- 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.
1 of 4
- 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.
2 of 4
- 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.
3 of 4
- Relaxation of the ventricles marks the end of the cardia cycle.
- The process begins again.
4 of 4