The Cardiac Cycle

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Updated on November 11, 2019

The cardiac cycle is the sequence of events that occurs when the heart beats. As the heart beats, it circulates blood through pulmonary and systemic circuits of the body. There are two phases of the cardiac cycle: The diastole phase and the systole phase. In the diastole phase, heart ventricles relax and the heart fills with blood. In the systole phase, the ventricles contract and pump blood out of the heart to arteries. One cardiac cycle is completed when the heart chambers fill with blood and blood is pumped out of the heart.

The Cardiovascular System

The cardiac cycle is vital to proper cardiovascular system function. Comprised of the heart and circulatory system, the cardiovascular system transports nutrients to and removes gaseous waste from the cells of the body. The cardiac cycle provides the "muscle" needed to pump blood throughout the body. Blood vessels act as pathways that transport blood to various destinations.

The driving force behind the cardiac cycle is the electrical system known as cardiac conduction. This powers the cardiovascular system. Specialized tissues called heart nodes send nerve impulses that disperse throughout the heart wall to make the heart muscle contract.

Cardiac Cycle Phases

The events of the cardiac cycle described below trace the path of blood from when it enters the heart to when it is pumped out of the heart to the rest of the body. Periods of contraction and pumping are systole and periods of relaxation and filling are diastole. The atria and ventricles of the heart both go through diastole and systole phases and diastole and systole phases occur simultaneously.

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Ventricular Diastole

Diagram of the heart during the diastole phase of the cardiac cycle. — Mariana Ruiz Villarreal / Wikimedia Commons / Public Domain

During the ventricular diastole period, the atria and heart ventricles are relaxed and the atrioventricular valves are open. Oxygen-depleted blood returning to the heart from the body following the last cardiac cycle passes through the superior and inferior vena cavae and flows to the right atrium.

The open atrioventricular valves (tricuspid and mitral) allow blood to pass through the atria to the ventricles. Impulses from the sinoatrial (SA) node travel to the atrioventricular (AV) node and the AV node sends a signal that triggers both atria to contract. As a result of this contraction, the right atrium empties its contents into the right ventricle. The tricuspid valve, located between the right atrium and right ventricle, prevents blood from flowing back into the right atrium.

Ventricular Systole

Diagram of the heart during the systole phase of the cardiac cycle. — Mariana Ruiz Villarreal / Wikimedia Commons / Public Domain

At the beginning of the ventricular systole period, the right ventricle, which is filled with blood passed on from the right atrium, receives impulses from fiber branches (Purkinje fibers) carrying electrical impulses that cause it to contract. As this occurs, the atrioventricular valves close and the semilunar valves (pulmonary and aortic valves) open.

Ventricular contraction causes oxygen-depleted blood from the right ventricle to be pumped to the pulmonary artery. The pulmonary valve prevents blood from flowing back into the right ventricle. The pulmonary artery carries de-oxygenated blood along the pulmonary circuit to the lungs. There, the blood collects oxygen and returns to the left atrium of the heart through the pulmonary veins.

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Atrial Diastole

In the atrial diastole period, the semilunar valves close and the atrioventricular valves open. Oxygenated blood from the pulmonary veins fills the left atrium while blood from the venae cavae fills the right atrium. The SA node contracts again triggering both atria to do the same.

Atrial contraction causes the left atrium to empty its contents into the left ventricle and the right atrium to empty its contents into the right ventricle. The mitral valve, located between the left atrium and left ventricle, prevents oxygenated blood from flowing back into the left atrium.

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Atrial Systole

During the atrial systole period, the atrioventricular valves close and the semilunar valves open. The ventricles receive impulses to contract. Oxygenated blood in the left ventricle is pumped to the aorta and the aortic valve prevents the oxygenated blood from flowing back into the left ventricle. Oxygen-depleted blood is also pumped from the right ventricle to the pulmonary artery at this time.

The aorta branches out to provide oxygenated blood to all parts of the body through systemic circulation. After its tour through the body, de-oxygenated blood is returned to the heart via the venae cavae.