1. The Cardiac Cycle

2.  0.0 – 0.05 seconds: Atrial depolarisation: a wave of depolarisation spreads over the atria from the Sino Atrial Node initiating the contraction of the atria. (the P wave on the ECG)  0.0-0.1 seconds: Atrial Systole: The atria start to contract. Blood pressure in the atria is higher than that in the ventricles. Blood moves through the atrioventricular valves into the ventricles. The Cardiac Cycle

3. 0.1 seconds: Ventricular depolarisation: a wave of depolarisation spreads from the Atrio Ventricular node down the Purkinje fibres in the bundles of His. The wave then spreads out from the bottom of the ventricles upwards. This is seen as the QRS wave on the ECG. The wave of depolarisation initiates the contraction of the ventricles The Cardiac Cycle

4.  0.1 seconds: The pressure in the atria falls below that in the ventricles. This causes the atrioventricular valves to slam shut causing the first heart sound, Lub. 0.1- 0.15 Seconds: The contracting ventricles cause the pressure in the ventricles to rise; however the volume of the ventricles does not change as the semi lunar valves are closed The Cardiac Cycle

5.  0.15 seconds: The pressure in the ventricle rises above that in the aorta allowing the semilunar valves to open  0.15- 0.4 Seconds: Ventricular systole: With the pressure in the ventricles higher than in the arteries the pressure in the arteries rises as they fill with blood and the volume of the ventricles falls. The Cardiac Cycle

6.  0.4 seconds: The ventricles empty and their pressure falls below that in the arteries causing the semilunar valves to slam shut, this causes the second heart sound, Dub. The Cardiac Cycle

7.  0.4- 0.45 seconds: The pressure in the ventricle continues to fall. At 0.45 the pressure in the ventricle falls below that in the atria and the atrioventricular valves open.  0.45 – 0.8 Seconds: Diastole: The atria and ventricles are in diastole, relaxed and recovering for the next contraction. During this time the atria are loaded with blood by the veins. The Cardiac Cycle