- Atria's pressure increases as the volume of blood increases and contracts forcing the blood out of the atria and into the ventricles via the AV valves.
- The AV valves then snap shut to prevent back flow of blood
- Pressure builds in the ventricles as the volume of blood increases and ventricles contract. The contraction starts at the apex of the heart pushing the blood towards the arteries via the semi lunar valves
- The ventricles and atria are all relaxed.
- Blood flows into the atria and through the ventricles via the AV valves
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Co-ordination of the heart
- SAN acts as a pacemaker and initiates a wave of excitation that spreads along the atrial wall.
- The wave of excitation then reaches the AVN where it is shortly delayed to allow the atria time to fully contract before the ventricles.
- The wave of excitation then spreads down the septum to the bundle of His and purkyne fibers spreading along the ventricular walls causing the ventricles to contract starting from the apex and upwards
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