The Cardiac Cycle

1. Systole and Diastole
2. Co-ordination of the Heart
3. Wiggers Diagram
4. The Cardiac Cycle
   
   • Stroke Volume
5. Quiz

• There are seven stages of the cardiac cycle, split into systole and diastole.
• The conduction system consists of the SA node, the AV node, the Bundle of His and the Purkinje fibres.
• The cardiac cycle can be used to explain the different heart sounds heard (closing of valves) and the changes seen in an ECG.

The heart is an amazing pump, being able to pump 5 litres per minute, 7000 litres per day. The stages of pumping are split into diastole and systole.

Systole is when the ventricles of the heart contract to pump blood into the pulmonary or systemic circulation.

Diastole is when the ventricles relax and fill with blood.

Image - The way blood pressure is shown in clinical practice. The top number is the pressure within the vessels during systole. The bottom number is the pressure within the vessels during diastole

SimpleMed original by Dr. Marcus Judge

Co-ordination of the Heart

There is a conduction system in the heart that sends electrical impulses across the myocardium (muscular tissue of the heart) to control the contraction of the atria and ventricles. The steps of the system are:

1. Pacemaker cells located within the sinoatrial (SA) node in the wall of the right atrium spontaneously depolarise, generating an electrical impulse which spreads across the atria and causes them to contract.

2. There is a fibrous band of tissue that separates the atria from the ventricles. This prevents the impulse from spreading directly to the ventricles.

3. Instead, the impulse reaches the atrioventricular (AV) node which is located between the atria and is the only physiological path through the fibrous band.

4. The AV node delays the pulse for a short period before transmitting the impulse down the bundle of His (specialised conduction fibres).

5. Once reaching the end of the bundle of His, the impulse continues to spread through the ventricle walls via the Purkinje fibres.

6. The impulse then spreads up the walls of the ventricles and causes them to contract.

This system allows the ventricles to contract in a coordinated fashion from the apex upwards towards the base of the heart to maximise the amount of blood ejected - similar to squeezing a tube of toothpaste from the bottom rather than the middle.

The regulation of the cardiac cycle and the mechanism of the action potentials is discussed in detail in the article on Cardiac Conduction.

Preview the The Cardiac Cycle quiz