To understand the cardiac cycle, it is useful to have a basic understand of the heart's structure. The heart is the organ responsible for moving blood throughout the body. The heart muscle is called the myocardium and is approximately the size of a fist. It is composed of four chambers: the left ventricle, the right ventricle, the left atrium and the right atrium. There are also a number of blood vessels attached to the heart. The inferior and superior vena cavas bring deoxygenated blood back to the heart. The pulmonary arteries take deoxygenated blood from the heart to the lungs to be oxygenated. The pulmonary veins bring oxygenated blood back to the heart. The aorta, the largest blood vessel in the human body, carries oxygenated blood to the rest of the body. The cardiac cycle refers to a set of contractions that occur in the heart that can be broken into two major stages: the diastolic and the systolic phases.
Diastole refers to a period in the cardiac cycle when the left and right ventricles are relaxed. During this phase, blood is allowed to pass from the left and right atriums into the left and right ventricles. Deoxygenated blood that accumulated in the right atrium from the superior and inferior vena cava, as well as deoxygenated blood from the heart, moves into the right ventricle. Oxygenated blood that accumulated in the left atrium from the pulmonary veins moves into the left ventricle. As the pressure increases in the ventricles, it causes the valves between the ventricles and the atriums (called atrioventricular valves) to close, which prevents the blood from moving back into the atriums. At this juncture, the diastolic phase is over.
Systole refers to the contraction of the heart muscles. Systole is set in motion by an electrical signal sent from muscle fibres in the S-A node (sino-atrial node) that moves through the entire heart. This signal triggers the right atrium, followed by the left atrium, to contract and push the blood remaining in them into the ventricles. The electrical signal then reaches the ventricles. The right ventricle contracts first, forcing blood into the pulmonary artery and to the lungs to be oxygenated. The left ventricle then contracts which drives blood into the aorta and out to the body. The muscle fibres that drive the ventricular contraction tire quickly and the ventricles collapse, sealing all the valves leading into or out of the ventricles. This triggers the atriums to begin filling again. When blood once again begins to fill the ventricles, the cardiac cycle has come full circle. The cardiac cycle takes approximately one second to complete.