute. The AV node contracts at the rate of 50 beats /minute. The ventricle muscle fibres contract at the rate of 36 beats / minute. The fibres from the bundle of His contract in between 50 to 36 beats / minute. So the SA nodes contract at the fastest rate and it is a 'Pace maker'. That means SA node decides the rate of contraction of your heart. It is supplied by sympathetic as well as parasympathetic innervation to rise or lower the heart rate. In AV node you have the conduction of the impulse slowed down for a movement. So that the ventricles should contract 'after' the contraction of the atria. This helps you to have proper pumping of the blood in forward direction.
Cardiac muscles has intrinsic rhythmicity. That mean the muscle fibre contract rhythmically. The different types of cardiac muscle fibres contract at at different rhythm. SA node contract at the rate of 72 beats / minute. The other atrial muscle fibres contract at the rate of 60 beats / minute. The AV node contracts at the rate of 50 beats /minute. The ventricle muscle fibres contract at the rate of 36 beats / minute. The fibres from the bundle of His contract in between 50 to 36 beats / minute. So the SA nodes contract at the fastest rate and it is a 'Pace maker'. That means SA node decides the rate of contraction of your heart. It is supplied by sympathetic as well as parasympathetic innervation to rise or lower the heart rate. In AV node you have the conduction of the impulse slowed down for a movement. So that the ventricles should contract 'after' the contraction of the atria. This helps you to have proper pumping of the blood in forward direction.