Cardiac cycle

 The cardiac cycle is divided into two basic phases: systole (contraction and ejection) and diastole (relaxation and filling).



Diastole is the time when the ventricles are not contracting but rather relaxed. Blood is passively flowing into the left ventricle (LV) and right ventricle (RV) from the left atrium (RA) for the majority of this time (see figure to the right). Mitral and tricuspid atrioventricular valves separate the atria from the ventricles, allowing blood to flow through them. Through the inferior vena cava (IVC) and superior vena cava (SVC), the RA receives venous blood from the body. Through four pulmonary veins that enter the LA, oxygenated blood from the lungs reaches the LA. Both atria contract at the conclusion of diastole, bringing additional blood into the ventricles.

The period of time known as systole is when the left and right ventricles contract and release blood into the pulmonary artery and aorta, respectively. The aortic and pulmonic valves open during systole to allow ejection into the aorta and pulmonary artery, respectively. During systole, the atrioventricular valves close; consequently, there is no blood entering the ventricles; However, the pulmonary veins and vena cavae continue to allow blood to enter the atria.

Changes in aortic pressure (AP), left ventricular pressure (LVP), left atrial pressure (LAP), left ventricular volume (LV Vol), and heart sounds during a single cardiac contraction and relaxation cycle are depicted in the cardiac cycle diagram to the right. The electrocardiogram is linked to these changes over time. The Health Education Assets Library offers a tutorial and online video on the cardiac cycle: Video of the cardiac cycle

A pressure-measuring catheter is inserted into the aorta from a peripheral artery to measure aortic pressure, and a catheter is inserted into the left ventricle to measure changes in intraventricular pressure as the heart beats to obtain left ventricular pressure. A catheter is rarely used to directly measure left atrial pressure; However, the pulmonary capillary wedge pressure can be recorded to estimate the left atrial pressure indirectly. Using echocardiography, radionuclide imaging, or a specialized volume conductance catheter that is inserted within the ventricle, changes in the volume of the ventricle can be evaluated in real time.


 The cardiac cycle is typically divided into seven phases to provide a deeper understanding of systole and diastole. The electrocardiogram's P wave, which indicates atrial depolarization and is the final phase of diastole, signals the beginning of the first phase. Systole is represented by phases 2-4, while early and mid-diastole are represented by phases 5-7. The appearance of the subsequent P wave, which signals the beginning of a new cycle, concludes the final phase of the cardiac cycle.

By clicking on any one of the seven phases listed below, you can obtain comprehensive descriptions of each phase.

Isovolumetric Relaxation, Atrial Contraction, Isovolumetric Contraction, Rapid Ejection, Reduced Ejection, Rapid Filling, and Reduced Filling are the first seven phases.

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