Heart and Blood Flow

Cardiac Cycle

The cardiac cycle is one contraction-relaxation event of the heart. During this cycle, changes in electrical activity cause the pressures and volumes of the heart chambers to change as blood moves in and out of the heart, valves open and close to direct blood flow, and heart sounds are generated from the closing of the valves.

The cardiac cycle identifies all of the events that occur during one contraction-relaxation event of the heart. The events of the cardiac cycle are typically illustrated in a figure often referred to as a Wiggers diagram, named after Dr. Carl Wiggers, the physiologist who developed the figure.

The cardiac cycle consists of two periods: diastole and systole. Diastole describes the period when heart muscle is in the relaxation phase. Systole is the period when heart muscle contracts, generating force. The cardiac cycle occurs in several stages:

1. Ventricular filling: During diastole, ventricles passively fill with blood from the atria. The semilunar valves are closed because the ventricular pressure is lower than pressures in the aorta and pulmonary trunk. At the end of this phase the atria contract, completing the filling of the ventricles.

2. Isovolumetric contraction: At the start of systole, ventricles contract, AV valves close, and ventricular pressure increases. All four valves are closed, and blood volume in the ventricles does not change. This phase ends when the ventricular pressure exceeds the pressure in the arteries and forces the semilunar valves to open.

3. Ventricular ejection: Semilunar valves open, and blood is ejected into the aorta and pulmonary trunk.

4. Isovolumetric relaxation: At the start of ventricular diastole, the ventricles relax, and ventricular pressure drops. The semilunar valves close, and the AV valves remain closed until the pressure in the atria exceeds the pressure in the ventricles.

During the contraction cycle, two major heart sounds can be heard, the S1 and S2. The first sound, S1, is heard as a "lubb" sound—this sound is made as AV valves close at the start of ventricular systole. The closure of the semilunar valves (the aortic and pulmonary valves) occurs at the start of ventricular diastole and is heard as "dupp," the second heart sound (S2). Abnormal heart sounds may indicate problems with the heart valves. Heart murmurs are heard as "whoosh" sounds, which indicate backflow through heart valves that may not be closing correctly.

Wiggers Diagram

The events of the cardiac cycle occur in different stages. Electrical activity, pressures and volumes of the atria and ventricles, and the opening and closing of the heart valves are all coordinated to keep blood flowing unidirectionally through the heart.