The Cardiac Cycle Relate the events during the cardiac cycle to the maintenance of blood flow through the heart Describe the relationship between pressure and volume changes in the heart and aorta and events in the cardiac cycle Discuss the roles of the sinoatrial node, atrioventricular node and bundle of His

Cardiac Muscle Cardiac muscle is able to contract spontaneously without nervous or hormonal stimulation (if provided woth oxygen, nutrients and the right mixture of mineral salts) These contractions are called myogenic.

The Pacemaker – the SAN The cycle starts at the sinoatrial node (SA node or SAN). This is a small patch of tissue that has its own natural rhythm of contraction. The rhythm is made faster or slower by nervous impulses and hormones. The SAN determines the rate of contraction of the rest of the cardiac muscle.

Cardiac Impulses The SAN generates waves of electrical impulses over the atria. When these reach the junction between the atria and ventricles they are delayed by the atrioventricular septum. This delay gives time for the waves of contraction to spread over the atria.

Atrioventricular node (AV node) The atrioventricular septum is a layer of non-conducting connective tissue. It completely separates the atria from the ventricles except for a region in the right atrium called the atrioventricular node (AV node) The AV node is the only route of transmission for the cardiac impulse. It acts as a second pacemaker region, relaying the cardiac impulse to the ventricles

What if the SAN misfunctions? The AV node will then take over but it is slower than the inherent rhythm

Systole (contraction) The cardiac cycle is relayed from the AV node over the ventricles through a bundle of specialised muscle fibres called the bundle of His. This bundle branches into other fibres called the Purkyne fibres (or Purkinje fibres)

Ventricular systole (contraction) The cardiac impulse passing through the Purkyne fibres causes a wave of contraction. It starts at the apex of the heart (bottom) and passes rapidly over the ventricles (ventricular systole).

Regions close to the AV node have thin Purkyne fibres which carry impulses more slowly than the thick fibres that supply the more distant parts of the ventricles. This ensures that all parts of the ventricles contract more or less simultaneously.

Diastole (relaxation) The whole heart realxes after ventricular systole, allowing blood from the veins to fill up the heart. This is called diastole.

Pressure changes in the heart The diagram shows the pressure changes during a cardiac cycle on the left side of the heart Note that the maximal pressure in the left ventricle is far higher than in the atrium This is because the ventricle has to work hard to pump blood to all parts of the body (except the lungs) The atrium only pumps blood into the ventricle.

Electrical change in the heart: the ECG Cardiac muscle contracts as a result of electrical stimulation. A wave of electrical charge is initiated in the pacemaker region and spreads across the heart This generates an electric current in the body fluids around the heart The currents can be detected ob the body surface using recording electrodes.

Electrocardiogram ECG A cathode ray oscilloscope or chart recorder is used to show the ECG The P wave is caused by atrial systole The QRS wave is caused by ventricular systole The T wave coincides with ventricular diastole

Heart Rate This can be calculated from the interval between one P wave and the next.

Check your understanding 1. List the sequence of main events during the cardiac cycle 2. Why does the pressure in the left ventricle reach a higher peak than the blood pressure in the left atrium? 3. Which part of the heart normally initiates the cardiac cycle?