Cardiac cycle

Definition

The cardiac cycle is the sequence of events that occur when the heart beats. The cycle has two main phases: diastole, when the heart ventricles are relaxed, and systole, when the ventricles contract. One cardiac cycle is defined as the contraction of the two atria followed by contraction of the two ventricles.

Description

The heart is a muscular organ that works as a pumping system. It takes in blood with reduced levels of oxygen from the veins (deoxygenated blood), and delivers it to the lungs for oxygenation. When it receives the oxygenated blood back from the lungs, it pumps the blood back into the arteries to be distributed throughout the body.

The heart is divided into four hollow chambers, two on the left and two on the right. The right chambers are the right atrium and the right ventricle. They receive blood from the veins. The left chambers are the left atrium and the left ventricle. They receive blood from the pulmonary circulation, and the left ventricle forces blood into the systemic circulation.

In a cardiac cycle, blood enters the right atrium of the heart from the superior and inferior vena cavae, and flows across the tricuspid valve into the right ventricle. From the right ventricle the blood flows into the pulmonary artery, which is separated from the ventricle by the pulmonary valve. After oxygenation in the lungs, blood returns to the heart via four pulmonary veins that enter the left atrium. From the left atrium, blood flows across the mitral valve and into the left ventricle. From the left ventricle blood is ejected across the aortic valve into the aorta. Together, the mitral and triscupsid valves are known as the atrioventricular valves and the aortic and pulmonary valves as the semilunar valves.

From a mechanical point of view, the cardiac cycle is due to blood movement occurring as a result of pressure differences within the chambers of the heart. In order for blood to flow through a blood vessel or across and heart valve, there must be a force acting on the blood. This force is provided by the difference in blood pressure (a pressure gradient) across these structures by the contractions of the heart. Each heart beat, or cardiac cycle, is divided into two phases of contraction and relaxation, stimulated by electrical impulses from the sinoatrial node (SA node), a patch of tissue in the heart that sets the rate of contractions. It contracts itself and then sends nerve impulse to the atria. The time during which ventricular contraction occurs is called systole. The time between ventricular contractions, during which ventricular filling occurs, is called diastole (also known as the relaxation phase).

In early diastole, the ventricles relax, the semilunar valves close, the atrioventricular valves open and the ventricles fill with blood. In mid diastole, the atria and ventricles are relaxed, the semilunar valves are closed, the atrioventricular valves are open, and the ventricles keep filling with blood. In late diastole, the SA node sends an electrical impulse to the atria, which causes the atria to contract and the ventricles to fill with more blood. The electrical signal that causes contraction moves from the atria toward the ventricles. Before it does, though, it reaches the atrioventricular node (AV node). The AV node delays the signal so that the ventricle can contract all at once rather than a little bit at a time.

Prior to systole, the electrical signal passes from the AV node down the AV bundle, also known as the bundle of His, to the Purkinje fibers. The fibers allow the fast spread of the electrical signal to all parts of the ventricles, and the electrical signal causes the ventricles to contract. Systole begins with the closure of the atrioventricular valves. During systole, the ventricles contract, the semi- lunar valves open, and blood is pumped from the ventricles to the aorta.

Blood pressure is highest during systole, and lowest during diastole. It has two components, the systolic and diastolic pressure. Normal systolic pressure for an adult is 120 mm Hg, and normal diastolic pressure is 80 mm Hg. These values are commonly recorded as 120/80.

The normal heart beats at a rate of about 72 beats per minute (with a range of 60-100 beats per minute), but can vary with normal daily activity.

The cardiac cycle produces well-known sounds that can be clearly heard with a stethoscope . The first heart sound is associated with closure of the atrioventricular valves and signals the start of ventricular systole. The second heart sound is associated with the closure of the semilunar valves and indicates the start of ventricular diastole. A third heart sound is due to the rapid phase of ventricular filling, and a fourth heart sound is due to atrial systole. The two last sounds are usually not loud enough to hear, so neither of them is heard under normal exam procedures.

Role in human health

The role of the cardiac cycle is essential to maintain life, as the heart distributes the oxygen-carrying blood required for the functioning of the body.

Common diseases and disorders

Although diseases of the valves are common, and other cardiac disorders can also disrupt normal blood flow, they do not cause an abnormal cardiac cycle per se. Since the normal sequence of events occurs, even though it's impaired, it is considered a normal cardiac cycle (but not a normal heart beat). Most of the common abnormalities in the cardiac cycle are caused by disturbances in electrical conduction of the heart.

Disturbances in the electrical cycle are known as arrhythmias. However, two types of disturbances, sinus tachycardia and sinus bradycardia, do not affect the sequence of events, because they speed up (tachycardia)

or slow down (bradycardia) the beating of the heart. The other arrthymias have one of two origins. They can be caused by a lack of normal electrical conduction. For example, a lack of electrical signaling from the sinoatrial node can lead to the placement of an exogenous pacemaker (something that causes the heart to beat other than the sinoatrial node). The other cause of arrhythmias is abnormally strong ectopic (in the wrong place) electrical activity. For example, premature atrial contractions can be caused by excessive electrical activity somewhere in the atrium other than the sinoatrial node. Some arrhythmias may be a combination of the two.

Cardiac cycle disturbances can be complex, but can be dissected by an electrocardiogram (EKG or ECG). They can be caused by a wide variety of problems, including, but not limited to, coronary heart disease, damage to the heart muscle secondary to a heart attack, genetic heart defects, valvular disease of the heart, and medications. Following is a list of common arrhythmias. Atrial arrhythmias:

• paroxysmal atrial tachycardia
• multifocal atrial tachycardiaatrial fibrillation
• atrial flutter

Ventricular arrhythmias:

• premature ventricular contractions (PVCs)
• ventricular tachycardia
• ventricular fibrillation

Arrhythmias arising because of abnormalities in the AV node, Purkinje fibers, or bundle of His:

• nodal rhythm
• first degree block
• second degree block
• third degree block
• bundle branch block

Resources

BOOKS

Guyton, A. C., G. Guyton, and F. Hall. Textbook of Medical Physiology, Ninth ed. Philadelphia: W.B. Saunders Co.,1995.

KEY TERMS

Aorta —The largest artery of the body that originates from the left ventricle of the heart, arches over the heart to the left, and descends just in front of the spinal column. The aorta divides into three arteries: the brachiocephalic artery (that supplies blood to the brain and head), the left carotid artery, and the left subclavian artery.

Aortic valve —The heart valve that divides the left ventricle and the aorta. Blood from the left ventricle is ejected across the aortic valve into the aorta for further distribution. It opens during contraction of the left ventricle and closes afterwards to prevent the backward flow of blood from the aorta.

Arterial blood pressure —The ejection of blood into the aorta by the left ventricle results in the characteristic aortic pressure pulse. The maximum of the aortic pressure pulse is called the systolic pressure and the lowest pressure in the aorta is called the diastolic pressure.

Arterial pulse pressure —The arterial pulse pressure is the difference between the systolic and diastolic arterial pressures.

Artery —Blood vessel that carries blood away from the heart for distribution throughout the body.

Atrioventricular node (AV node) —Nodal tissue located on the right side of the partition that divides the atria, near the bottom of the right atrium. It delays impulses from the SA node, thus allowing the atria to empty themselves.

Atrioventricular valves —Heart valves located between the atria and the ventricles. They are the mitral valve and the tricuspid valve.

Diastole —The time in between ventricular contractions during which ventricular filling occurs. Also called the relaxation phase.

Diastolic pulse pressure —The pressure exerted on the walls of the arteries during diastole. A normal value ranges around 90 mmHg.

Heart —In humans, the heart is divided into four chambers: the right atrium and ventricle and the left atrium and ventricle. Blood flows from the veins into the right atrium, then to the right ventricle and into the lungs for oxygenation, from where it is returned to the left atrium, then to the left ventricle for distribution to the body via arteries.

Mitral valve —The heart valve that divides the left atrium and the left ventricles. It opens during contraction of the left atrium to allow blood flow into the left ventricle and closes to prevent the backward flow of blood to the left atrium.

Oxygenation —The process of adding oxygen to something. Oxygen is added to the blood in the lungs. Oxygen contained in the inhaled air is delivered to the blood, where the oxygen binds to a protein called hemoglobin (which functions as an oxygen-carrier).

Pulmonary artery —Short blood vessel that carries deoxygenated blood from the heart to the lungs.

Pulmonary vein —One of four blood vessels that carry oxygenated blood from the lungs to the heart.

Pulmonary valve —The heart valve that divides the right ventricle from the pulmonary artery.

Semilunar valves —Heart valves shaped like a half- moon that located between the aorta and the left ventricle and between the pulmonary artery and the right ventricle. They are the aortic valve and the pulmonary valve.

Sinoatrial node (SA node) —Also called the pacemaker of the heart. It consists of nodal tissue located in the upper wall of the right atrium. It controls the rate of contraction of the heart by generating nerve impulses that travel throughout the heart wall causing both atria to contract.

Systole —The time during which ventricular contraction occurs.

Systolic pulse pressure —The pressure exerted on the walls of the arteries during the contraction of the heart. A normal value ranges around 150 mmHg.

Tricuspid valve —The heart valve, named for its three cusps, that divides the right atrium from the right ventricle. Blood flows from the right atrium, across the tricuspid valve, and into the right ventricle. When closed, it prevents the blood from flowing back into the right atrium.

Vein —Blood vessel that returns blood to the heart from the body. All the veins from the body converge into two major veins that lead to the right atrium of the heart. These veins are the superior vena cava and the inferior vena cava. The pulmonary vein carries the blood from the right ventricle of the heart into the lungs.

Tortora, Gerard, and Sandra Grabowski. "The Cardiovascular System: The Heart." In Principles of Anatomy and Physiology, 8th ed. New York: Harper Collins, 1996.

ORGANIZATIONS

American Heart Association National Center. 7272 Greenville Avenue, Dallas, Texas 75231. (800) AHA-USA1. <http://www.americanheart.org>.

OTHER

Anderson, R. The Cardiac Cycle. Windows/Macintosh CD- rom, 1996.

Cardiovascular Physiology Web Resource. The Cardiac Cycle. <http://www.oucom.ohiou.edu/CVPhysiology/HD002.htm>.

Monique Laberge, PhD

Cardiac Cycle

Definition

The cardiac cycle is the sequence of events that occur when the heart beats. The cycle has two main phases: diastole, when the heart ventricles are relaxed; and systole, when the ventricles contract. One cardiac cycle is defined as the contraction of the two atria followed by contraction of the two ventricles.

Description

The heart is a muscular organ that works as a pumping system. It takes in blood with reduced levels of oxygen from the veins (deoxygenated blood), and delivers it to the lungs for oxygenation. When it receives the oxygenated blood back from the lungs, it pumps the blood back into the arteries to be distributed throughout the body.

The heart is divided into four hollow chambers, two on the left and two on the right. The right chambers are the right atrium and the right ventricle. They receive blood from the veins. The left chambers are the left atrium and the left ventricle. They receive blood from the pulmonary circulation, and the left ventricle forces blood into the systemic circulation.

In a cardiac cycle, blood enters the right atrium of the heart from the superior and inferior vena cavae, and flows across the tricuspid valve into the right ventricle. From the right ventricle the blood flows into the pulmonary artery, which is separated from the ventricle by the pulmonary valve. After oxygenation in the lungs, blood returns to the heart via four pulmonary veins that enter the left atrium. From the left atrium, blood flows across the mitral valve and into the left ventricle. From the left ventricle blood is ejected across the aortic valve into the aorta. Together, the mitral and triscupsid valves are known as the atrioventricular valves and the aortic and pulmonary valves as the semilunar valves.

From a mechanical point of view, the cardiac cycle is due to blood movement occurring as a result of pressure differences within the chambers of the heart. In order for blood to flow through a blood vessel or across and heart valve, there must be a force acting on the blood. This force is provided by the difference in blood pressure (a pressure gradient) across these structures by the contractions of the heart. Each heart beat, or cardiac cycle, is divided into two phases of contraction and relaxation, stimulated by electrical impulses from the sinoatrial node (SA node), a patch of tissue in the heart that sets the rate of contractions. It contracts itself and then sends nerve impulse to the atria. The time during which ventricular contraction occurs is called systole. The time between ventricular contractions, during which ventricular filling occurs, is called diastole (also known as the relaxation phase).

In early diastole, the ventricles relax, the semilunar valves close, the atrioventricular valves open and the ventricles fill with blood. In mid diastole, the atria and ventricles are relaxed, the semilunar valves are closed, the atrioventricular valves are open, and the ventricles keep filling with blood. In late diastole, the SA node sends an electrical impulse to the atria, which causes the atria to contract and the ventricles to fill with more blood. The electrical signal that causes contraction moves from the atria toward the ventricles. Before it does, though, it reaches the atrioventricular node (AV node). The AV node delays the signal so that the ventricle can contract all at once rather than a little bit at a time.

Prior to systole, the electrical signal passes from the AV node down the AV bundle, also known as the bundle of His, to the Purkinje fibers. The fibers allow the fast spread of the electrical signal to all parts of the ventricles, and the electrical signal causes the ventricles to contract. Systole begins with the closure of the atrioventricular valves. During systole, the ventricles contract, the semilunar valves open, and blood is pumped from the ventricles to the aorta.

Blood pressure is highest during systole, and lowest during diastole. It has two components, the systolic and diastolic pressure. Normal systolic pressure for an adult is 120 mm Hg, and normal diastolic pressure is 80 mm Hg. These values are commonly recorded as 120/80.

The normal heart beats at a rate of about 72 beats per minute (with a range of 60-100 beats per minute), but can vary with normal daily activity.

The cardiac cycle produces well-known sounds that can be clearly heard with a stethoscope. The first heart sound is associated with closure of the atrioventricular valves and signals the start of ventricular systole. The second heart sound is associated with the closure of the semilunar valves and indicates the start of ventricular diastole. A third heart sound is due to the rapid phase of ventricular filling, and a fourth heart sound is due to atrial systole. The two last sounds are usually not loud enough to hear, so neither of them is heard under normal exam procedures.

Role in human health

The role of the cardiac cycle is essential to maintain life, as the heart distributes the oxygen-carrying blood required for the functioning of the body.

Common diseases and disorders

Although diseases of the valves are common, and other cardiac disorders can also disrupt normal blood flow, they do not cause an abnormal cardiac cycle per se. Since the normal sequence of events occurs, even though it is impaired, it is considered a

KEY TERMS

Aorta— The largest artery of the body that originates from the left ventricle of the heart, arches over the heart to the left, and descends just in front of the spinal column. The aorta divides into three arteries: the brachiocephalic artery (that supplies blood to the brain and head), the left carotid artery, and the left subclavian artery.

Aortic valve— The heart valve that divides the left ventricle and the aorta. Blood from the left ventricle is ejected across the aortic valve into the aorta for further distribution. It opens during contraction of the left ventricle and closes afterwards to prevent the backward flow of blood from the aorta.

Arterial blood pressure— The ejection of blood into the aorta by the left ventricle results in the characteristic aortic pressure pulse. The maximum of the aortic pressure pulse is called the systolic pressure and the lowest pressure in the aorta is called the diastolic pressure.

Arterial pulse pressure— The arterial pulse pressure is the difference between the systolic and diastolic arterial pressures.

Artery— Blood vessel that carries blood away from the heart for distribution throughout the body.

Atrioventricular node (AV node)— Nodal tissue located on the right side of the partition that divides the atria, near the bottom of the right atrium. It delays impulses from the SA node, thus allowing the atria to empty themselves.

Atrioventricular valves— Heart valves located between the atria and the ventricles. They are the mitral valve and the tricuspid valve.

Diastole— The time in between ventricular contractions during which ventricular filling occurs. Also called the relaxation phase.

Diastolic pulse pressure— The pressure exerted on the walls of the arteries during diastole. A normal value ranges around 80 mmHg.

Heart— In humans, the heart is divided into four chambers: the right atrium and ventricle and the left atrium and ventricle. Blood flows from the veins into the right atrium, then to the right ventricle and into the lungs for oxygenation, from where it is returned to the left atrium, then to the left ventricle for distribution to the body via arteries.

Mitral valve— The heart valve that divides the left atrium and the left ventricles. It opens during contraction of the left atrium to allow blood flow into the left ventricle and closes to prevent the backward flow of blood to the left atrium.

Oxygenation— The process of adding oxygen to something. Oxygen is added to the blood in the lungs. Oxygen contained in the inhaled air is delivered to the blood, where the oxygen binds to a protein called hemoglobin (which functions as an oxygen-carrier).

Pulmonary artery— Short blood vessel that carries deoxygenated blood from the heart to the lungs.

Pulmonary vein— One of four blood vessels that carry oxygenated blood from the lungs to the heart.

Pulmonary valve— The heart valve that divides the right ventricle from the pulmonary artery.

Semilunar valves— Heart valves shaped like a half-moon that located between the aorta and the left ventricle and between the pulmonary artery and the right ventricle. They are the aortic valve and the pulmonary valve.

Sinoatrial node (SA node)— Also called the pacemaker of the heart. It consists of nodal tissue located in the upper wall of the right atrium. It controls the rate of contraction of the heart by generating nerve impulses that travel throughout the heart wall causing both atria to contract.

Systole— The time during which ventricular contraction occurs.

Systolic pulse pressure— The pressure exerted on the walls of the arteries during the contraction of the heart. A normal value ranges around 120 mmHg.

Tricuspid valve— The heart valve, named for its three cusps, that divides the right atrium from the right ventricle. Blood flows from the right atrium, across the tricuspid valve, and into the right ventricle. When closed, it prevents the blood from flowing back into the right atrium.

Vein— Blood vessel that returns blood to the heart from the body. All the veins from the body converge into two major veins that lead to the right atrium of the heart. These veins are the superior vena cava and the inferior vena cava. The pulmonary vein carries the blood from the right ventricle of the heart into the lungs.

normal cardiac cycle (but not a normal heart beat). Most of the common abnormalities in the cardiac cycle are caused by disturbances in electrical conduction of the heart.

Disturbances in the electrical cycle are known as arrhythmias. However, two types of disturbances, sinus tachycardia and sinus bradycardia, do not affect the sequence of events, because they speed up (tachycardia) or slow down (bradycardia) the beating of the heart. The other arrthymias have one of two origins. They can be caused by a lack of normal electrical conduction. For example, a lack of electrical signaling from the sinoatrial node can lead to the placement of an exogenous pacemaker (something that causes the heart to beat other than the sinoatrial node). The other cause of arrhythmias is abnormally strong ectopic (in the wrong place) electrical activity. For example, premature atrial contractions can be caused by excessive electrical activity somewhere in the atrium other than the sinoatrial node. Some arrhythmias may be a combination of the two.

Cardiac cycle disturbances can be complex, but can be dissected by an electrocardiogram (EKG or ECG). They can be caused by a wide variety of problems, including, but not limited to, coronary heart disease, damage to the heart muscle secondary to a heart attack, genetic heart defects, valvular disease of the heart, and medications. Following is a list of common arrhythmias.

Atrial arrhythmias:

• paroxysmal atrial tachycardia
• multifocal atrial tachycardia
• atrial fibrillation
• atrial flutter

Ventricular arrhythmias:

• premature ventricular contractions (PVCs)
• ventricular tachycardia
• ventricular fibrillation

Arrhythmias arising because of abnormalities in the AV node, Purkinje fibers, or bundle of His:

• nodal rhythm
• first degree block
• second degree block
• third degree block
• bundle branch block

Resources

BOOKS

Guyton, A. C., G. Guyton, and F. Hall. Textbook of Medical Physiology, Ninth ed. Philadelphia: W.B. Saunders Co., 1995.

Tortora, Gerard, and Sandra Grabowski. "The Cardiovascular System: The Heart." In Principles of Anatomy and Physiology, 8th ed. New York: Harper Collins, 1996.

ORGANIZATIONS

American Heart Association National Center. 7272 Greenville Avenue, Dallas, Texas 75231. (800) AHA-USA1. 〈http://www.americanheart.org〉.

OTHER

Anderson, R. The Cardiac Cycle. Windows/Macintosh CD-rom, 1996.

Cardiovascular Physiology Web Resource. The Cardiac Cycle. 〈http://www.oucom.ohiou.edu/CVPhysiology/HD002.htm〉.

Cardiac cycle

Resource

The normal muscular contractions of the heart are called the cardiac cycle. The heart consists of two pairs of hollow muscular chambers through which blood is pumped. One pair—the right atrium and right ventricle—pumps blood through the lungs. The other pair—the left atrium and left ventricle—pumps blood through the rest of the body, including the arteries and veins of the heart itself. Each atrium serves to fill its corresponding ventricle, which then does the actual work of pumping the blood out of the heart. The pairs of chambers pump at the same time. In each pair, the atrium contracts first, to fill the ventricle. This is why the heart makes a characteristic lub-DUB sound: the “lub” is the sound of valves shutting when the atria (which are smaller and weaker) contract, and the louder “DUB” is the sound of valves shutting when the ventricles, which have just been filled by the atria, contract. Valves at the entry and exit of every heart chamber keep blood from flowing the wrong way.

In humans, the cardiac cycle can be divided into two major phases, the systolic phase and the diastolic phase. Systole (pronounced SIS-tole-ee) occurs during the systolic phase, when the ventricles of the heart contract. Accordingly, systole results in the highest pressures within the systemic and pulmonary circulatory systems. Diastole (pronounced die-ASS-tole-ee) occurs during the diastolic phase, when the right and left ventricles relax and fill.

The cardiac cycle cannot be described as a linear series of events associated with the flow of blood through the four chambers. One cannot accurately describe the cardiac cycle by simply tracing the path of blood from the right atrium, into the right ventricle, into the pulmonary circulation, the venous pulmonary return to the left atrium, and finally the ejection into the aorta and systemic circulation by the contraction of the left ventricle. In reality, the cardiac cycle is a coordinated series of events that take place simultaneously on both the right pulmonary circuit and left systemic circuit of the heart.

The cardiac cycle begins with a period of rapid ventricular filling. The right atrium fills with deoxygenated blood from the superior vena cava, the inferior vena cava, and the coronary venous return (e.g., the coronary sinus and smaller coronary veins). At the same time, the pulmonary veins return oxygenated blood from the lungs to the left atrium. During the early diastolic phase of the cardiac cycle, both ventricles relax and fill from their respective atrial sources. The atrio-ventricular valves (the tricuspid valve is located between the right atrium and right ventricle; the mitral valve is between the left atrium and left ventricle) open and allow blood to flow from the atria into the ventricles.

The flow of blood through the atrio-ventricular valves is unidirectional and as volume-related pressure increases within the ventricles, the atrioventricular valves close to prevent backflow from the ventricles into the atria.

At the onset of the systolic phase, specialized cardiac muscle fibers within the sino-atrial node (S-A node) contract and send an electrical signal propagated throughout the heart. In a sweeping fashion, the right atrium contracts and forces the final volume of blood into the right ventricle. The left atrium contracts and contributes the final 20% of volume to the left ventricle.

The S-A node signal is delayed by the atrioventricular node to allow the full contraction of the atria that allows the ventricles to reach their maximum volume. A sweeping right to left wave of ventricular contraction then pumps blood into the pulmonary and systemic circulatory systems. The semilunar valves that separate the right ventricle from the pulmonary artery and the left ventricle from the aorta open shortly after the ventricles begin to contract. The opening of the semilunar valves ends a brief period of isometric (constant volume) ventricular contraction and initiates a period of rapid ventricular ejection.

As muscle fibers contract, they lose their ability to contract forcefully (i.e., the greatest force of muscular contraction in the ventricle occurs earlier in the contraction phase and decreases as contraction proceeds). When ventricular pressures fall below their respective attached arterial pressures, the semilunar pulmonary and aortic valves close. At the end of systole, the semi-lunar valves shut to prevent the backflow of blood into the ventricles.

After emptying, both ventricles collapse to undergo a period of repolarization and refilling. The receptivity of the ventricles to filling corresponding lowers atrial pressures and allows them to fill from their respective venous sources. At the outset, ventricular pressures remain greater than atrial pressures and the atrioventricular valves remain closed. Because the volume of blood in the ventricle is once again static— closed off by both the atrio-ventricular and semilunar valves—this period is described as isometric (same volume) relaxation.

The cardiac cycle is complete with the onset of another period of rapid ventricular filling that takes place when atrial pressures exceed ventricular pressures and the atrio-ventricular valves open to allow rapid filling.

It is the opening and snapping shut of the atrioventricular and semilunar pulmonary and aortic valves that creates the familiar pattern of sound associated with the cardiac cycle. Because the right to left contractions of the atria and ventricles are sweeping, the opening and closings of the right side and left side valves are separated by a short interval. The first heart sound results from the closure of the atrio-ventricular valves. The second heart sound results from the closure of the semilunar valves.

The electrical events associated with cardiac cycle are measured with the electrocardiogram (EKG or ECG). Many heart problems can be diagnosed by examining an ECG.

See also Action potential; Circulatory system; Heart diseases; Heart, embryonic development and changes at birth; Heart, rhythm control and impulse conduction; Nerve impulses and conduction of impulses; Nervous system; Neuron.

Resources

BOOKS

Chizner, Michael. Clinical Cardiology Made Ridiculously Simple. Miami, FL: Medmaster, 2006.

Vetrovec, Inc. Cardiology for the Primary Care Physician. Philadelphia: W.B. Saunders Company, 2006.

OTHER

Klabunde, R.E. “Cardiac Cycle.” Cardiovascular Physiology Concepts. July 17, 2006. <http://www.cvphysiology.com/Heart%20Disease/HD002.htm> (accessed October 21, 2006).

Brenda Wilmoth Lerner

Cardiac cycle

The coordinated and rhythmic series of muscular contractions associated with the heart comprise the cardiac cycle.

In humans, the cardiac cycle can be subdivided into two major phases, the systolic phase and the diastolic phase. Systole occurs when the ventricles of the heart contract. Accordingly, systole results in the highest pressures within the systemic and pulmonary circulatory systems. Diastole is the period between ventricular contractions when the right and left ventricles relax and fill.

The cardiac cycle cannot be described as a linear series of events associated with the flow of blood through the four chambers. One can not accurately describe the cardiac cycle by simply tracing the path of blood from the right atrium, into the right ventricle, into the pulmonary circulation, the venous pulmonary return to the left atrium, and finally the ejection into the aorta and systemic circulation by the contraction of the left ventricle. In reality, the cardiac cycle is a coordinated series of events that take place simultaneously on both the right pulmonary circuit and left systemic circuit of the heart.

The cardiac cycle begins with a period of rapid ventricular filling. The right atrium fills with deoxygenated blood from the superior vena cava, the inferior vena cava, and the coronary venous return (e.g., the coronary sinus and smaller coronary veins ). At the same time, the pulmonary veins return oxygenated blood from the lungs to the left atrium. During the early diastolic phase of the cardiac cycle, both ventricles relax and fill from their respective atrial sources. The atrio-ventricular valves (the tricuspid valve is located between the right atrium and right ventricle; the mitral valve is between the left atrium and left ventricle) open and allow blood to flow from the atria into the ventricles.

The flow of blood through the atrio-ventricular valves is unidirectional and as volume related pressure increases within the ventricles, the atrioventricular valves close to prevent backflow from the ventricles into the atria.

At the onset of the systolic phase, specialized cardiac muscle fibers within the sino-atrial node (S-A node) contract and send an electrical signal propagated throughout the heart. In a sweeping fashion, the right atrium contracts and forces the final volume of blood into the right ventricle. The left atrium contracts and contributes the final 20% of volume to the left ventricle.

The S-A node signal is delayed by the atrioventricular node to allow the full contraction of the atria that allows the ventricles to reach their maximum volume. A sweeping right to left wave of ventricular contraction then pumps blood into the pulmonary and systemic circulatory systems. The semilunar valves that separate the right ventricle from the pulmonary artery and the left ventricle from the aorta open shortly after the ventricles begin to contract. The opening of the semilunar valves ends a brief period of isometric (constant volume) ventricular contraction and initiates a period of rapid ventricular ejection.

As muscle fibers contract, they lose their ability to contract forcefully (i.e., the greatest force of muscular contraction in the ventricle occurs earlier in the contraction phase and decreases as contraction proceeds). When ventricular pressures fall below their respective attached arterial pressures, the semilunar pulmonary and aortic valves close. At the end of systole, the semilunar valves shut to prevent the backflow of blood into the ventricles.

After emptying, both ventricles collapse to undergo a period of repolarization and refilling. The receptivity of the ventricles to filling corresponding lowers atrial pressures and allows them to fill from their respective venous sources. At the outset, ventricular pressures remain greater than atrial pressures and the atrioventricular valves remain closed. Because the volume of blood in the ventricle is once again static—closed off by both the atrio-ventricular and semilunar valves—this period is described as isometric (same volume) relaxation.

The cardiac cycle is complete with the onset of another period of rapid ventricular filling that takes place when atrial pressures exceed ventricular pressures and the atrio-ventricular valves open to allow rapid filling.

It is the opening and snapping shut of the atrio-ventricular and semilunar pulmonary and aortic valves that creates the familiar pattern of sound associated with the cardiac cycle. Because the right to left contractions of the atria and ventricles are sweeping, the opening and closings of the right side and left side valves are separated by a short interval. The first heart sound results from the closure of the atrio-ventricular valves. The second heart sound results from the closure of the semilunar valves.

The electrical events associated with cardiac cycle are measured with the electrocardiogram (EKG).

See also Action potential; Circulatory system; Heart diseases; Heart, embryonic development and changes at birth; Heart, rhythm control and impulse conduction; Nerve impulses and conduction of impulses; Nervous system; Neuron.

Resources

books

Gilbert, Scott F. Developmental Biology. 6th ed. Sunderland, MA: Sinauer Associates, Inc., 2000.

Guyton, Arthur C., and John E. Hall. Textbook of Medical Physiology. 10th ed. Philadelphia: W.B. Saunders Co., 2000.

Thibodeau, Gary A., and Kevin T. Patton. Anatomy & Physiology. 5th ed. St. Louis: Mosby, 2002.

Other

Klabunde, R. E. "Cardiac Cycle." Cardiovascular Physiology Concepts. January 17, 2003 [cited January 22, 2003] <http://www.cv-physiology.com/Heart%20Disease/HD002.htm>.

Brenda Wilmoth Lerner

cardiac cycle The series of muscular contractions which squeeze blood through the heart. In Amphibia and higher vertebrates, blood is pumped through a double circulatory system. The right side of the heart pumps blood through the pulmonary circuit to reach the lungs, the left side pumps oxygenated blood through the systemic circuit which supplies the body. Deoxygenated blood enters the right atrium via the vena cava and is pumped into the right ventricle. The right ventricle pumps the blood into the pulmonary artery which divides and leads to both lungs. Oxygenated blood returns from the lungs via the pulmonary vein to the left atrium and in turn to the left ventricle. From the left ventricle, the most muscular of the heart chambers, the blood is pumped around the body where it becomes deoxygenated and returns to the right atrium. Amphibia have only one ventricle, which must pump blood around both circuits; deoxygenated blood is kept separate from oxygenated blood by a combination of timing and complex architecture. During the phase of muscle relaxation (diastole), blood flows freely into the atrium until it is full whereupon the sino-atrial node sends out an electrical pulse causing the cardiac muscle of the atrium to contract (systole). Blood is forced into the atrium, which is in diastole, until it is full, whereupon the atrioventricular node sends out an electrical pulse along the Purkinje fibres around the right ventricle and the bundle of His (specialized muscle fibres) around the left ventricle, causing the ventricle to enter systole and force blood out into the arteries of the circulatory system. Nerves of the sympathetic nervous system from the spinal cord connect to the sino-atrial node and can speed up the rate of heartbeat, while the vagus nerve from the medulla also connected to the sino-atrial node slows it down.