Cardiac cycle
The cardiac cycle is the performance of the human heart from the beginning of one heartbeat to the beginning of the next. It consists of two periods: one during which the heart muscle relaxes and refills with blood, called diastole, following a period of robust contraction and pumping of blood, called systole. After emptying, the heart relaxes and expands to receive another influx of blood returning from the lungs and other systems of the body, before again contracting.
Assuming a healthy heart and a typical rate of 70 to 75 beats per minute, each cardiac cycle, or heartbeat, takes about 0.8 second to complete the cycle. Duration of the cardiac cycle is inversely proportional to the heart rate.
Description
There are two atrial and two ventricle chambers of the heart; they are paired as the left heart and the right heart—that is, the left atrium with the left ventricle, the right atrium with the right ventricle—and they work in concert to repeat the cardiac cycle continuously. At the start of the cycle, during ventricular diastole–early, the heart relaxes and expands while receiving blood into both ventricles through both atria; then, near the end of ventricular diastole–late, the two atria begin to contract, and each atrium pumps blood into the ventricle below it. During systole#Ventricular [systole and Wiggers diagram|ventricular systole] the ventricles contract and vigorously pulse two separated blood supplies from the heart—one to the lungs and one to all other body organs and systems—while the two atria relax. This precise coordination ensures that blood is efficiently collected and circulated throughout the body.The mitral and tricuspid valves, also known as the atrioventricular, or AV valves, open during ventricular diastole to permit filling. Late in the filling period the atria begin to contract forcing a final crop of blood into the ventricles under pressure—see cycle diagram. Then, prompted by electrical signals from the sinoatrial node, the ventricles start contracting, and as back-pressure against them increases the AV valves are forced to close, which stops the blood volumes in the ventricles from flowing in or out; this is known as the isovolumic contraction stage.
Due to the contractions of the systole, pressures in the ventricles rise quickly, exceeding the pressures in the trunks of the aorta and the pulmonary arteries and causing the requisite valves to open—which results in separated blood volumes being ejected from the two ventricles. This is the ejection stage of the cardiac cycle; it is depicted as the ventricular systole–first phase followed by the ventricular systole–second phase. After ventricular pressures fall below their peak and below those in the trunks of the aorta and pulmonary arteries, the aortic and pulmonary valves close again—see, at the right margin, Wiggers diagram, blue-line tracing.
Next is the isovolumic relaxation, during which pressure within the ventricles begin to fall significantly, and thereafter the atria begin refilling as blood returns to flow into the right atrium and into the left atrium. As the ventricles begin to relax, the mitral and tricuspid valves open again, and the completed cycle returns to ventricular diastole and a new "Start" of the cardiac cycle.
Throughout the cardiac cycle, blood pressure increases and decreases. The movements of cardiac muscle are coordinated by a series of electrical impulses produced by specialized pacemaker cells found within the sinoatrial node and the atrioventricular node. Cardiac muscle is composed of myocytes which initiate their internal contractions without receiving signals from external nerves—with the exception of changes in the heart rate due to metabolic demand.
In an electrocardiogram, electrical systole initiates the atrial systole at the P wave deflection of a steady signal; and it starts contractions.
Cardiac cycle and Wiggers diagram
The cardiac cycle involves four major stages of activity: 1) "isovolumic relaxation", 2) inflow, 3) "isovolumic contraction", 4) "ejection". Stages 1 and 2 together—"isovolumic relaxation" plus inflow —comprise the ventricular diastole period, including atrial systole, during which blood returning to the heart flows through the atria into the relaxed ventricles. Stages 3 and 4 together—"isovolumic contraction" plus "ejection"—are the ventricular systole period, which is the simultaneous pumping of separate blood supplies from the two ventricles, one to the pulmonary artery and one to the aorta. Notably, near the end of the diastole, the atria begin contracting, then pump blood into the ventricles; this pressurized delivery during ventricular relaxation is called the atrial systole.The closure of the aortic valve causes a rapid change in pressure in the aorta called the incisura. This short sharp change in pressure is rapidly attenuated down the arterial tree. The pulse wave form is also reflected from branches in the arterial tree and gives rise to a dicrotic notch in main arteries. The summation of the reflected pulse wave and the systolic wave may increase pulse pressure and help tissue perfusion. With increasing age, the aorta stiffens and can become less elastic which will reduce peak pulse in the periphery.
Physiology
The heart is a four-chambered organ consisting of right and left halves, called the right heart and the left heart. The upper two chambers, the left and right atria, are entry points into the heart for blood-flow returning from the circulatory system, while the two lower chambers, the left and right ventricles, perform the contractions that eject the blood from the heart to flow through the circulatory system. Circulation is split into pulmonary circulation—during which the right ventricle pumps oxygen-depleted blood to the lungs through the pulmonary trunk and arteries; or the systemic circulation—in which the left ventricle pumps/ejects newly oxygenated blood throughout the body via the aorta and all other arteries.Heart electrical conduction system
In a healthy heart all activities and rests during each individual cardiac cycle, or heartbeat, are initiated and orchestrated by signals of the heart's electrical conduction system, which is the "wiring" of the heart that carries electrical impulses throughout the body of cardiomyocytes, the specialized muscle cells of the heart. These impulses ultimately stimulate heart muscle to contract and thereby to eject blood from the ventricles into the arteries and the cardiac circulatory system; and they provide a system of intricately timed and persistent signaling that controls the rhythmic beating of the heart muscle cells, especially the complex impulse-generation and muscle contractions in the atrial chambers.The rhythmic sequence of this signaling across the heart is coordinated by two groups of specialized cells, the sinoatrial node, which is situated in the upper wall of the right atrium, and the atrioventricular node located in the lower wall of the right heart between the atrium and ventricle. The sinoatrial node, often known as the cardiac pacemaker, is the point of origin for producing a wave of electrical impulses that stimulates atrial contraction by creating an action potential across myocardium cells.
Impulses of the wave are delayed upon reaching the AV node, which acts as a gate to slow and to coordinate the electrical current before it is conducted below the atria and through the circuits known as the bundle of His and the Purkinje fibers—all which stimulate contractions of both ventricles. The programmed delay at the AV node also provides time for blood volume to flow through the atria and fill the ventricular chambers—just before the return of the systole, ejecting the new blood volume and completing the cardiac cycle.