Coordinated sequence of contractions and relaxations by the heart muscle which causes blood to move from the atria, into the ventricles and then the arteries
Systole
Muscle contractions
Diastole
Muscle relaxation
Cardiac cycle
1. Atrial systole
2. Ventricular systole first phase
3. Ventricular systole second phase
4. Ventricular diastole early
5. Ventricular diastole late
Atrial systole
1. Atria contract while ventricles relax
2. Decreases volume inside atria which increases pressure
3. Increased pressure forces atrioventricular valves open and pushes blood into relaxed ventricles
Ventricular systole first phase
1. Ventricles contract while atria are relaxed
2. Decreases volume inside ventricles which increases pressure
3. Pressure is higher in ventricles compared to atria, which forces atrioventricular valves closed
4. Closure of AV valve prevents back-flow of blood into atria
Ventricular systole second phase
1. Ventricular pressure rises and exceeds pressure in arteries
2. Opens semilunar valves, ejecting blood
Ventricular diastole early
1. Atria and ventricles are relaxed so pressure is low in both chambers
2. Pressure is higher in arteries than in heart chambers, forcing semilunar valves closed to prevent blood flowing backwards into ventricles
3. Blood is returned to the heart and the atria fill with blood
