Cardiac cycle 2525843 216828697 2008-06-03T12:18:57Z Mtcv 21196 /* Ventricular systole */ png → svg [[Image:Cardiac Cycle Left Ventricle.PNG|thumb|right|400px|Cardiac events occurring in the cardiac cycle. Two complete cycles are illustrated.]] '''Cardiac cycle''' is the term referring to all or any of the events related to the flow of blood that occur from the beginning of one heartbeat to the beginning of the next.<ref name="Medical Physiology">Guyton, A.C. & Hall, J.E. (2006) ''Textbook of Medical Physiology'' (11th ed.) Philadelphia: Elsevier Saunder ISBN 0-7216-0240-1</ref> The frequency of the cardiac cycle is the [[heart rate]]. Every single 'beat' of the [[heart]] involves three major stages: '''atrial [[systole (medicine)|systole]]''', '''ventricular systole''' and '''complete cardiac [[diastole]].''' The term ''diastole'' is synonymous with relaxation of a muscle. Throughout the cardiac cycle, the [[blood pressure]] increases and decreases. The cardiac cycle is co-ordinated by a series of electrical impulses that are produced by specialised heart cells founds within the sino-atrial node and the atrioventricular node. The cardiac muscle is composed of [[myocytes]] which initiate their own contraction without assistance of external nerves (with the exception of modifying the heart rate due to metabolic demand). ==Atrial systole== [[Image:Heart diastole.png|thumb|right|Atrial systole]] '''Atrial systole''' is the contraction of the heart muscle (''myocardia'') of the left and right [[Atrium (heart)|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'''. 70% of the blood flows passively down to the ventricles, so the atria do not have to contract a great amount.<ref>Advanced Biology for You - Gareth Williams</ref> Atrial kick is absent if there is loss of normal electrical conduction in the heart, such as during [[atrial fibrillation]], [[atrial flutter]], and [[third degree heart block|complete heart block]]. Atrial kick is also different in character depending on the condition that the heart is in like stiff heart, that 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 [[Electrocardiogram|ECG]]. The wave of bipolarization 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== [[Image:Heart systole.svg|thumb|155px|right|Ventricular systole]] '''Ventricular systole''' is the contraction of the muscles (''myocardia'') of the left and right [[Ventricle (heart)|ventricle]]s. The graph of aortic pressure throughout the cardiac cycle displays a small dip which coincides with the aortic valve closure. The dip in the graph is immediately followed by a brief rise then gradual decline. The small rise in the graph is known as the "dicrotic notch" or "incisure", and represents a transient increase in aortic pressure. 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.<ref>[http://facstaff.elon.edu/shouse/physiology/frog/plethys/frogplethys.html Plethysmograph<!-- Bot generated title -->]</ref> <ref>[http://faculty.ucc.edu/biology-potter/heart.htm The Heart<!-- Bot generated title -->]</ref> <ref>[http://www.susqu.edu/facstaff/r/richard/ECGlab.html Human Cardiopulmonary Physiology<!-- Bot generated title -->]</ref> ===Detection of ventricular systole=== ====Heart sounds==== {{Main|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 "lub-dub" sound made by the heart as it beats. Formally, this sound is known as the ''First Heart Tone'', or '''S<sub>1</sub>'''. 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-dub" (the ''Second Heart Tone'', or '''S<sub>2</sub>'''), is caused by the closure of the aortic and pulmonic 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 closes. Similarly, as the pressure in the right ventricle falls below the pressure in the pulmonary artery, the pulmonary valve closes. The second heart sound is also two components, A2, 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. ====Electrocardiogram==== In an [[electrocardiogram]], electrical systole of the ventricles begins at the beginning of the [[QRS complex]]. ==Complete cardiac diastole== [[Image:Heart diastole.png|thumb|right|Cardiac 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 (medicine)|systole]]. When the pressure in the left ventricle drops to below the pressure in the left atrium, the [[mitral valve]] opens, and the left ventricle fills with blood that was accumulating in the left atrium. Likewise, when the pressure in the right ventricle drops below that in the right atrium, the [[tricuspid valve]] opens, and the right ventricle fills with blood that was accumulating in the right atrium. ==Regulation of the cardiac cycle== Cardiac muscle has [[Cardiac muscle automaticity|automaticity]], which means that it is self-exciting. This is in contrast with [[skeletal 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 heart accelerate heart rate and the vagus nerve decelerates heart rate. The rhythmic sequence of contractions is coordinated by the [[sinoatrial]] (SA) and [[atrioventricular node|atrioventricular]] (AV) nodes. The sinoatrial node, often known as the ''[[cardiac pacemaker]]'', 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]]. 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]], 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]] *[[Blood pressure]] *[[Cardiac action potential]] *[[Cardiac muscle]] *[[Cardiac output]] *[[Electrocardiogram]] *[[Heart]] *[[Systolic array]] (computer architecture) *[[Ventricle (heart)|Ventricle]] ==References== <div class="references-small" style="-moz-column-count:2; column-count:2;"> <references/> </div> ==External links== *[http://www.interactivephysiology.com/demo/systems/buildframes.html?cardio/cardcycl/01 Interactive cardiac cycle]. ''Interactivephysiology.com'' *[http://library.med.utah.edu/kw/pharm/hyper_heart1.html Cardiac cycle]. ''University of Utah School of Medicine'' *[http://www.med.ucla.edu/wilkes/intro.html Heart sounds]. ''UCLA School of Medicine'' {{Cardiovascular physiology}} [[Category:Physiology]] [[Category:Cardiology]] [[ar:الدورة القلبية]] [[cs:Srdeční rytmus]] [[da:Hjertecyklus]] [[es:Ciclo cardíaco]] [[pl:Cykl pracy serca]] [[pt:Ciclo cardíaco]]