10.7: Cardiac Cycle

The cardiac cycle describes the heart’s phases of contraction and relaxation that drive blood flow throughout the body.

Master this section and you'll be able to:

• Summarize the events of the cardiac cycle
• Compare atrial and ventricular systole and diastole
• Relate the ECG waves to the electrical and mechanical events of the cardiac cycle.
• Explain how pressure changes within the heart control valve opening and blood flow.

The cardiac cycle refers to the complete sequence of mechanical events that occur in the heart during one heartbeat. It begins with atrial contraction and concludes with ventricular relaxation. These mechanical events are initiated and regulated by the heart’s electrical activity, as illustrated in the electrocardiogram (ECG), which displays a repeating pattern of waves — the P wave, QRS complex, and T wave — corresponding to distinct phases of cardiac excitation and recovery. 

Heart rate is the term used to describe the frequency of the cardiac cycle.

The period of contraction that the heart undergoes while it pumps blood is called systole. The period of relaxation that occurs as the chambers fill with blood is called diastole. Both the atria and ventricles undergo systole and diastole, and it is essential that these components be carefully regulated and coordinated to ensure blood is pumped efficiently to the body.

Fluids, whether gases or liquids, are materials that flow according to pressure gradients — that is, they move from regions that are higher in pressure to regions that are lower in pressure. Accordingly, when the heart chambers are relaxed (diastole), blood will flow into the atria from the veins, which are higher in pressure. As blood flows into the atria, the pressure will rise, so the blood will initially move passively from the atria into the ventricles. When the action potential triggers the muscles in the atria to contract (atrial systole), the pressure within the atria rises further, pumping blood into the ventricles. During ventricular systole, pressure rises in the ventricles, pumping blood into the pulmonary trunk from the right ventricle and into the aorta from the left ventricle. Again, as you consider this flow and relate it to the conduction pathway, the elegance of the system should become apparent.

Figure \(\PageIndex{1}\): Overview of the Cardiac Cycle. The cardiac cycle begins with atrial systole and progresses to ventricular systole, atrial diastole, and ventricular diastole, when the cycle begins again. Correlations to the ECG are highlighted.

Interactive Element

Phases of the Cardiac Cycle

The cardiac cycle describes the complete sequence of events that occur in the heart during one heartbeat—from the beginning of atrial contraction to the end of ventricular relaxation. It includes alternating periods of contraction (systole) and relaxation (diastole) that move blood efficiently through the heart and into the circulation.

Ventricular and Atrial Diastole

At the start of the cycle, both the atria and ventricles are relaxed. Blood flows into the right atrium from the superior and inferior venae cavae and the coronary sinus, and into the left atrium from the four pulmonary veins. The atrioventricular (AV) valves — the tricuspid and mitra l— are open, allowing blood to passively flow into the ventricles. About 70–80 percent of ventricular filling occurs during this time. The pulmonary and aortic semilunar valves are closed, preventing any backflow into the ventricles.

Atrial Systole and Diastole

Depolarization of the atria (seen as the P wave on an ECG) triggers atrial contraction, or atrial systole. The atria contract from top to bottom, increasing pressure and pushing the remaining 20–30 percent of blood into the ventricles — this is known as the atrial kick. Atrial systole lasts about 100 milliseconds and ends just before ventricular contraction begins. The atria then relax and enter atrial diastole.

Ventricular Systole

Ventricular systole (represented by the QRS complex on the ECG) occurs in two phases and lasts about 270 milliseconds. At the start, the ventricles contain roughly 130 mL of blood, called the end-diastolic volume (EDV) or preload.

• Isovolumic contraction phase: The ventricles contract, pressure rises, and the tricuspid and mitral valves close, producing the first heart sound (“lub”). Because the semilunar valves remain closed, no blood leaves the heart, so ventricular volume remains constant.

• Ventricular ejection phase: Once ventricular pressure exceeds that in the pulmonary trunk and aorta, the semilunar valves open and blood is ejected. Both ventricles pump the same amount of blood, called the stroke volume, typically 70–80 mL. About 50–60 mL remain in the ventricle after contraction, known as the end-systolic volume (ESV).

Ventricular Diastole

Ventricular relaxation follows repolarization of the ventricles, represented by the T wave on the ECG, and lasts about 430 milliseconds.

• Isovolumic relaxation phase: As the ventricles relax, pressure falls below that in the pulmonary trunk and aorta, causing the semilunar valves to close — producing the second heart sound (“dup”). The AV valves remain closed, so ventricular volume does not change.

• Late ventricular diastole: As relaxation continues, ventricular pressure drops below atrial pressure. The tricuspid and mitral valves open, allowing blood to flow from the atria into the ventricles. Both chambers are again in diastole, the semilunar valves remain closed, and the cycle begins anew.

Figure \(\PageIndex{2}\): Relationship between the Cardiac Cycle and ECG. Initially, both the atria and ventricles are relaxed (diastole). The P wave represents depolarization of the atria and is followed by atrial contraction (systole). Atrial systole extends until the QRS complex, at which point, the atria relax. The QRS complex represents depolarization of the ventricles and is followed by ventricular contraction. The T wave represents the repolarization of the ventricles and marks the beginning of the ventricular relaxation.