19.3 Cardiac Cycle
3 min read•june 18, 2024
The cardiac cycle is the heart's rhythmic dance of contraction and relaxation. It's a crucial process that keeps blood flowing through our bodies, delivering oxygen and nutrients to every cell. Understanding this cycle helps us grasp how our hearts work tirelessly to keep us alive.
Blood pressure, flow, and resistance play key roles in this intricate ballet. As the heart's chambers contract and relax, valves open and close, creating the familiar "lub-dub" sounds we associate with a heartbeat. These phases and sounds reveal the complex coordination required for efficient blood circulation.
Cardiac Cycle
Blood pressure and cardiovascular flow
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- Blood pressure drives blood flow through the cardiovascular system
- Higher blood pressure leads to greater blood flow ()
- Lower blood pressure reduces blood flow ()
- Pressure gradient determines direction and rate of blood flow
- Blood flows from high pressure to low pressure areas
- Larger pressure differences result in faster blood flow
- Resistance affects blood pressure and flow rate
- Increased resistance from narrowed blood vessels raises blood pressure and reduces flow ()
- Decreased resistance from dilated blood vessels lowers blood pressure and increases flow ()
- , a product of heart rate and , influences blood pressure and flow
- Higher cardiac output raises blood pressure and enhances flow (exercise)
- Lower cardiac output lowers blood pressure and reduces flow (heart failure)
- , the pressure that the ventricle must overcome to eject blood, affects cardiac output
Phases of cardiac cycle
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- Atria contract, pumping blood into ventricles
- Atrioventricular (AV) valves open, allowing atrial-to-ventricular blood flow
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- Ventricles contract, ejecting blood from the heart
- AV valves close, preventing blood backflow into atria
- Semilunar valves (aortic and pulmonary) open, enabling blood flow into and
- Begins with phase
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- Atria relax and fill with blood from and
- AV valves remain closed
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- Ventricles relax and fill with blood from atria
- Semilunar valves close, preventing blood backflow into ventricles
- AV valves open, facilitating atrial-to-ventricular blood flow
- Begins with phase
Systole vs diastole in heart chambers
- Atrial
- Atrial contraction phase
- Occurs concurrently with ventricular
- Atrial diastole
- Atrial relaxation and filling phase
- Occurs concurrently with ventricular systole
- Ventricular systole
- Ventricular contraction phase
- Occurs concurrently with atrial diastole
- Ventricular diastole
- Ventricular relaxation and filling phase
- Occurs concurrently with atrial systole
Heart sounds and valve actions
- First heart sound (, "lub")
- Results from AV valve (mitral and tricuspid) closure
- Marks the beginning of ventricular systole
- Second heart sound (, "dub")
- Results from semilunar valve (aortic and pulmonary) closure
- Marks the end of ventricular systole and start of ventricular diastole
- Third heart sound (, "lub-dub-dee")
- May be audible in some individuals (children, athletes)
- Occurs during early ventricular filling due to rapid blood inflow
- Fourth heart sound (, "dee-lub-dub")
- May be audible in individuals with stiff ventricles (reduced compliance)
- Occurs during late ventricular filling due to forceful atrial contraction
Ventricular volumes and cardiac function
- : The volume of blood remaining in the ventricle after contraction
- : The volume of blood in the ventricle just before contraction
- : The initial stretching of cardiac muscle fibers before contraction, typically represented by end-diastolic volume
- : Describes how the heart automatically adjusts to changes in , increasing stroke volume as end-diastolic volume increases
Key Terms to Review (49)
Afterload: Afterload is the pressure that the ventricles must generate to eject blood during systole. It is a critical component of cardiac function, influencing how effectively the heart can pump blood into the arteries. A higher afterload can lead to increased workload on the heart, which may affect stroke volume and cardiac output, making it a key factor in understanding the dynamics of blood flow and cardiac physiology.
Aorta: The aorta is the largest blood vessel in the human body, originating from the left ventricle of the heart and serving as the main conduit for oxygenated blood to be distributed throughout the body. This crucial artery plays a central role in the heart's anatomy, the cardiac cycle, cardiac physiology, and the development of the cardiovascular system during fetal circulation.
Aortic valve: The aortic valve is a one-way valve located between the left ventricle of the heart and the aorta, which prevents blood from flowing back into the heart once it has been pumped out. It consists of three cusps that open and close with each heartbeat.
Aortic Valve: The aortic valve is one of the four heart valves responsible for regulating the flow of blood through the heart. It is located between the left ventricle and the aorta, and its primary function is to ensure that blood flows in the correct direction during the cardiac cycle.
Atherosclerosis: Atherosclerosis is a chronic, inflammatory condition characterized by the buildup of plaque within the arteries, leading to narrowing and hardening of the blood vessels. This condition is a major contributor to cardiovascular disease and can have significant implications for the central nervous system, autonomic system, cardiac cycle, and vascular homeostasis.
Atrial Diastole: Atrial diastole is the relaxation phase of the atria, the upper chambers of the heart, during the cardiac cycle. This phase allows the atria to fill with blood returning from the body and lungs, preparing for the next contraction.
Atrial Systole: Atrial systole is the contraction of the atria, the upper chambers of the heart, that occurs during the cardiac cycle. This contraction helps to fill the ventricles, the lower chambers of the heart, with blood prior to ventricular contraction.
Atrioventricular Valves: The atrioventricular (AV) valves are one-way valves located between the atria and ventricles of the heart. They ensure the proper flow of blood from the atria to the ventricles during the cardiac cycle, preventing backflow.
Cardiac Output: Cardiac output is the volume of blood pumped by the heart per minute, which is a crucial measure of the heart's pumping ability and a key indicator of cardiovascular function. It is the product of heart rate and stroke volume, and it plays a central role in the delivery of oxygenated blood to the body's tissues.
Cardiac output (CO): Cardiac output is the volume of blood the heart pumps through the circulatory system in one minute. It is calculated by multiplying the stroke volume (the amount of blood pumped by one ventricle during a contraction) by the heart rate (the number of beats per minute).
Descending aorta: The descending aorta is the portion of the aorta, the largest artery in the body, that runs down through the chest (thoracic aorta) and abdomen (abdominal aorta), delivering oxygen-rich blood to the lower parts of the body. It follows the arch of the aorta and extends to where it bifurcates into the two common iliac arteries in the lower abdomen.
Diastole: Diastole is the phase of the cardiac cycle during which the heart muscle relaxes and allows the chambers, specifically the ventricles, to fill with blood. It occurs right after systole, the phase of heart contraction, completing a full heart beat cycle.
End diastolic volume (EDV): End Diastolic Volume (EDV) is the volume of blood in a ventricle at the end of filling, or diastole, before the heart contracts. It represents the maximum volume of blood that the ventricle will pump out during systole.
End systolic volume (ESV): End systolic volume is the volume of blood left in a ventricle of the heart after it has contracted and before it begins to fill again. It represents the lowest volume of blood in the ventricle at any point in the cardiac cycle.
End-Diastolic Volume: End-diastolic volume (EDV) refers to the volume of blood present in the ventricles of the heart at the end of the diastolic phase, just before ventricular contraction. It is a crucial measure of cardiac function and a key component in understanding the cardiac cycle.
End-Systolic Volume: End-systolic volume (ESV) is the volume of blood remaining in the ventricles of the heart at the end of the systolic (contraction) phase of the cardiac cycle. It represents the amount of blood that the heart has not been able to pump out during the contraction phase.
Frank-Starling Law: The Frank-Starling law, also known as the Frank-Starling mechanism, is a fundamental principle that describes the relationship between the volume of blood entering the heart (preload) and the force of contraction of the heart's ventricles. It explains how the heart can automatically adjust its output to meet the body's changing demands for blood flow.
Heart sounds: Heart sounds are the noises generated by the beating heart and the resultant flow of blood through it. They are primarily caused by the closing of heart valves and can be heard with a stethoscope.
Hypertension: Hypertension is a chronic medical condition where the blood pressure in the arteries is persistently elevated, imposing increased workload on the cardiovascular system. It often requires management to prevent cardiovascular diseases and other complications.
Hypertension: Hypertension, also known as high blood pressure, is a chronic medical condition characterized by an abnormally elevated blood pressure within the arteries. It is a significant risk factor for various cardiovascular diseases, including heart attack, stroke, and kidney failure.
Hypotension: Hypotension is defined as abnormally low blood pressure, which can result in symptoms like dizziness, fainting, or shock. This condition is significant as it can be influenced by various factors such as medications, hormonal regulation, and heart function, affecting overall body perfusion and health.
Isovolumetric Contraction: Isovolumetric contraction is a phase of the cardiac cycle where the ventricles contract, but there is no change in ventricular volume. This occurs as the ventricles generate enough pressure to overcome the pressure in the aorta and pulmonary arteries, but the atrioventricular (AV) valves remain closed, preventing blood from entering or leaving the ventricles.
Isovolumetric Relaxation: Isovolumetric relaxation is a phase of the cardiac cycle where the ventricles relax, but the atrioventricular (AV) valves remain closed, resulting in no change in ventricular volume. This phase occurs immediately after the ventricles have completed their contraction and ejection of blood.
Isovolumic contraction: Isovolumic contraction is a phase in the cardiac cycle where the heart's ventricles contract without changing volume because all heart valves are closed. This phase occurs right after the closure of the mitral and tricuspid valves and before the aortic and pulmonary valves open.
Isovolumic ventricular relaxation phase: The isovolumic ventricular relaxation phase is a stage of the cardiac cycle where the ventricles relax but all heart valves are closed, preventing blood from flowing into or out of the ventricles. This phase occurs right after the ventricular ejection phase and ends when the mitral valve opens.
Mitral valve: The mitral valve is a crucial structure in the heart that separates the left atrium from the left ventricle, preventing the backflow of blood during contraction. This valve plays a key role in ensuring that oxygen-rich blood flows efficiently from the lungs to the rest of the body. Its proper function is vital for maintaining effective circulation and overall cardiovascular health.
Murmur: A murmur is an unusual sound heard between heartbeats, often resembling a whooshing or swishing noise, caused by turbulent blood flow in the heart or near it. It can be a sign of various heart conditions, ranging from harmless to serious.
Preload: Preload is the degree of stretch of the cardiac muscle fibers at the end of diastole, just before contraction. It directly influences the force of myocardial contraction according to Frank-Starling law.
Preload: Preload refers to the amount of blood filling the ventricles of the heart prior to contraction. It is a crucial determinant of the heart's ability to pump blood effectively and efficiently, as it directly influences the force of ventricular contraction.
Pulmonary artery: The pulmonary artery is a large blood vessel that transports deoxygenated blood from the right ventricle of the heart to the lungs for oxygenation. Unlike other arteries, it carries blood low in oxygen content.
Pulmonary Artery: The pulmonary artery is a major blood vessel that carries deoxygenated blood from the right ventricle of the heart to the lungs, where the blood becomes oxygenated. It is a crucial component in the circulatory system, playing a vital role in the cardiac cycle and fetal circulation.
Pulmonary valve: The pulmonary valve is a one-way valve located between the right ventricle and the pulmonary artery that prevents blood from flowing back into the heart once it has been ejected into the pulmonary circulation. It opens to allow deoxygenated blood to be pumped from the heart to the lungs for oxygenation.
Pulmonary Valve: The pulmonary valve is one of the four major valves in the human heart. It is located at the base of the pulmonary artery, responsible for regulating the flow of deoxygenated blood from the right ventricle to the lungs for oxygenation.
Pulmonary veins: Pulmonary veins are blood vessels that carry oxygenated blood from the lungs back to the left atrium of the heart. Unlike most veins, which carry deoxygenated blood, pulmonary veins are unique because they transport blood rich in oxygen.
Pulmonary Veins: The pulmonary veins are blood vessels that carry oxygenated blood from the lungs to the heart's left atrium. They play a crucial role in the cardiovascular and respiratory systems, facilitating the exchange of gases and the circulation of blood throughout the body.
S1: S1, or the first heart sound, is the sound produced by the closure of the atrioventricular (AV) valves at the beginning of ventricular contraction. It signifies the onset of systole, which is when the ventricles contract to pump blood out of the heart, and is crucial for assessing heart function during the cardiac cycle.
S2: S2 is a key term in the context of the cardiac cycle, referring to the second heart sound that is produced during ventricular systole. It is an important indicator of the proper functioning of the heart's valves and the timing of the cardiac cycle.
S3: S3 refers to the third heart sound, a low-pitched, soft sound that can be heard during diastole, the relaxation phase of the cardiac cycle. It is associated with the rapid filling of the ventricles and is a normal finding in children and some young adults, but may indicate certain pathological conditions in adults.
S4: S4 is a key component of the cardiac cycle, representing the fourth heart sound that can be detected during diastole. It is associated with the rapid filling phase of the ventricles and provides important information about the heart's function and health.
Stroke Volume: Stroke volume is the amount of blood pumped from the ventricles of the heart with each contraction. It is a crucial factor in determining cardiac output, which is the total volume of blood pumped by the heart per minute. Stroke volume is an important concept in understanding the heart's ability to effectively circulate blood throughout the body.
Stroke volume (SV): Stroke volume is the amount of blood pumped by the left ventricle of the heart in one contraction. It varies depending on bodily demands, such as during exercise or rest.
Systole: Systole is the phase of the cardiac cycle during which the heart muscles contract and pump blood from the chambers into the arteries. It primarily occurs in two stages: atrial systole (contraction of the atria) and ventricular systole (contraction of the ventricles).
Tricuspid valve: The tricuspid valve is a valve in the heart located between the right atrium and the right ventricle; it prevents the backflow of blood as it moves from the atrium to the ventricle. It consists of three flaps or cusps, which give it its name.
Tricuspid Valve: The tricuspid valve is one of the four heart valves that regulate blood flow through the heart. It is located between the right atrium and the right ventricle, ensuring that blood flows in the correct direction during the cardiac cycle.
Vasodilation: Vasodilation is the process by which blood vessels widen due to the relaxation of vascular smooth muscle, allowing increased blood flow and decreased blood pressure. This mechanism plays a critical role in regulating body temperature, enhancing oxygen delivery during physical activity, and supporting various physiological responses.
Venae Cavae: The venae cavae are the two large veins that return deoxygenated blood from the body to the heart. They are the superior vena cava, which drains blood from the upper body, and the inferior vena cava, which drains blood from the lower body. These vessels play a crucial role in the cardiac cycle by delivering blood to the right atrium of the heart.
Ventricular Diastole: Ventricular diastole is the relaxation and filling phase of the cardiac cycle, where the ventricles of the heart expand and fill with blood in preparation for the next contraction. This phase is a critical component of the overall heart function, as it ensures that the ventricles are adequately filled with oxygenated blood before the next systolic contraction.
Ventricular ejection phase: The ventricular ejection phase is the stage of the cardiac cycle where the heart contracts to pump blood out of the ventricles and into the aorta and pulmonary artery. It occurs after the ventricles are filled with blood during diastole.
Ventricular Systole: Ventricular systole is the contraction phase of the ventricles in the cardiac cycle, during which the ventricles pump blood out of the heart. It is a crucial component of the heart's pumping mechanism, responsible for the ejection of blood into the pulmonary and systemic circulatory systems.