Co-ordination of the Cardiac CycleSBI3U

• The heart is made of cardiac muscle.

• When the cells receive an electrical impulse they contract - causing a heartbeat.

• Cardiac muscle is myogenic - it can contract on its own, without needing nerve impulses.

Sinoatrial node (SA node)

• This specialized node is found on the upper inside wall of the right atrium.

• The SA node is known as the pacemaker of the heart and initiates a heartbeat every 0.85 seconds.

• This signal travels across the atria causing them to contract and load the ventricles with blood.

• Ventricles are electrically insulated from atria - so they don’t contract yet.

Atrio-ventricular node (AV node)

• The AV node is located on the bottom surface of the right atria and is responsible for initiating the contraction of the ventricles.

• Electrical impulse passes to ventricles via AV node and the Bundle of His. They pass the impulse to the base of the ventricles (~ 0.1 s delay).

• The bundle of His is a group of fibres that conduct impulses to Purkyne fibres which carry impulses to left & right ventricles.

• Ventricles then contract from the bottom upwards.

No impulse

• Cardiac muscle relaxes = diastole

Electrocardiograph

P = atrial systole

QRS = ventricular systole

T = ventricular diastole

• Records electrical activity of heart to monitor heart function.

• A Pacemaker generates electrical currents in body fluids around the heart - detected by electrodes to produce ECG

• Heart muscle:depolarises (loses electrical charge) when it contractsrepolarises (regains charge) when it relaxes

Ventricular fibrillation is an abnormal heart rhythm that is disorganized and irregular.

Ventricular tachycardia is a rapid, regular heart rhythm that originates in the lower chambers of the heart.