| Cardiac cycle | |
|---|---|
| Organisms | Animalia |
| Biological system | Circulatory system |
| Health | Beneficial |
| Action | Involuntary |
| Method | Blood is allowed to enter relaxed ventricle chamber from vein through venous valve. Heart muscle contracts ventricle and blood is expelled through arterial valve to artery. |
| Outcome | Circulation of blood per minute (Humans) |
| Duration | 0.6–1 second (Humans) |
| *Animalia with the exception ofPorifera,Cnidaria,Ctenophora,Platyhelminthes,Bryozoan,Amphioxus. | |
Thecardiac cycle is the performance of thehuman heart from the beginning of one heartbeat to the beginning of the next.[1] It consists of two periods: one during which the heart muscle relaxes and refills with blood, calleddiastole, following a period of robust contraction and pumping of blood, calledsystole.[1] 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.[2][3] Duration of the cardiac cycle is inversely proportional to the heart rate.[4]
There are twoatrial and twoventricle chambers of the heart; they are paired as theleft heart and theright 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 (see cycle diagram at right margin).[1] At the start of the cycle, duringventricular diastole–early, the heart relaxes and expands while receiving blood into both ventricles through both atria; then, near the end ofventricular diastole–late, the two atria begin to contract (atrial systole), and each atrium pumps blood into the ventricle below it.[5] Duringventricular systole the ventricles contract and vigorously pulse (or eject) 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 (atrial diastole). This precise coordination ensures that blood is efficiently collected and circulated throughout the body.[1][2]
Themitral andtricuspid valves, also known as theatrioventricular, or AV valves, open during ventricular diastole to permit filling. Late in the filling period the atria begin to contract (atrial systole) forcing a final crop of blood into the ventricles under pressure—see cycle diagram. Then, prompted by electrical signals from thesinoatrial node, the ventricles start contracting (ventricular systole), 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 theisovolumic contraction stage.[1][6]
Due to the contractions of the systole, pressures in the ventricles rise quickly, exceeding the pressures in the trunks of theaorta and thepulmonary arteries and causing the requisite valves (theaortic andpulmonary valves) to open—which results in separated blood volumes beingejected from the two ventricles.[1] This is the ejection stage of the cardiac cycle; it is depicted (see circular diagram) as theventricular systole–first phase followed by theventricular systole–second phase.[2] After ventricular pressures fall below their peak(s) 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.[2]
Next is theisovolumic relaxation, during which pressure within the ventricles begin to fall significantly, and thereafter the atria begin refilling as blood returns to flow into theright atrium (from thevena cavae) and into theleft atrium (from thepulmonary veins).[1] 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.[1][2]
Throughout the cardiac cycle,blood pressure increases and decreases. The movements ofcardiac muscle are coordinated by a series of electrical impulses produced by specializedpacemaker cells found within thesinoatrial node and theatrioventricular node. Cardiac muscle is composed ofmyocytes which initiate their internal contractions without receiving signals from external nerves—with the exception of changes in theheart rate due tometabolic demand.[1][2]
In anelectrocardiogram, electrical systole initiates the atrial systole at the P wave deflection of a steady signal; and it starts contractions (systole).[2]
The cardiac cycle involves four major stages of activity: 1) "isovolumic relaxation", 2) inflow, 3) "isovolumic contraction", 4) "ejection".[1][2] Stages 1 and 2 together—"isovolumic relaxation" plus inflow (equals "rapid inflow", "diastasis", and "atrial systole")—comprise the ventricular diastole period, including atrial systole, during which blood returning to the heart flows through the atriainto the relaxed ventricles.[1][2] Stages 3 and 4 together—"isovolumic contraction" plus "ejection"—are the ventricular systole period, which is the simultaneous pumping of separate blood suppliesfrom the two ventricles, one to the pulmonary artery and one to the aorta.[1][2] Notably, near the end of the diastole, the atria begin contracting, then pump blood into the ventricles; this pressurized delivery during ventricular relaxation (ventricular diastole) is called theatrial systole.[1][2]
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.
Theheart is a four-chamberedorgan consisting of right and left halves, called theright heart and theleft heart. The upper two chambers, the left and rightatria, are entry pointsinto the heart for blood-flow returning from thecirculatory system, while the two lower chambers, the left and rightventricles, perform the contractions that eject the bloodfrom the heart to flow through the circulatory system. Circulation is split intopulmonary circulation—during which the right ventricle pumps oxygen-depleted blood to the lungs through thepulmonary trunk and arteries; or thesystemic circulation—in which the left ventricle pumps/ejects newly oxygenated blood throughout the body via theaorta and all other arteries.[1][2]
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 ofcardiomyocytes, the specialized muscle cells of the heart.[1][2] These impulses ultimately stimulate heart muscle to contract and thereby to eject blood from the ventricles into the arteries and the cardiaccirculatory 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 (orsinus rhythm) of this signaling across the heart is coordinated by two groups of specialized cells, thesinoatrial (SA) node, which is situated in the upper wall of the right atrium, and theatrioventricular (AV) node located in the lower wall of theright heart between the atrium and ventricle.[1][2] The sinoatrial node, often known as thecardiac pacemaker, is the point of origin for producing a wave of electrical impulses that stimulates atrial contraction by creating anaction potential across myocardium cells.[7][8]
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 thebundle of His and thePurkinje 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 (contractions), ejecting the new blood volume and completing the cardiac cycle.[8] (See Wiggers diagram: "Ventricular volume" tracing (red), at "Systole" panel.)
Cardiac diastole is the period of the cardiac cycle when, after contraction, the heart relaxes and expands while refilling with blood returning from thecirculatory system.[1][2] Bothatrioventricular (AV) valves open to facilitate the 'unpressurized' flow of blood directly through the atria into both ventricles, where it is collected for the next contraction. This period is best viewed at the middle of the Wiggers diagram—see the panel labeled "diastole". Here it shows pressure levels in both atria and ventricles as near-zero during most of the diastole. (See gray and light-blue tracings labeled "atrial pressure" and "ventricular pressure"—Wiggers diagram.) Here also may be seen the red-line tracing of "Ventricular volume", showing an increase in blood volume from the low plateau of the "isovolumic relaxation" stage to the maximum volume occurring in the "atrial systole" sub-stage.[1][2]
Atrial systole is the contracting ofcardiac muscle cells of bothatria following electrical stimulation and conduction of electrical currents across the atrial chambers (see above,Physiology).[1][2] While nominally a component of the heart's sequence of systolic contraction and ejection, atrial systole actually performs the vital role of completing the diastole, which is to finalize the filling of both ventricles with blood while they are relaxed and expanded for that purpose. Atrial systole overlaps the end of the diastole, occurring in the sub-period known asventricular diastole–late (see cycle diagram). At this point, the atrial systole applies contraction pressure to 'topping-off' the blood volumes sent to both ventricles; this atrial contraction closes the diastole immediately before the heart again begins contracting and ejecting bloodfrom the ventricles (ventricular systole) to the aorta and arteries.[1][2]
Ventricular systole is the contractions, following electrical stimulations, of theventricular syncytium of cardiac muscle cells in the left and rightventricles. Contractions in the right ventricle providepulmonary circulation by pulsing oxygen-depleted blood through the pulmonary valve then through the pulmonary arteries to the lungs. Simultaneously, contractions of the left ventricular systole providesystemic circulation of oxygenated blood to all body systems by pumping blood through the aortic valve, the aorta, and all the arteries. (Blood pressure is routinely measured in the larger arteries off the left ventricle during the left ventricular systole).[1][2]