1. The Cardiac Cycle

2. Each individual pump of the heart represents one heart beat.
Cardiac Cycle

3. The cardiac cycle represents the mechanical events of one heartbeat.
0.8 seconds
72 times a minute

4. The cardiac cycle consists of two phases that represent the contraction and relaxation of the heart muscle.
Diastole
0.5sec
relaxation
Systole
0.3sec
contraction

5. Cardiac Diastole - the heart relaxes for 0.5 seconds,
during which the sequence of events
in images A & B occurs.
A. The atria fill with blood while the atrioventricular (bicuspid & tricuspid) valves are closed.
B. The atrioventricular valves are pushed open by rising atrial pressure and the ventricles start to fill with blood. The semi-lunar valves are closed throughout.

6. Atrial Systole - First the SA Node initiates an electrical
impulse that travels across the atrial walls.
This is followed by a wave-like contraction across
the myocardium of each atrium. The effect of the atrial
systole is to force all remaining blood past the
atrioventricular valves (bicuspid and tricuspid) into
the lower chambers. The semi-lunar valves
(pulmonary and aortic valves are closed during this activity).
As shown in image C.
C. The Atria contract, forcing
remaining blood down into
the ventricles.

7. Ventricular Systole - The electrical impulse passes from the AV Node to the Purkinje Fibres. Once ventricular myocardium is stimulated, a second wave-like contraction spreads across the ventricular walls, whist the atrioventricular valves remain closed.
Pressure inside the ventricles continues to rise. The ventricles continue to contract until the continued increase in pressure pushes open the semi-lunar valves. The result is that blood flows into the pulmonary (lungs) and systemic (around the body) circulatory systems. As shown in image D.
D. The ventricles contract and
atrioventricular valves close.
Ventricular contraction forces
open the semi-lunar valves,
so that blood is ejected into
pulmonary artery and aorta.

8. Cardiac Impulse
Myogenic
sino-atrial node (SA)
AV node
Bundle of His
Purkinje fibres

9. Structures involved in the conduction of the cardiac impulse