Cardiac Cycle

contraction of the heart …

systole

relaxation of the heart

diastole

atrial systole

• the walls of the atria contract - atrial volume decreases, atrial pressure increases

• Pressure in the atria rises above that in the ventricles, forcing the (AV) valves open

• Blood is forced into the ventricles

  • Slight increase in ventricular pressure and chamber volume as the ventricles receive the blood from the atria

• The ventricles are relaxed at this point; ventricular diastole coincides with atrial systole

Ventricular systole

• Walls of the ventricles contract

  • Ventricular volume decreases

  • Ventricular pressure increases

• The pressure in the ventricles rises above that in the atria

  • The AV valves are forced to close, preventing back flow of blood

• The pressure in the ventricles rises above that in the aorta and pulmonary artery

  • The semilunar (SL) valves are forced open so blood is forced into the arteries and out of the heart

• During this period:

  • the atria are relaxing; atrial diastole coincides with ventricular systole

  • the blood flow to the heart continues, so the relaxed atria begin to fill with blood again

Diastole

• Ventricles and atria are both relaxed

• Pressure in the ventricles drops below that in the aorta and pulmonary artery, forcing the SL valves to close

• Atria continue to fill with blood  

  • Blood returns to the heart via the vena cava and pulmonary vein

• Pressure in the atria rises above that in the ventricles, forcing the AV valves open

• Blood flows passively into the ventricles without need for atrial systole

• The cycle then begins again with atrial systole

table