1.4 Cardiac Cycle and Heart Sounds
3 min read•august 1, 2024
The cardiac cycle is the rhythmic sequence of heart muscle contraction and relaxation. It's a crucial process that pumps blood throughout the body, maintaining life. Understanding the cycle's phases and associated heart sounds is key to grasping how the heart functions.
This topic dives into and , pressure and volume changes, and the distinct phases of the cardiac cycle. It also covers heart sounds, which provide valuable insights into heart health and function. These concepts are fundamental to understanding the heart's role in the cardiovascular system.
Systole vs Diastole
Defining Systole and Diastole
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- Systole is the contraction phase of the cardiac cycle during which the heart muscle contracts and pumps blood out of the heart
- Diastole is the relaxation phase of the cardiac cycle during which the heart muscle relaxes and the chambers fill with blood
- The cardiac cycle consists of one complete sequence of systole and diastole which occurs with each heartbeat (60-100 beats per minute in adults)
Timing and Duration of Systole and Diastole
- Systole lasts for a shorter period than diastole, typically about one-third of the total cardiac cycle duration
- In a normal resting (60-100 bpm), systole lasts approximately 0.3-0.4 seconds, while diastole lasts about 0.4-0.7 seconds
- The relative durations of systole and diastole change with heart rate; as heart rate increases, diastole shortens more than systole
Pressure and Volume Changes in the Heart
Ventricular Pressure and Volume Changes
- During systole, ventricular pressure increases as the contract, causing the aortic and pulmonary valves to open and blood to be ejected from the heart
- During diastole, ventricular pressure decreases as the ventricles relax, allowing the mitral and tricuspid valves to open and blood to flow from the into the ventricles
- Ventricular volume decreases during systole as blood is ejected () and increases during diastole as the ventricles fill with blood (end-diastolic volume)
Atrial Pressure and Volume Changes
- Atrial pressure and volume changes are less pronounced than ventricular changes, with a slight increase in pressure during and a decrease during
- During atrial systole, atria contract and pump blood into the ventricles, causing a small increase in atrial pressure (a-wave)
- During atrial diastole, atria relax and fill with blood from the venae cavae and pulmonary veins, leading to a gradual increase in atrial volume and a slight decrease in pressure
Phases of the Cardiac Cycle
Ventricular Phases
- : Ventricles begin to contract, all valves are closed, and ventricular pressure rises without a change in volume
- : Aortic and pulmonary valves open, and blood is ejected from the ventricles into the aorta and pulmonary arteries
- : Ventricles begin to relax, all valves are closed, and ventricular pressure decreases without a change in volume
- : Mitral and tricuspid valves open, allowing blood to flow from the atria into the ventricles (rapid filling phase followed by diastasis)
Atrial Phases
- Atrial systole: Atria contract, pumping blood into the ventricles (occurs at the end of ventricular diastole)
- Atrial diastole: Atria relax and fill with blood from the venae cavae and pulmonary veins (occurs during ventricular systole and most of ventricular diastole)
Heart Sounds: Origin and Significance
First and Second Heart Sounds
- The (S1) is caused by the closure of the mitral and tricuspid valves at the beginning of ventricular systole (lub sound)
- The (S2) is caused by the closure of the aortic and pulmonary valves at the end of ventricular systole (dub sound)
- S1 and S2 are normally clear, distinct sounds that can be heard using a stethoscope over the precordium
Clinical Significance of Heart Sounds
- Heart sounds can be used to assess the function of the heart valves and the timing of the cardiac cycle events
- Abnormal heart sounds, such as murmurs or extra sounds, may indicate underlying cardiac pathology, such as valve stenosis, regurgitation, or septal defects
- Splitting of S1 or S2 can occur in various conditions affecting the synchrony of (bundle branch blocks, atrial septal defect)
- Additional heart sounds, like S3 and S4, may be heard in certain pathological conditions (heart failure, hypertrophic cardiomyopathy)
Key Terms to Review (25)
Arrhythmia: Arrhythmia refers to any irregularity in the heart's rhythm, which can manifest as an irregular heartbeat, a change in the speed of the heartbeat, or a complete disruption in the normal sequence of electrical impulses. This condition can significantly impact the efficiency of the heart's pumping action and is closely tied to the cardiac conduction system's ability to generate and propagate electrical signals, as well as its role in the cardiac cycle and heart sounds.
Atria: The atria are the two upper chambers of the heart, consisting of the right atrium and the left atrium. These chambers are crucial for receiving blood returning to the heart, playing an essential role in the overall structure and function of the cardiovascular system. The atria facilitate the initial phase of blood flow through the heart, helping to ensure efficient circulation throughout the body.
Atrial Diastole: Atrial diastole is the phase of the cardiac cycle during which the atria of the heart relax and fill with blood from the veins. This crucial period allows the chambers to expand, creating a pressure difference that facilitates blood flow into the atria from the superior and inferior vena cava as well as the pulmonary veins. It sets the stage for efficient blood flow into the ventricles, significantly influencing overall heart function and rhythm.
Atrial Systole: Atrial systole refers to the contraction of the atria in the heart, which occurs at the end of the diastolic phase of the cardiac cycle. This contraction pushes blood into the ventricles, completing the filling process before ventricular systole begins. Atrial systole plays a crucial role in ensuring that the ventricles are adequately filled with blood, optimizing the efficiency of the heart's pumping action.
Atrioventricular valves: Atrioventricular valves are specialized structures located between the atria and ventricles of the heart, specifically the tricuspid valve on the right side and the mitral (bicuspid) valve on the left side. These valves ensure unidirectional blood flow from the atria to the ventricles while preventing backflow during ventricular contraction. Their proper function is crucial for maintaining an effective cardiac cycle and generating the characteristic heart sounds during each heartbeat.
Blood flow turbulence: Blood flow turbulence refers to the chaotic and irregular movement of blood within the cardiovascular system, typically occurring when blood flows through narrowed or obstructed vessels. This turbulence can create eddies and swirls, leading to increased friction and resistance, which can impact overall circulation. Understanding this concept is essential as it relates directly to heart sounds and the efficiency of blood flow during the cardiac cycle.
Blood pressure: Blood pressure is the force exerted by circulating blood on the walls of blood vessels, primarily arteries, during the cardiac cycle. It is a vital indicator of cardiovascular health, reflecting the efficiency of the heart and the resistance of blood vessels. Understanding blood pressure involves its relationship with heart function, blood components, homeostatic mechanisms, and the dynamics of blood flow regulation.
Cardiac output: Cardiac output is the volume of blood that the heart pumps per minute, reflecting the efficiency of the heart as a pump. It is determined by two primary factors: heart rate, which is how often the heart beats, and stroke volume, which is the amount of blood ejected with each beat. Understanding cardiac output is essential because it relates directly to how well blood circulates through the body's tissues, impacting everything from oxygen delivery to organ function.
Diastole: Diastole refers to the phase of the cardiac cycle when the heart muscles relax, allowing the chambers of the heart to fill with blood. During this period, both the atria and ventricles undergo relaxation, which is crucial for efficient blood flow and maintaining proper circulation throughout the body. The timing and coordination of diastole are essential for optimal function of the heart's chambers and valves, affecting systemic and pulmonary circulation.
Electrocardiogram: An electrocardiogram (ECG or EKG) is a test that records the electrical activity of the heart over a period of time using electrodes placed on the skin. This diagnostic tool helps in understanding the heart's rhythm, size, and any potential damage to the heart muscle, linking closely to the cardiac cycle and the heart's conduction system. By visualizing the electrical impulses that trigger each heartbeat, an ECG provides vital information about the heart’s function and health.
First heart sound: The first heart sound, often denoted as S1, is the sound produced by the closure of the atrioventricular (AV) valves at the beginning of ventricular contraction, marking the start of systole. This sound signifies the transition from diastole to systole and is critical for understanding the cardiac cycle and heart sounds as a whole.
Heart Rate: Heart rate refers to the number of times the heart beats per minute, reflecting the frequency of cardiac contractions. It plays a crucial role in determining how effectively blood is pumped throughout the body, influencing oxygen delivery and overall cardiovascular health. Monitoring heart rate is vital for understanding various physiological responses during different activities, such as rest, exercise, and stress.
Isovolumetric Ventricular Contraction: Isovolumetric ventricular contraction is a phase in the cardiac cycle during which the ventricles contract with no change in volume, as all heart valves are closed. This phase occurs immediately after the ventricles begin to contract, leading to a rise in pressure within the ventricles while the blood remains contained, preparing for the ejection of blood into the arteries once the pressure exceeds that in the aorta and pulmonary artery.
Isovolumetric Ventricular Relaxation: Isovolumetric ventricular relaxation is a phase in the cardiac cycle where the ventricles relax after systole, and all heart valves are closed, resulting in no change in ventricular volume. This period occurs between the closure of the aortic and pulmonary valves and the opening of the mitral and tricuspid valves. During this time, pressure within the ventricles decreases, leading to a drop in myocardial tension while preparing for the next filling phase.
Phonocardiography: Phonocardiography is a non-invasive technique used to record the sounds produced by the heart during its functioning, specifically focusing on heart sounds and murmurs. This method plays a crucial role in understanding the cardiac cycle and detecting abnormal heart sounds that may indicate various cardiac conditions. By visualizing these sounds, phonocardiography helps in diagnosing and monitoring heart health.
Second Heart Sound: The second heart sound, often denoted as S2, is a crucial component of the cardiac cycle, produced by the closure of the semilunar valves (aortic and pulmonary valves) at the end of ventricular systole. This sound signals the transition from ventricular contraction to relaxation, playing a vital role in the overall rhythm of the heart and ensuring proper blood flow throughout the cardiovascular system.
Semilunar valves: Semilunar valves are crescent-shaped structures located at the exits of the ventricles of the heart, specifically at the aorta and pulmonary arteries. They play a crucial role in the cardiac cycle by preventing backflow of blood into the ventricles after contraction, ensuring unidirectional blood flow throughout the cardiovascular system. These valves open and close in response to pressure changes during heartbeats, contributing to the characteristic sounds associated with heart function.
Stroke volume: Stroke volume is the amount of blood ejected by the heart's left ventricle during one contraction. This measurement is crucial as it reflects the efficiency of the heart in pumping blood and is influenced by factors such as the heart's chamber structure and the valves that regulate blood flow, as well as the phases of the cardiac cycle which dictate when and how much blood is pushed out with each heartbeat.
Systole: Systole is the phase of the cardiac cycle during which the heart muscles contract, pumping blood out of the chambers. This contraction is crucial for maintaining blood flow throughout the body and is directly linked to the function of the cardiac chambers and valves, as well as the heart's electrical activity. Understanding systole is key to comprehending how blood is circulated through both systemic and pulmonary pathways.
Third Heart Sound: The third heart sound, commonly referred to as S3, is a low-frequency sound that occurs shortly after the second heart sound (S2) during the rapid filling phase of the ventricles in the cardiac cycle. This sound is typically associated with the early diastole when blood flows into the ventricles from the atria, and its presence can indicate various physiological or pathological conditions depending on the context.
Valve Closure: Valve closure refers to the process by which the heart valves close to prevent the backflow of blood during the cardiac cycle. This mechanism is crucial for maintaining proper blood flow direction through the heart's chambers and ensuring efficient circulation throughout the body. The timing and coordination of valve closure are essential for generating the characteristic heart sounds and are closely linked to the functional dynamics of the cardiac chambers.
Valvular Insufficiency: Valvular insufficiency is a heart condition where one or more of the heart valves do not close properly, leading to a backflow of blood into the chambers of the heart. This condition can result in various symptoms such as fatigue, shortness of breath, and palpitations, affecting the efficiency of the heart's pumping action. When the heart valves fail to function correctly, it can disrupt the normal cardiac cycle, leading to abnormal heart sounds during auscultation.
Ventricles: Ventricles are the two lower chambers of the heart responsible for pumping blood out of the heart. They play a crucial role in the circulatory system by receiving blood from the atria and forcing it into the arteries, thus ensuring proper blood flow throughout the body and lungs. The left ventricle pumps oxygenated blood to the body, while the right ventricle sends deoxygenated blood to the lungs for oxygenation.
Ventricular ejection: Ventricular ejection is the phase of the cardiac cycle during which blood is expelled from the ventricles into the arteries. This process occurs after the ventricles have filled with blood and contracted, generating enough pressure to open the semilunar valves and propel blood into the pulmonary artery and aorta. It is crucial for maintaining circulation and ensuring that oxygenated blood reaches the body while deoxygenated blood is sent to the lungs for oxygenation.
Ventricular Filling: Ventricular filling is the phase in the cardiac cycle where blood flows from the atria into the ventricles, preparing the heart for contraction. This process occurs during diastole, when the heart muscle relaxes, allowing the ventricles to expand and receive blood from the atria. Proper ventricular filling is crucial for maintaining adequate stroke volume and cardiac output, impacting overall heart function and circulation.