the cardiac cycle

what is the cardiac cycle?

the series of events that take place in one heart beat, including muscle contraction (systole) and relaxation (diastole)
it is a continuous process

what changes occur during the cardiac cycle?

volume and pressure changes

how does volume change in the cardiac cycle?

contraction of the heart muscle causes a decrease in volume in the corresponding chamber of the heart, which then increases again when the muscle relaxes

how does volume cause pressure to change?

when volume decreases, pressure increases
when volume increases, pressure decreases

how do pressure changes affect valves?

valves open when the pressure of blood behind them is greater than the pressure in front of them
they close when the pressure of blood in front of them is greater than the pressure behind them

what occurs during atrial systole to the atria?

the walls of the atria contract; atrial volume decreases, atrial pressure increases 
pressure in the atria rises above that in the ventricles, forcing the atrioventricular (AV) valves open

what occurs during atrial systole to the ventricles?

blood is forced into the ventricles, slight increase in ventricular pressure + volume as ventricles receive blood from atria
ventricles are relaxed at this point; ventricular diastole coincides with atrial systole

what occurs during ventricle systole to the ventricles?

the walls of the ventricles contract; ventricular volume decreases, ventricular pressure increases
pressure in the ventricles rises above that in the atria, forcing the AV valves to close, preventing back flow of blood
pressure in the ventricles rises above that in the aorta and pulmonary artery, forcing the semilunar (SL) valves open so blood is forced into the arteries and out of the heart

what occurs during ventricular systole to the atria?

the atria are relaxing; atrial diastole coincides with ventricular systole
blood flow to the heart continues, so the relaxed atria begin to fill with blood again

what occurs during diastole to the ventricles?

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

what occurs during diastole to the atria?

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 of atrial systole
the cycle then begins again with atrial systole