Cardiac cycle
Encyclopedia
The cardiac cycle is a term referring to all or any of the events related to the flow or blood pressure
Blood pressure
Blood pressure is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. When used without further specification, "blood pressure" usually refers to the arterial pressure of the systemic circulation. During each heartbeat, BP varies...

 that occurs from the beginning of one heartbeat
Heart sounds
Heart sounds, or heartbeats, are the noises generated by the beating heart and the resultant flow of blood through it...

 to the beginning of the next. The frequency of the cardiac cycle is described by the heart rate
Heart rate
Heart rate is the number of heartbeats per unit of time, typically expressed as beats per minute . Heart rate can vary as the body's need to absorb oxygen and excrete carbon dioxide changes, such as during exercise or sleep....

. Each beat of the heart involves five major stages. The first two stages, often considered together as the "ventricular filling" stage, involve the movement of blood from atria into ventricles. The next three stages involve the movement of blood from the ventricles to the pulmonary artery (in the case of the right ventricle) and the aorta (in the case of the left ventricle).

The first, "early diastole
Diastole
Diastole is the period of time when the heart fills with blood after systole . Ventricular diastole is the period during which the ventricles are relaxing, while atrial diastole is the period during which the atria are relaxing...

", is when the semilunar valves close, the atrioventricular (AV) valves
Heart valve
A heart valve normally allows blood flow in only one direction through the heart. The four valves commonly represented in a mammalian heart determine the pathway of blood flow through the heart...

 open, and the whole heart is relaxed. The second, "atrial systole
Systole (medicine)
Systole is the contraction of the heart. Used alone, it usually means the contraction of the left ventricle.In all mammals, the heart has 4 chambers. The left and right ventricles pump together. The atria and ventricles pump in sequence...

", is when the atrium contracts, the AV valves open, and blood flows from atrium to the ventricle. The third, "isovolumic ventricular contraction", is when the ventricles begin to contract, the AV and semilunar valves close, and there is no change in volume. The fourth, "ventricular ejection", is when the ventricles are empty and contracting, and the semilunar valves are open. During the fifth stage, "Isovolumic ventricular relaxation", pressure decreases, no blood enters the ventricles, the ventricles stop contracting and begin to relax, and the semilunar valves close due to the pressure of blood in the aorta.

Throughout the cardiac cycle, blood pressure
Blood pressure
Blood pressure is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. When used without further specification, "blood pressure" usually refers to the arterial pressure of the systemic circulation. During each heartbeat, BP varies...

 increases and decreases. The cardiac cycle is coordinated by a series of electrical impulses that are produced by specialized heart cells found within the sinoatrial node
Sinoatrial node
The sinoatrial node is the impulse-generating tissue located in the right atrium of the heart, and thus the generator of normal sinus rhythm. It is a group of cells positioned on the wall of the right atrium, near the entrance of the superior vena cava...

 and the atrioventricular node
Atrioventricular node
The atrioventricular node is a part of the electrical control system of the heart that coordinates heart rate. It electrically connects atrial and ventricular chambers...

. The cardiac muscle is composed of myocytes which initiate their own contraction without help of external nerves (with the exception of modifying the heart rate due to metabolic demand). Under normal circumstances, each cycle takes approximately one second.

Anatomical basis of the cardiac cycle

The heart is a four-chambered organ
Organ (anatomy)
In biology, an organ is a collection of tissues joined in structural unit to serve a common function. Usually there is a main tissue and sporadic tissues . The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are...

 consisting of right and left halves. Two of the chambers, the left and right atria, are entry-points into the heart, while the other two chambers, the left and right ventricles
Ventricle (heart)
In the heart, a ventricle is one of two large chambers that collect and expel blood received from an atrium towards the peripheral beds within the body and lungs. The Atria primes the Pump...

, are responsible for contractions that send the blood through the circulation. The circulation is split into the pulmonary and systemic
Systemic
Systemic refers to something that is spread throughout, system-wide, affecting a group or system such as a body, economy, market or society as a whole. Systemic may also refer to:-In medicine:...

 circulation. The right ventricle's role is to pump deoxygenated blood to the lungs through the pulmonary trunk and pulmonary arteries. The left ventricle's role is to pump now oxygenated blood to the body through the aorta.

Importantly, the right and left ventricles contract simultaneously, and so in consideration of the cardiac cycle the events that are occurring on one side of the heart are equivalent to the events occurring on the other side of the heart. However, the ventricles
Ventricle (heart)
In the heart, a ventricle is one of two large chambers that collect and expel blood received from an atrium towards the peripheral beds within the body and lungs. The Atria primes the Pump...

 contract shortly after the atria. The sino-atrial node sends out electrical waves of excitation to both atria, and it is prevented from flowing into the ventricles
Ventricle (heart)
In the heart, a ventricle is one of two large chambers that collect and expel blood received from an atrium towards the peripheral beds within the body and lungs. The Atria primes the Pump...

 by strands of non-conducting fibrous tissue situated laterally from the tricuspid/bicuspid valves to the septum. These waves of excitation travel towards the septum and into the atrio-ventricular node, where they are held for roughly 0.1 seconds. They are then discharged down the bundle of his
Bundle of His
The bundle of His, known as the AV bundle or atrioventricular bundle, is a collection of heart muscle cells specialized for electrical conduction that transmits the electrical impulses from the AV node to the point of the apex of the fascicular branches...

, then down the purkinje tissue, which are both situated inside the septum
Septum
In anatomy, a septum is a wall, dividing a cavity or structure into smaller ones.-In human anatomy:...

. The waves flow down towards the apex of the heart and are then discharged into the ventricles, causing them to contract (ventricular systole) This creates the well known beat of the heart.

Atrial systole

Atrial systole is the contraction of the heart muscle (myocardia) of the left and right atria. Normally, both atria contract at the same time. The term systole is synonymous with contraction (movement or shortening) of a muscle. Electrical systole is the electrical activity that stimulates the myocardium of the chambers of the heart to make them contract. This is soon followed by Mechanical systole, which is the mechanical contraction of the heart.

As the atria contract, the blood pressure in each atrium increases, forcing additional blood into the ventricles. The additional flow of blood is called atrial kick.

80% of the blood flows passively down to the ventricles, so the atria do not have to contract a great amount.

Atrial kick is absent if there is loss of normal electrical conduction in the heart, such as during atrial fibrillation
Atrial fibrillation
Atrial fibrillation is the most common cardiac arrhythmia . It is a common cause of irregular heart beat, identified clinically by taking a pulse. Chaotic electrical activity in the two upper chambers of the heart result in the muscle fibrillating , instead of achieving coordinated contraction...

, atrial flutter
Atrial flutter
Atrial flutter is an abnormal heart rhythm that occurs in the atria of the heart. When it first occurs, it is usually associated with a fast heart rate or tachycardia , and falls into the category of supra-ventricular tachycardias. While this rhythm occurs most often in individuals with...

, and complete heart block
Third degree heart block
-Presentation:Third-degree AV block, also known as complete heart block, is a medical condition in which the impulse generated in the SA node in the atrium does not propagate to the ventricles....

. Atrial kick is also different in character depending on the condition of the heart, such as stiff heart, which is found in patients with diastolic dysfunction.

Detection of atrial systole

Electrical systole of the atria begins with the onset of the P wave on the ECG
Electrocardiogram
Electrocardiography is a transthoracic interpretation of the electrical activity of the heart over a period of time, as detected by electrodes attached to the outer surface of the skin and recorded by a device external to the body...

.
The wave of bipolarization (or depolarization) that stimulates both atria to contract at the same time is due to sinoatrial node which is located on the upper wall of the right atrium.

Ventricular systole

Ventricular systole is the contraction of the muscles (myocardia) of the left and right ventricle
Ventricle (heart)
In the heart, a ventricle is one of two large chambers that collect and expel blood received from an atrium towards the peripheral beds within the body and lungs. The Atria primes the Pump...

s.

At the later part of the ejection phase, although the ventricular pressure falls below the aortic pressure, the aortic valve remains patent because of the inertial energy of the ejected blood.

The graph of aortic pressure throughout the cardiac cycle displays a small dip (the "incisure" or "dicrotic notch") which coincides with the aortic valve closure. The dip in the graph is immediately followed by a brief rise (the "dicrotic wave") then gradual decline. Just as the ventricles enter into diastole, the brief reversal of flow from the aorta back into the left ventricle causes the aortic valves to shut. This results in the slight increase in aortic pressure caused by the elastic recoil of the semilunar valves and aorta.

Heart sounds

The closing of the mitral and tricuspid valves (known together as the atrioventricular valves) at the beginning of ventricular systole cause the first part of the "lubb-dubb" sound made by the heart as it beats. Formally, this sound is known as the First Heart Tone, or S1. This first heart tone is created by the closure of mitral and tricuspid valve and is actually a two component sound, M1, T1.

The second part of the "lub-dubb" (the Second Heart Tone, or S2), is caused by the closure of the aortic and pulmonary valves at the end of ventricular systole. As the left ventricle empties, its pressure falls below the pressure in the aorta, and the aortic valve
Aortic valve
The aortic valve is one of the valves of the heart. It is normally tricuspid , although in 1% of the population it is found to be congenitally bicuspid . It lies between the left ventricle and the aorta....

 closes. Similarly, as the pressure in the right ventricle falls below the pressure in the pulmonary artery, the pulmonary valve
Pulmonary valve
The pulmonary valve is the semilunar valve of the heart that lies between the right ventricle and the pulmonary artery and has three cusps. Similar to the aortic valve, the pulmonary valve opens in ventricular systole, when the pressure in the right ventricle rises above the pressure in the...

 closes. The second heart sound is also two components, A2 and P2. The aortic valve closes earlier than the pulmonary valve and they are audibly separated from each other in the second heart sound. This "splitting" of S2 is only audible during inhalation. However, some cardiac conduction abnormalities such as left bundle branch block
Left bundle branch block
Left bundle branch block is a cardiac conduction abnormality seen on the electrocardiogram . In this condition, activation of the left ventricle is delayed, which results in the left ventricle contracting later than the right ventricle....

 (LBBB) allow the P2 sound to be heard before the A2 sound during expiration. With LBBB, inhalation brings A2 and P2 closer together where they cannot be audibly distinguished.

Electrocardiogram

In an electrocardiogram
Electrocardiogram
Electrocardiography is a transthoracic interpretation of the electrical activity of the heart over a period of time, as detected by electrodes attached to the outer surface of the skin and recorded by a device external to the body...

, electrical systole of the ventricles begins at the beginning of the QRS complex
QRS complex
The QRS complex is a name for the combination of three of the graphical deflections seen on a typical electrocardiogram . It is usually the central and most visually obvious part of the tracing. It corresponds to the depolarization of the right and left ventricles of the human heart...

.

Diastole

Cardiac Diastole is the period of time when the heart relaxes after contraction in preparation for refilling with circulating blood. Ventricular diastole is when the ventricles are relaxing, while atrial diastole is when the atria are relaxing. Together they are known as complete cardiac diastole.

During ventricular diastole, the pressure in the (left and right) ventricles drops from the peak that it reaches in systole
Systole (medicine)
Systole is the contraction of the heart. Used alone, it usually means the contraction of the left ventricle.In all mammals, the heart has 4 chambers. The left and right ventricles pump together. The atria and ventricles pump in sequence...

. When the pressure in the left ventricle drops to below the pressure in the left atrium, the mitral valve
Mitral valve
The mitral valve is a dual-flap valve in the heart that lies between the left atrium and the left ventricle...

 opens, and the left ventricle fills with blood that was accumulating in the left atrium. The isovolumic relaxation time
Isovolumic relaxation time
Isovolumic relaxation time is an interval in the cardiac cycle, from the aortic component of the second heart sound, that is, closure of the aortic valve, to onset of filling by opening of the mitral valve...

 (IVRT) is the interval from the aortic component of the second heart sound, that is, closure of the aortic valve
Aortic valve
The aortic valve is one of the valves of the heart. It is normally tricuspid , although in 1% of the population it is found to be congenitally bicuspid . It lies between the left ventricle and the aorta....

, to onset of filling by opening of the mitral valve. Likewise, when the pressure in the right ventricle drops below that in the right atrium, the tricuspid valve
Tricuspid valve
The tricuspid valve, or right atrioventricular valve, is on the right dorsal side of the mammalian heart, between the right atrium and the right ventricle. The normal tricuspid valve usually has three leaflets and three papillary muscles. They are connected to the papillary muscles by the chordae...

 opens, and the right ventricle fills with blood that was accumulating in the right atrium.
During diastole the pressure within the right ventricle is lower than that in aorta, allowing blood to circulate in the heart itself via the coronary arteries.

Regulation of the cardiac cycle

Cardiac muscle has automaticity, which means that it is self-exciting. (You could also call it "myogenic" tissue. Meaning a tissue able of creating its own excitement.) This is in contrast with skeletal muscle
Skeletal muscle
Skeletal muscle is a form of striated muscle tissue existing under control of the somatic nervous system- i.e. it is voluntarily controlled. It is one of three major muscle types, the others being cardiac and smooth muscle...

, which requires either conscious or reflex nervous stimuli for excitation. The heart's rhythmic contractions occur spontaneously, although the rate of contraction can be changed by nervous or hormonal influences, exercise and emotions. For example, the sympathetic nerves to accelerate heart rate and the vagus nerve
Vagus nerve
The vagus nerve , also called pneumogastric nerve or cranial nerve X, is the tenth of twelve paired cranial nerves...

 decelerates heart rate.

The rhythmic sequence of contractions is coordinated by the sinoatrial (SA) and atrioventricular
Atrioventricular node
The atrioventricular node is a part of the electrical control system of the heart that coordinates heart rate. It electrically connects atrial and ventricular chambers...

 (AV) nodes. The sinoatrial node, often known as the cardiac pacemaker
Cardiac pacemaker
right|thumb|350px|Image showing the cardiac pacemaker which is the SA nodeThe contraction of heart muscle in all animals with hearts is initiated by chemical impulses. The rate at which these impulses fire controls the heart rate...

, is located in the upper wall of the right atrium and is responsible for the wave of electrical stimulation that initiates atrial contraction by creating an action potential
Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...

. Once the wave reaches the AV node, situated in the lower right atrium, it is delayed there before being conducted through the bundles of His and back up the Purkinje fibers
Purkinje fibers
For the nervous cells, see Purkinje cellPurkinje fibers are located in the inner ventricular walls of the heart, just beneath the endocardium...

, leading to a contraction of the ventricles. The delay at the AV node allows enough time for all of the blood in the atria to fill their respective ventricles. In the event of severe pathology, the AV node can also act as a pacemaker; this is usually not the case because their rate of spontaneous firing is considerably lower than that of the pacemaker cells in the SA node and hence is overridden.

See also

  • Apex beat
    Apex beat
    The apex beat, also called the point of maximum impulse , is the furthermost point outwards and downwards from the sternum at which the cardiac impulse can be felt...

  • Blood pressure
    Blood pressure
    Blood pressure is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. When used without further specification, "blood pressure" usually refers to the arterial pressure of the systemic circulation. During each heartbeat, BP varies...

  • Cardiac action potential
    Cardiac action potential
    In electrocardiography, the cardiac action potential is a specialized action potential in the heart, necessary for the electrical conduction system of the heart....

  • Cardiac muscle
    Cardiac muscle
    Cardiac muscle is a type of involuntary striated muscle found in the walls and histologic foundation of the heart, specifically the myocardium. Cardiac muscle is one of three major types of muscle, the others being skeletal and smooth muscle...

  • Cardiac output
    Cardiac output
    Cardiac output is the volume of blood being pumped by the heart, in particular by a left or right ventricle in the time interval of one minute. CO may be measured in many ways, for example dm3/min...

  • Electrocardiogram
    Electrocardiogram
    Electrocardiography is a transthoracic interpretation of the electrical activity of the heart over a period of time, as detected by electrodes attached to the outer surface of the skin and recorded by a device external to the body...

  • Heart
    Heart
    The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...

  • Systolic array
    Systolic array
    In computer architecture, a systolic array is a pipe network arrangement of processing units called cells. It is a specialized form of parallel computing, where cells , compute data and store it independently of each other.thumb|240px...

     (computer architecture)
  • Ventricle
    Ventricle (heart)
    In the heart, a ventricle is one of two large chambers that collect and expel blood received from an atrium towards the peripheral beds within the body and lungs. The Atria primes the Pump...


External links

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