The atria are relaxed. The valves between the atria and ventricles are closed.
The right atrium fills will deoxygenated blood from the venae cavae. The left atrium fills with oxygenated blood from the pulmonary veins.
As the atria fill more pressure is put onto the valves, so they start to open.
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Atrial systole
Nerve impulses generated in the sino-atrial node (SAN) spread out throught the muscles in the atria.
The atria contract. Their volume decreases so the pressure of the blood inside increases.
Blood is forced through the valves into the ventricles.
The impulses from the SAN stimulate the atrio-ventricular node (AVN) in the septum separating the atria.
Impulses from the AVN pass along the Bundle of His which is made up of conductive tissue called Purkyne or Purkinje Fibres.
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Ventricular systole
The wave of impulses from the bundle of His stimulates the muscles of the ventricles at their apex.
The ventricles contract from the apex upwards. Their volume decreases so the pressure of the blood inside increases.
This forces shut the valves separating the atria and the ventricles, (preventing backflow into the atria) and opens the valves guarding the openings of the arteries.
Blood is forced through the pulmonary artery and the aorta.
The elasticity of the artery walls allows for the increase in the volume of blood. Back flow into the ventricles is prevented by the semi-lunar valves.
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Ventricular diastole
Relaxation of the ventricles marks the end of the cardia cycle.
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