Diastole and Systole Phases of the Cardiac Cycle

Diastole and Systole Phases of the Cardiac Cycle

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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. In the diastole phase, the heart ventricles are relaxed and the heart fills with blood. In the systole phase, the ventricles contract and pump blood out of the heart and to arteries. One cardiac cycle is completed when the heart chambers fill with blood and blood is then pumped out of the heart.

Cardiovascular System

The cardiac cycle is vital to proper cardiovascular system function. Comprised of the heart and the circulatory system, the cardiovascular system transports nutrients to and removes gaseous waste from the cells of the body. The cardiac cycle of the heart provides the "muscle" needed to pump blood throughout the body, while blood vessels act as pathways to transport blood to various destinations. The driving force behind the cardiac cycle is cardiac conduction. Cardiac conduction is the electrical system that powers the cardiac cycle and the cardiovascular system. Specialized tissue called heart nodes send nerve impulses that travel throughout the heart wall causing the heart muscle to contract.

Cardiac Cycle Phases

The events of the cardiac cycle described below trace the path of blood as it enters the heart, pumps to the lungs, travels back to the heart and is pumped out to the rest of the body. It is important to note that the events that occur in the first and second diastole periods actually happen at the same time. The same is also true for the events of the first and second systole periods.

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First Diastole Period

Mariana Ruiz Villarreal / Wikimedia Commons / Public Domain

During the first diastole period, the atria and ventricles are relaxed and the atrioventricular valves are open. Oxygen-depleted blood returning to the heart from the body passes through the superior and inferior vena cavae and flows to the right atrium. The open atrioventricular valves (tricuspid and mitral valves) 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 send signals that trigger both atria to contract. As a result of the 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.

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First Systole Period

Mariana Ruiz Villarreal / Wikimedia Commons / Public Domain

At the beginning of the first systole period, the right ventricle is filled with blood passed on from the right atrium. The ventricles receive impulses from fiber branches (Purkinje fibers), which carry electrical impulses to the ventricles causing them 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 oxygen-depleted blood along the pulmonary circuit to the lungs. There, blood picks up oxygen and is returned to the left atrium of the heart by the pulmonary veins.

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Second Diastole Period

In the second diastole period, the semilunar valves close and the atrioventricular valves open. Oxygenated blood from the pulmonary veins fills the left atrium (blood from the venae cavae is also filling the right atrium at this time). The SA node contracts again triggering both atria to contract. Atrial contraction causes the left atrium to empty its contents into the left ventricle (the right atrium is also emptying blood into the right ventricle at this time). 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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Second Systole Period

During the second systole period, the atrioventricular valves close and the semilunar valves open. The ventricles receive impulses and 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 being 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, oxygen-depleted blood is returned to the heart via the venae cavae.