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The cardiac cycle is the performance of the
human heart The heart is a cardiac muscle, muscular Organ (biology), organ in most animals, which pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste ...

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 Image:Heart diastole.png, upright=1.5, Heart performance during ventricular diastole: early diastole is a suction mechanism that draws blood 'down' from the left atrium (pink) and right atrium (blue) into each of the two ventricles. Then, in late ve ...
, following a period of robust contraction and pumping of blood, dubbed
systole The systole ( ) is the part of the cardiac cycle The cardiac cycle is the performance of the human heart The heart is a cardiac muscle, muscular Organ (biology), organ in most animals, which pumps blood through the blood vessels of ...
. After emptying, the heart immediately relaxes and expands to receive another influx of blood returning from the lungs and other systems of the body, before again contracting to pump blood to the lungs and those systems. A normally performing heart must be fully expanded before it can efficiently pump again. Assuming a healthy heart and a typical rate of 70 to 75 beats per minute, each cardiac cycle, or heartbeat, takes about 0.8 seconds to complete the cycle. There are two
atrial The atrium (Latin ātrium, “entry hall”) is the upper chamber through which blood enters the ventricles of the heart The heart is a cardiac muscle, muscular Organ (biology), organ in most animals, which pumps blood through the blood ...
and two ventricle chambers of the heart; they are paired as the
left heart The heart is a muscular organ in most animals, which pumps blood through the blood vessel The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cel ...
and the
right heart The heart is a muscular MUSCULAR (DS-200B), located in the United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain,Usage is mixed. The Guardian' and Telegraph' use ...
—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). 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 ( ''atrial systole''), and each atrium pumps blood into the ventricle below it. During ''ventricular systole'' the ventricles are contracting and vigorously pulsing (or ejecting) two separated blood supplies from the heart—one to the lungs and one to all other body organs and systems—while the two atria are relaxed ( ''atrial diastole''). This precise coordination ensures that blood is efficiently collected and circulated throughout the body. The
mitral The mitral valve (), also known as the bicuspid valve or left atrioventricular valve, is one of the four heart valve A heart valve is a that allows in one direction through the chambers of the . Four valves are usually present in a mammalian h ...

mitral
and
tricuspid The tricuspid valve, or right atrioventricular valve, is on the right dorsal side of the mammalian heart, at the superior portion of the right ventricle A ventricle is one of two large chambers toward the bottom of the heart The heart is ...
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 (atrial systole) forcing a final crop of blood into the ventricles under pressure—see cycle diagram. Then, prompted by electrical signals from the
sinoatrial node The sinoatrial node (also known as the sinuatrial node, SA node or sinus node) is a group of Cell (biology), cells known as pacemaker cells, located in the wall of the atrium (heart), right atrium of the heart. These cells can produce an electric ...
, 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 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 The aorta ( ) is the main and largest artery An artery (plural arteries) () is a blood vessel The blood vessels are the components of the circulatory system The circulatory system, also called the cardiovascular system or the vas ...

aorta
and the
pulmonary The lungs are the primary organs of the respiratory system The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animal ...

pulmonary
arteries and causing the requisite valves (the
aortic The aorta ( ) is the main and largest artery in the human body, originating from the Ventricle (heart), left ventricle of the heart and extending down to the abdomen, where it aortic bifurcation, splits into two smaller arteries (the common ilia ...

aortic
and
pulmonary The lungs are the primary organs of the respiratory system The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animal ...

pulmonary
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 (see circular diagram) as the ''ventricular systole–first phase'' followed by the ''ventricular systole–second phase''. 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 right margin,
Wiggers diagram A Wiggers diagram, named after its developer, Carl Wiggers, is a standard diagram that is used in teaching cardiovascular physiology, cardiac physiology. In the Wiggers diagram, the X-axis is used to plot time, while the Y-axis contains all of the ...

Wiggers diagram
, blue-line tracing. Now follows 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 The atrium (Latin ātrium, “entry hall”) is the upper chamber through which blood enters the ventricles of the heart The heart is a cardiac muscle, muscular Organ (biology), organ in most animals, which pumps blood through the blood ...
(from the
vena cavae The venae cavae (; from the Latin for "hollow veins", singular "vena cava" ) are two large veins (venous trunks) that return deoxygenated blood from the body into the heart. In humans there are the superior vena cava and the inferior vena cava, an ...
) and into the
left atrium The atrium (Latin ātrium, “entry hall”) is the upper chamber through which blood enters the Ventricle (heart), ventricles of the heart. There are two atria in the human heart – the left atrium receives blood from the pulmonary (lung) circula ...
(from the
pulmonary vein The pulmonary veins are the vein Veins are blood vessels in humans, and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary vein, pulmonary a ...
s). 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 Blood pressure (BP) is the pressure Pressure (symbol: ''p'' or ''P'') is the force In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motio ...

blood pressure
increases and decreases. The movements of
cardiac muscle Cardiac muscle (also called heart muscle or myocardium) is one of three types of vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biol ...

cardiac muscle
are coordinated by a series of electrical impulses produced by specialised pacemaker cells found within the
sinoatrial node The sinoatrial node (also known as the sinuatrial node, SA node or sinus node) is a group of Cell (biology), cells known as pacemaker cells, located in the wall of the atrium (heart), right atrium of the heart. These cells can produce an electric ...
and the
atrioventricular node The atrioventricular node or AV node is a part of the electrical conduction system of the heart The electrical conduction system of the heart transmits signals generated usually by the sinoatrial node to cause contraction of the heart muscle. Th ...
. Cardiac muscle is composed of myocytes which initiate their internal contractions without applying to external nerves—with the exception of changes in the
heart rate Heart rate is the speed of the heartbeat Heartbeat or heartbeats may refer to: Physiology *Cardiac cycle, of the heart *Contraction of the cardiac muscle, muscles of the heart, or a perceived effect of it, such as: **Heart sounds, the noises gene ...

heart rate
due to
metabolic Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life Life is a characteristic that distinguishes physical entities that have biological processes, such as Cell signaling, signaling and self-sustaining ...
demand. In an
electrocardiogram Electrocardiography is the process of producing an electrocardiogram (ECG or EKG). It is a graph of voltage Voltage, electric potential difference, electromotive force emf, electric pressure or electric tension is the difference in elec ...

electrocardiogram
, electrical systole initiates the atrial systole at the P wave deflection of a steady signal; and it starts contractions (systole).


The cardiac cycle and Wiggers diagram

The cardiac cycle involves four major stages of activity: 1) "Isovolumic relaxation", 2) Inflow, 3) "Isovolumic contraction", 4) "Ejection". (See Wiggers diagram, which presents the stages, label-wise, in 3,4,1,2 order, left-to-right.) Moving from the left along the Wiggers diagram shows the activities within four stages during a single cardiac cycle. (See the consecutive panels labeled, at bottom-right, "Diastole" then "Systole"). 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 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 pumping blood into the ventricles; this pressurized delivery during ventricular relaxation (ventricular diastole) is called the ''atrial systole'', aka ''atrial kick''. The time-wise increases and decreases of the heart's blood volume (see Wiggers diagram), are also instructive to follow. The red-line tracing of "Ventricular volume" provides an excellent track of the two periods and four stages of one cardiac cycle. Starting with the Diastole period: the low-volume plateau of "Isovolumic relaxation" stage, followed by a rapid rise and two slower rises, all components of the "Inflow stage"—increasing to the high-volume plateau of the "Isovolumic contraction" stage; (find the label at left side of diagram). Then, the Systole, including the high "Isovolumic contraction" stage to the rapid decrease in blood volume (i.e., the vertical drop of the red-line tracing) which signifies the emptying of the ventricles during the "Ejection" stage of the completed cycle—all equal to one heartbeat.


Stages

Valve configurations during main stages of the cardiac cycle+ Notes: Stages 1, 2a, and 2b together comprise the "Diastole" period; stages 3 and 4 together comprise the "Systole" period. + based on Ganong Barrett, et al. ''Ganong's Review of Medical Physiology'' 24e (2012) Table 30-3 # Rapid-filling inflow produced by atrial systole during "ventricular diastole–late" * Atrioventricular (AV) valves= tricuspid valve; mitral valve Semilunar valves= pulmonary valve; aortic valve Dicrotic notch- rebounding of aorta, helps perfuse coronary arteries, with increasing age the aorta stiffens and less elasticity hence the notch may be less and problems arise perfusing coronary arteries


Physiology

The
heart The heart is a muscular MUSCULAR (DS-200B), located in the United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain,Usage is mixed. The Guardian' and Telegraph' use ...

heart
is a four-chambered
organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly cate ...
consisting of right and left halves, called the
right heart The heart is a muscular MUSCULAR (DS-200B), located in the United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain,Usage is mixed. The Guardian' and Telegraph' use ...
and the
left heart The heart is a muscular organ in most animals, which pumps blood through the blood vessel The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cel ...
. The upper two chambers, the left and right
atriaAtria may refer to: *Atrium (heart) The atrium (Latin ātrium, “entry hall”) is the upper chamber through which blood enters the Ventricle (heart), ventricles of the heart. There are two atria in the human heart – the left atrium receives bloo ...
, are entry points ''into'' the heart for blood-flow returning from the
circulatory system The circulatory system, also called the cardiovascular system or the vascular system, is an organ system An organ system is a biological system A biological system is a complex network Network and networking may refer to: Arts, entertai ...
, 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 The mammalian heart is divided between the systemic and the pulmonary circulation, generally agreed upon as left and right sided circuits. The right circuit is the portion of the circulatory system The circulatory system, also called the ca ...
—during which the right ventricle pumps oxygen-depleted blood to the lungs through the
pulmonary trunk A pulmonary artery is an artery An artery (plural arteries) () is a blood vessel The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, ...
and arteries; or the
systemic circulation The circulatory system, also called the cardiovascular system or the vascular system, is an organ system that permits blood to circulate and transport nutrients (such as amino acids and electrolytes), oxygen, carbon dioxide, hormones, and bloo ...
—in which the left ventricle pumps/ejects newly oxygenated blood throughout the body via the
aorta The aorta ( ) is the main and largest artery An artery (plural arteries) () is a blood vessel The blood vessels are the components of the circulatory system The circulatory system, also called the cardiovascular system or the vas ...

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
cardiomyocyte Cardiac muscle cells or cardiomyocytes (also known as myocardiocytes or cardiac myocytes) are the muscle cells ( myocytes) that make up the cardiac muscle (heart muscle). Each myocardial cell contains myofibrils, which are specialized organelles ...
s, 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 The circulatory system, also called the cardiovascular system or the vascular system, is an organ system An organ system is a biological system A biological system is a complex network Network and networking may refer to: Arts, entertai ...
; 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 (or
sinus rhythm A sinus rhythm is any cardiac rhythm in which depolarisation of the cardiac muscle begins at the sinus node. It is characterised by the presence of correctly oriented P waves on the electrocardiogram Electrocardiography is the process of ...
) of this signaling across the heart is coordinated by two groups of specialized cells, the sinoatrial (SA) node, which is situated in the upper wall of the right atrium, and the atrioventricular (AV) node located in the lower wall of the
right heart The heart is a muscular MUSCULAR (DS-200B), located in the United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain,Usage is mixed. The Guardian' and Telegraph' use ...
between the atrium and ventricle. The sinoatrial node, often known as the ''
cardiac pacemaker 350px, Image showing the cardiac pacemaker or SA node, the normal pacemaker within the electrical conduction system of the heart. The contraction of cardiac muscle (heart The heart is a muscle, muscular Organ (anatomy), organ in most a ...
'', is the point of origin for producing a wave of electrical impulses that stimulates atrial contraction by creating an
action potential In physiology Physiology (; ) is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence ...

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 Discovered in 1893 by Swiss Swiss may refer to: * the adjectival form of Switzerland *Swiss people Places *Swiss, Missouri *Swiss, North Carolina *Swiss, West Virginia *Swiss, Wisconsin Other uses *Swiss-system tournament, in various games and s ...
and the
Purkinje fibers The Purkinje fibers ( ; Purkinje tissue or subendocardial branches) are located in the inner Ventricle (heart), ventricular walls of the heart, just beneath the endocardium in a space called the subendocardium. The Purkinje fibers are specializ ...

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 (contractions), ejecting the new blood volume and completing the cardiac cycle. (See Wiggers diagram: "Ventricular volume" tracing (red), at "Systole" panel.)


Diastole and systole in the cardiac cycle

Cardiac diastole is the period of the cardiac cycle when, after contraction, the heart relaxes and expands while refilling with blood returning from the
circulatory system The circulatory system, also called the cardiovascular system or the vascular system, is an organ system An organ system is a biological system A biological system is a complex network Network and networking may refer to: Arts, entertai ...
. Both atrioventricular (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 increase in blood-volume from the low plateau of the "Isovolumic relaxation" stage to the maximum volume occurring in the "Atrial systole" sub-stage.


Atrial systole

Atrial systole is the contracting of
cardiac muscle Cardiac muscle (also called heart muscle or myocardium) is one of three types of vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biol ...

cardiac muscle
cells of both
atriaAtria may refer to: *Atrium (heart) The atrium (Latin ātrium, “entry hall”) is the upper chamber through which blood enters the Ventricle (heart), ventricles of the heart. There are two atria in the human heart – the left atrium receives bloo ...
following electrical stimulation and conduction of electrical currents across the atrial chambers (see above, ''Physiology''). 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 as ''ventricular 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 kick'' closes the diastole immediately before the heart again begins contracting and ejecting blood ''from'' the ventricles (ventricular systole) to the aorta and arteries. Atrial kick is absent or disrupted if there is loss of normal electrical conduction in the heart, such as caused by
atrial fibrillation Atrial fibrillation (AF or A-fib) is an abnormal heart rhythm (arrhythmia) characterized by the rapid and irregular beating of the atrial chambers of the heart. It often begins as short periods of abnormal beating, which become longer or cont ...

atrial fibrillation
,
atrial flutter Atrial flutter (AFL) is a common abnormal heart rhythm that starts in the atrial chambers of the heart The heart is a muscle, muscular Organ (anatomy), organ in most animals, which pumps blood through the blood vessels of the circulatory ...
, or
heart block Heart block (HB) is a disorder in the heart The heart is a muscle, muscular Organ (anatomy), organ in most animals, which pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the ...

heart block
. Atrial kick may also be degraded by any deterioration in the condition of the heart, such as "stiff heart" found in patients with
diastolic dysfunction Heart failure with preserved ejection fraction (HFpEF) is a form of heart failure in which the ejection fraction – the percentage of the volume of blood ejected from the left ventricle with each heartbeat divided by the volume of blood when the le ...
.


Ventricular systole

Ventricular systole is the contractions, following electrical stimulations, of the ventricular syncytium of cardiac muscle cells in the left and right ventricles. Contractions in the right ventricle provide
pulmonary circulation The mammalian heart is divided between the systemic and the pulmonary circulation, generally agreed upon as left and right sided circuits. The right circuit is the portion of the circulatory system The circulatory system, also called the ca ...
by pulsing oxygen-depleted blood through the pulmonary valve then through the pulmonary arteries to the lungs. Simultaneously, contractions of the left ventricular systole provide
systemic circulation The circulatory system, also called the cardiovascular system or the vascular system, is an organ system that permits blood to circulate and transport nutrients (such as amino acids and electrolytes), oxygen, carbon dioxide, hormones, and bloo ...
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).


See also

* *
Cardiac action potential The cardiac action potential is a brief change in voltage (membrane potential) across the sarcolemma, cell membrane of cardiac myocytes, heart cells. This is caused by the movement of charged atoms (called ions) between the inside and outside of th ...

Cardiac action potential
*
Cardiac output Cardiac output (CO), also known as heart output denoted by the symbols Q, or \dot Q_ , is a term used in cardiac physiologyCardiac physiology or heart function is the study of healthy, unimpaired function of the heart: involving blood flow; Ca ...


References


External links


Interactive cardiac cycle
''Interactivephysiology.com'' {{Cardiovascular physiology Cardiovascular physiology