Guyton and Hall Physiology Chapter 9 - Cardiac Muscle; The Heart as a Pump and Function of the Valves
93 Questions
6 Views

Guyton and Hall Physiology Chapter 9 - Cardiac Muscle; The Heart as a Pump and Function of the Valves

Created by
@PrizeMeerkat

Podcast Beta

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What is the volume of blood remaining in the ventricle at the end of systole known as?

  • Cardiac output
  • End-systolic volume (correct)
  • End-diastolic volume
  • Ejection volume

    • Which term is used to describe the fraction of the end-diastolic volume that is ejected during ventricular contraction?

  • Preload
  • Ejection fraction (correct)
  • Stroke volume
  • Cardiac index

    • How do semilunar valves differ from A-V valves during the ventricular contraction phase?

  • Semilunar valves close softly.
  • Semilunar valves prevent backflow only during diastole.
  • Semilunar valves snap closed due to high arterial pressures. (correct)
  • Semilunar valves open larger than A-V valves.

    • What occurs during the isovolumic contraction phase?

    <p>All heart valves are closed and pressure builds in the ventricles.</p> Signup and view all the answers

    What is a potential consequence of severe mitral valve dysfunction?

    <p>Reduced cardiac output.</p> Signup and view all the answers

    What primarily causes the c wave during ventricular contraction?

    <p>Bulging of the A-V valves backward</p> Signup and view all the answers

    What happens to ventricular pressure at the onset of the period of ejection?

    <p>It rises slightly above 80 mm Hg</p> Signup and view all the answers

    During which phase is 60% of the blood in the ventricles ejected?

    <p>Period of rapid ejection</p> Signup and view all the answers

    What is the primary function of the A-V valves during ventricular contraction?

    <p>Preventing backflow of blood into the ventricles</p> Signup and view all the answers

    What happens at the end of systole during the period of isovolumic relaxation?

    <p>Ventricular relaxation begins suddenly</p> Signup and view all the answers

    How much blood is typically ejected during the first third of the ejection period?

    <p>70%</p> Signup and view all the answers

    What phase follows the closing of the A-V valves during ventricular systole?

    <p>Isovolumic contraction</p> Signup and view all the answers

    What characterizes the period of slow ejection?

    <p>Remaining blood empties after rapid ejection</p> Signup and view all the answers

    What occurs immediately after the aortic valve closes during the cardiac cycle?

    <p>Isovolumetric relaxation occurs</p> Signup and view all the answers

    Which phase of the cardiac cycle is characterized by a gradual increase in ventricular pressure and no change in volume?

    <p>Isovolumetric contraction</p> Signup and view all the answers

    During which phase does the stroke volume get ejected from the ventricle?

    <p>Ejection phase</p> Signup and view all the answers

    What happens to the aortic valve when the left ventricular pressure exceeds aortic pressure?

    <p>It opens</p> Signup and view all the answers

    What is the primary reason the aortic valve closes at the end of the ejection phase?

    <p>Ventricular pressure falls below aortic pressure</p> Signup and view all the answers

    Which waveform represents the pressure changes in the left ventricle during the ejection phase?

    <p>Ventricular pressure wave</p> Signup and view all the answers

    During which phase does the volume of blood in the ventricle remain constant despite changes in pressure?

    <p>Isovolumetric contraction</p> Signup and view all the answers

    What characterizes the end-diastolic volume in the left ventricle?

    <p>The maximum volume of blood in the ventricle before contraction</p> Signup and view all the answers

    In which phase does the left ventricular pressure drop significantly as the ventricle returns to its relaxed state?

    <p>Isovolumetric relaxation</p> Signup and view all the answers

    Which of the following is NOT a function of the heart valves during the cardiac cycle?

    <p>Increasing systemic vascular resistance</p> Signup and view all the answers

    What determines the preload for cardiac contraction?

    <p>The volume of blood in the ventricle at the end of diastole</p> Signup and view all the answers

    During which phase does the aortic valve open and blood flows into the aorta?

    <p>Period of Ejection</p> Signup and view all the answers

    What occurs at point D in the cardiac cycle?

    <p>The aortic valve closes and pressure falls</p> Signup and view all the answers

    What describes the afterload during ventricular contraction?

    <p>The pressure in the aorta that the ventricle must overcome</p> Signup and view all the answers

    Which phase corresponds to the period of isovolumic relaxation?

    <p>Phase IV</p> Signup and view all the answers

    What occurs during the period of ejection in the ventricles?

    <p>Systolic pressure rises due to continued contraction</p> Signup and view all the answers

    Which of the following describes the changes during Phase III: Period of Ejection?

    <p>Ventricular volume decreases while pressure increases</p> Signup and view all the answers

    Which statement is true about the role of valves during the cardiac cycle?

    <p>The aortic valve closes at the start of diastole</p> Signup and view all the answers

    What effect does sympathetic nervous system stimulation have on ventricular pressure?

    <p>It increases the peak systolic pressure</p> Signup and view all the answers

    What happens to the volume of the ventricle at the end of the period of ejection?

    <p>The volume decreases significantly</p> Signup and view all the answers

    What causes the a wave in the atrial pressure curve?

    <p>Atrial contraction</p> Signup and view all the answers

    During which phase does the ventricular pressure rise abruptly?

    <p>Isovolumic contraction</p> Signup and view all the answers

    What occurs immediately after the ventricular contraction begins?

    <p>Isovolumic contraction occurs</p> Signup and view all the answers

    What is the main purpose of the aortic and pulmonary valves during ventricular contraction?

    <p>To open and release blood from ventricles</p> Signup and view all the answers

    What average increase in pressure occurs in the left atrium during atrial contraction?

    <p>7 to 8 mm Hg</p> Signup and view all the answers

    How much more blood can the heart typically pump compared to resting body requirements?

    <p>300% to 400%</p> Signup and view all the answers

    What is the effect of atrial failure on the cardiac output during exercise?

    <p>It leads to noticeable symptoms of heart failure</p> Signup and view all the answers

    During which period does no emptying of the ventricles occur despite contraction?

    <p>Isovolumic contraction</p> Signup and view all the answers

    When do the A-V valves close during the cardiac cycle?

    <p>At the beginning of isovolumic contraction</p> Signup and view all the answers

    What are the pressure waves in the atrial pressure curve identified as a, c, and v waves primarily associated with?

    <p>Atrial contractions and pressure changes</p> Signup and view all the answers

    What typically happens to atrial pressure during atrial contraction?

    <p>It increases by approximately 4 to 6 mm Hg in the right atrium.</p> Signup and view all the answers

    During which phase does the heart still manage to function effectively even if the atria fail to contract?

    <p>During resting conditions.</p> Signup and view all the answers

    What occurs during the period of isovolumic contraction?

    <p>Ventricular pressure rises, but volume remains unchanged for a short time.</p> Signup and view all the answers

    How does the heart typically perform if atrial contraction does not occur?

    <p>There will be no effect on pumping as the heart can function adequately.</p> Signup and view all the answers

    What is the significance of the a, c, and v waves in the atrial pressure curve?

    <p>They are primarily associated with atrial figures during contraction and relaxation phases.</p> Signup and view all the answers

    What causes the v wave during ventricular contraction?

    <p>Slow flow of blood into the atria from the veins</p> Signup and view all the answers

    During which part of the ejection period is the majority of blood (approximately 60%) expelled from the ventricles?

    <p>During the period of rapid ejection</p> Signup and view all the answers

    What primarily happens at the end of systole during the isovolumic relaxation phase?

    <p>Ventricular relaxation begins suddenly without change in volume</p> Signup and view all the answers

    What role do the A-V valves play during ventricular contraction?

    <p>Close to stop blood flow from atria during contraction</p> Signup and view all the answers

    What occurs when the left ventricular pressure rises slightly above 80 mm Hg?

    <p>Ventricular pressure pushes the semilunar valves open</p> Signup and view all the answers

    What is the typical ejection fraction percentage associated with healthy cardiac function?

    <p>0.6</p> Signup and view all the answers

    Which mechanism is responsible for the closure of the aortic and pulmonary valves after ventricular contraction?

    <p>High pressures in the arteries</p> Signup and view all the answers

    What is the consequence of reduced end-systolic volume in the ventricles?

    <p>Decreased stroke volume</p> Signup and view all the answers

    How do semilunar valves function differently from A-V valves during contraction?

    <p>They operate under higher pressure</p> Signup and view all the answers

    What role do the chordae tendineae and papillary muscles play in cardiac function?

    <p>Preventing backflow during ventricular contraction</p> Signup and view all the answers

    Which condition can severely compromise the function of the mitral valve?

    <p>Mitral valve prolapse</p> Signup and view all the answers

    What is the consequence of severe leakage during ventricular contraction?

    <p>Reduced ejection fraction</p> Signup and view all the answers

    What is the relationship between preload and end-diastolic volume?

    <p>Increased preload enhances end-diastolic volume</p> Signup and view all the answers

    What triggers the need for more blood or preload during early diastole?

    <p>Reduced end-systolic volume</p> Signup and view all the answers

    What primarily causes the higher velocity of blood ejection through the aortic and pulmonary valves?

    <p>Greater pressure differentials</p> Signup and view all the answers

    The calcium ions that enter during the plateau phase of cardiac muscle action potential are derived from the extracellular fluid.

    <p>True</p> Signup and view all the answers

    Fast sodium channels remain open for several seconds during the cardiac muscle action potential.

    <p>False</p> Signup and view all the answers

    The permeability of the cardiac muscle membrane for potassium ions increases immediately after the onset of the action potential.

    <p>False</p> Signup and view all the answers

    During Phase 2 of the cardiac action potential, both calcium and sodium ions flux into the cardiac muscle cell.

    <p>True</p> Signup and view all the answers

    The membrane potential of cardiac muscle cells reaches a value of approximately +20 millivolts before the sodium channels close.

    <p>True</p> Signup and view all the answers

    The pressure in the ventricle rises rapidly after the aortic valve opens.

    <p>False</p> Signup and view all the answers

    The end-systolic volume can decrease to as little as 5 to 10 ml in a healthy heart.

    <p>False</p> Signup and view all the answers

    The aortic and pulmonary valves are constructed with particularly strong fibrous tissue to withstand mechanical stress.

    <p>True</p> Signup and view all the answers

    The A-V valves are supported by chordae tendineae while the semilunar valves are not.

    <p>True</p> Signup and view all the answers

    The volume of blood that enters the ventricles during diastole can range from 100 to 120 ml in a healthy heart.

    <p>False</p> Signup and view all the answers

    The A-V valves open actively during ventricular contraction.

    <p>False</p> Signup and view all the answers

    The aortic valve closes due to a short period of backward flow of blood.

    <p>True</p> Signup and view all the answers

    Papillary muscles assist in the closure of the A-V valves by themselves.

    <p>False</p> Signup and view all the answers

    Diastolic pressure in the aorta can fall to around 80 mm Hg before the ventricle contracts again.

    <p>True</p> Signup and view all the answers

    The semilunar valves close passively during diastole.

    <p>True</p> Signup and view all the answers

    During systole, pressure in the aorta rises to about 100 mm Hg.

    <p>False</p> Signup and view all the answers

    The walls of arteries maintain low pressure throughout diastole.

    <p>False</p> Signup and view all the answers

    The opening and closing of heart valves are completely powered by muscle contractions.

    <p>False</p> Signup and view all the answers

    The maximum pressure in the aorta during ventricular contraction is referred to as diastolic pressure.

    <p>False</p> Signup and view all the answers

    During the cardiac cycle, the pressure in the aorta decreases steadily during diastole.

    <p>True</p> Signup and view all the answers

    Match the components of the cardiac muscle with their functions:

    <p>Intercalated discs = Allow coordination of contractions Atria = Receive blood from the body Ventricles = Pump blood to the lungs and body A-V bundle = Conduct electrical signals between atria and ventricles</p> Signup and view all the answers

    Match the terms with their corresponding characteristics in cardiac muscle action potentials:

    <p>Initial spike = Rapid depolarization Plateau phase = Prolonged depolarization Repolarization = Return to resting potential Action potential amplitude = About 105 millivolts</p> Signup and view all the answers

    Match the phases of the cardiac cycle with their descriptions:

    <p>Isovolumic contraction = No change in volume while pressure rises Period of ejection = Blood is expelled from the ventricles Isovolumic relaxation = Volume remains constant during pressure drop Atrial contraction = Fills the ventricles with blood</p> Signup and view all the answers

    Match the effects of the cardiac muscle contraction with their significance:

    <p>Longer contraction duration = Improves heart pumping efficiency Syncytial nature = Ensures coordinated contraction Fiber diameter = Increases force of contraction Electrical conduction system = Initiates and regulates heartbeats</p> Signup and view all the answers

    Match the actions during a heartbeat with their resulting events:

    <p>Atria contract = Push blood into ventricles Ventricles contract = Send blood to aorta and pulmonary artery Closing of A-V valves = Prevents backflow into atria Opening of semilunar valves = Allows blood to flow out of the heart</p> Signup and view all the answers

    Match the following components of cardiac muscle with their functions:

    <p>Calcium ions = Trigger contraction by interacting with troponin T tubules = Facilitate the diffusion of calcium ions into the sarcoplasm Sarcoplasmic reticulum = Stores calcium and releases it during contraction Ryanodine receptors = Activate calcium release from the sarcoplasmic reticulum</p> Signup and view all the answers

    Match the following phases of cardiac action potential with their descriptions:

    <p>Phase 4 = Resting membrane potential averaging -80 to -90 mV Refractory period = Duration when cardiac muscle is unresponsive to further stimulation Phase of ejection = Point when the heart expels blood to the aorta Activation phase = Calcium enters the cell, triggering muscle contraction</p> Signup and view all the answers

    Match the following terms related to signal conduction in cardiac muscle with their characteristics:

    <p>Velocity of conduction = About 0.3 to 0.5 m/sec in muscle fibers Purkinje fibers = Specialized system allowing rapid signal conduction at 4 m/sec Excitatory action potential = Responsible for initiating muscle contraction Skeletal muscle fibers = Conduct action potentials faster than cardiac muscle</p> Signup and view all the answers

    Match the following structures with their roles in cardiac muscle contraction:

    <p>Calcium release channels = Facilitate the release of calcium from the sarcoplasmic reticulum Sarcolemma = Membrane that houses voltage-dependent calcium channels Sarcoplasm = Site where calcium interacts with contractile proteins Calcium ions from T tubules = Contribute to calcium influx during action potentials</p> Signup and view all the answers

    Match the following responses in cardiac muscle during contraction with their triggers:

    <p>Calcium influx = Opens voltage-dependent calcium channels Calcium release from the sarcoplasmic reticulum = Initiated by activated ryanodine receptors Cross-bridge formation = Occurs through interaction of calcium with troponin Refractory period = Resulting from inability to restimulate the muscle during contraction</p> Signup and view all the answers

    Study Notes

    Cardiac Muscle Contraction

    • Period of Isovolumic (Isometric) Contraction:
      • The ventricles begin to contract, but no shortening of the muscle fibers occurs.
      • The pressure inside the ventricle increases until it equals the pressure in the aorta.
    • Period of Ejection:
      • The ventricular pressure exceeds the aortic pressure, forcing the semilunar valves open.
      • Blood is ejected from the ventricles into the aorta and pulmonary artery.
      • Approximately 60% of the blood in the ventricles at the end of diastole is ejected during systole.
      • The first third of ejection is called the period of rapid ejection, the remaining two thirds is the period of slow ejection.
    • Period of Isovolumic (Isometric) Relaxation:
      • Ventricular relaxation begins suddenly.
      • The aortic valve closes, and the ventricular pressure falls back to the diastolic pressure level.

    Ventricular Filling

    • During Diastole: While the A-V valves are closed, blood accumulates in the atria.
    • Atrial Contraction: This contraction results in bulging of the A-V valves into the atria, causing the "c" wave in the atrial pressure curve.
    • Filling of the Ventricles: As the A-V valves open, blood flows from the atria into the ventricles, causing the "v" wave in the atrial pressure curve.

    Function of the Ventricles as Pumps

    • Ventricular Preload: The end-diastolic pressure when the ventricle has become filled.
    • Ventricular Afterload: The pressure in the aorta.
    • Atrial Contribution to Ventricular Filling: Atria function as primer pumps, contributing 20% to ventricular pumping effectiveness.

    Outflow of Blood from the Ventricles During Systole

    • End-systolic Volume: The volume remaining in the ventricle after ejection, typically around 40-50 ml.
    • Ejection Fraction: The fraction of the end-diastolic volume ejected, usually around 60%.

    Heart Valves

    • Atrioventricular (A-V) Valves:
      • Prevent backflow of blood from the ventricles into the atria during ventricular contraction.
      • The mitral valve separates the left atrium from the left ventricle.
      • The tricuspid valve separates the right atrium from the right ventricle.
      • The papillary muscles and chordae tendineae prevent the valve leaflets from prolapsing back into the atria.
    • Semilunar Valves:
      • Prevent backflow of blood from the aorta and pulmonary artery into the ventricles during ventricular relaxation.
      • The aortic valve separates the left ventricle from the aorta.
      • The pulmonary valve separates the right ventricle from the pulmonary artery.
      • The semilunar valves snap closed due to the pressure in the arteries, contrasting with the softer closure of the A-V valves.

    Importance of Cardiac Valves

    • Properly functioning valves are essential for efficient blood flow.
    • Damaged valves can affect cardiac output and lead to heart failure.
    • Valve dysfunction can cause regurgitation (backflow of blood) or stenosis (narrowing of the valve opening) and can be treated through surgery or valve replacement.

    Heart Function

    • Heart beat: The heart beats so rapidly that it doesn't have time to fill completely before the next contraction.
    • Atria function: The atria act as primer pumps, increasing ventricular pumping effectiveness by 20%.
    • Cardiac output: The heart can pump 300% to 400% more blood than the resting body needs.
    • Atrial pressure waves: The atria experience pressure waves called 'a', 'c', and 'v' waves.
      • 'a' wave: Caused by atrial contraction, pressure rises by 4-6 mmHg in the right atrium and 7-8 mmHg in the left atrium.
      • 'c' wave: Occurs during ventricular contraction, caused by blood backflow into the atria and bulging of the A-V valves.
      • 'v' wave: Occurs towards the end of ventricular contraction due to slow blood flow into the atria while the A-V valves are closed.
    • Ventricular filling: The ventricles fill with blood during diastole due to the closed A-V valves.
    • Ventricular output: The ventricles need more volume (preload) or more filling from atrial contraction to maintain adequate cardiac output.

    Ventricular Pumping

    • Isovolumic contraction: When ventricular contraction begins, pressure rises sharply, closing the A-V valves. This period lasts for 0.02 to 0.03 seconds, during which the ventricle builds up enough pressure to open the semilunar valves.
    • Period of ejection: Once ventricular pressure exceeds 80 mmHg (left) or 8 mmHg (right), the semilunar valves open and blood is ejected into the aorta and pulmonary artery.
      • Rapid ejection: 60% of the end-diastolic volume is ejected, with 70% of this occurring during the first third of the ejection period.
      • Slow ejection: The remaining 30% of the end-diastolic volume is ejected during the next two thirds of the ejection period.
    • Isovolumic relaxation: At the end of systole, ventricular relaxation begins, reducing pressure and causing the semilunar valves to close.
      • End-systolic volume: The remaining blood in the ventricle (40-50 ml) after ejection.
      • Ejection fraction: The proportion of the end-diastolic volume that is ejected, usually around 60% (0.6).

    Heart Valves

    • A-V valves: Open during diastole to allow blood flow from atria to ventricles, close during systole to prevent backflow.
      • Mitral valve: Between the left atrium and ventricle, supports high pressure demands.
      • Tricuspid valve: Between the right atrium and ventricle.
    • Semilunar valves: Prevent backflow of blood into the ventricles from the aorta and pulmonary artery after the ventricles contract.
      • Aortic valve: Located between the left ventricle and the aorta.
      • Pulmonary valve: Located between the right ventricle and the pulmonary artery.

    Cardiac Function

    • Frank-Starling mechanism: When the heart stretches due to increased volume, the muscle contracts with more force because of optimal overlap of the actin and myosin filaments.
    • Cardiac efficiency: The ratio of work output to total chemical energy used.
    • Sympathetic nerve stimulation: Increases heart rate and strength of ventricular muscle contraction, boosting cardiac pumping by 30%.
    • Parasympathetic nerve stimulation: Decreases heart rate and strength of ventricular muscle contraction, reducing cardiac pumping by 30%.
      • Vagus nerve stimulation: Can briefly stop the heartbeat, allowing it to "escape" and beat at a rate of 20-40 beats per minute.

    Heart Wall Stress

    • Thickening of the left ventricular walls: This occurs in response to chronic high systolic pressure, helping to relieve wall stress.
    • Ventricular dilation (eccentric hypertrophy): Reduces cardiac efficiency as the ventricle requires more chemical energy for a given amount of work.

    Cardiac Muscle Action Potentials

    • Cardiac muscle cell action potentials differ from skeletal muscle cells in their duration and shape.
    • Cardiac muscle cell action potentials last for several tenths of a second due to the slow opening and prolonged closure of calcium channels.
    • During the plateau phase, calcium ions flow into the cell, maintaining depolarization and activating the contractile process.
    • Potassium permeability decreases during the plateau phase, contributing to the prolonged action potential.

    Heart Valves and Blood Flow

    • Atrioventricular (AV) valves prevent backflow from the ventricles to the atria during ventricular contraction.
    • Semilunar valves prevent backflow from the aorta and pulmonary artery to the ventricles during ventricular relaxation.
    • The AV valves require minimal backflow to close, while the semilunar valves require rapid backflow for a few milliseconds due to their heavier structure.
    • Papillary muscles attach to the AV valves via chordae tendineae, preventing valve prolapse and assisting in their closing.
    • The aortic valve opens during ventricular contraction, allowing blood to flow into the aorta and systemic arteries.
    • The elastic walls of the arteries stretch during systole, increasing pressure to about 120 mm Hg (systolic pressure)
    • During diastole, the elastic recoil of the arteries maintains pressure, ensuring continuous blood flow through the peripheral circulation.
    • The aortic valve closes at the end of systole, causing an incisura (notch) in the aortic pressure curve due to brief backflow before valve closure.
    • Aortic pressure falls gradually throughout diastole due to the continuous flow of blood through the peripheral circulation, reaching about 80 mm Hg (diastolic pressure) before the next contraction.
    • Stroke volume output, defined as the volume of blood ejected per heartbeat, can be increased by both increasing end-diastolic volume and decreasing end-systolic volume.

    Cardiac Performance and Neural Regulation

    • Cardiac stroke work is measured as the amount of energy required to pump a certain volume of blood against a given pressure.
    • Sympathetic nerves release norepinephrine, increasing heart rate, contractility, and stroke volume, leading to greater cardiac output.
    • Parasympathetic nerves release acetylcholine, decreasing heart rate and contractility, leading to reduced cardiac output.

    Cardiac Muscle Excitation-Contraction Coupling

    • Cardiac muscle shares the same excitation-contraction coupling mechanism as skeletal muscle.
    • Calcium ions are released from the sarcoplasmic reticulum and diffuse into the sarcoplasm from the T tubules.
    • The T tubules activate voltage-dependent calcium channels that initiate calcium release.
    • Calcium release channels, also known as ryanodine receptor channels, trigger the release of calcium into the sarcoplasm.
    • Sarcoplasmic calcium ions interact with troponin to initiate cross-bridge formation and muscle contraction.

    Cardiac Muscle Velocity of Signal Conduction

    • The velocity of conduction in atrial and ventricular muscle fibers is approximately 0.3 to 0.5 meters per second.
    • Conduction velocity in the specialized heart conductive system (Purkinje fibers) is up to 4 meters per second.

    Cardiac Muscle Refractory Period

    • Like all excitable tissues, cardiac muscle is refractory to restimulation during the action potential.

    Cardiac Cycle Events

    • The cardiac cycle includes systole (contraction) and diastole (relaxation).
    • The duration of the cardiac cycle is inversely proportional to the heart rate.
    • The P wave on an electrocardiogram represents atrial depolarization and precedes atrial contraction.
    • Atrial contraction contributes to ventricular filling, improving pumping effectiveness by up to 20%.

    Pressure Changes in the Atria

    • Atrial pressure waves (a, c, and v) are observed in the atrial pressure curve.
    • The "a" wave is caused by atrial contraction, while the "c" wave is related to ventricular contraction.
    • Inadequate atrial contraction can lead to reduced ventricular filling and increased preload requirement.

    Isovolumic Contraction in the Ventricles

    • During the initial phase of ventricular contraction, the pressure rises rapidly, closing the atrioventricular valves.
    • Ventricular pressure continues to build until it exceeds aortic pressure, opening the semilunar valves.
    • This initial period of contraction without volume change is called the period of isovolumic or isometric contraction.

    Ejection Fraction

    • The volume of blood ejected during systole is called the stroke volume.
    • The end-systolic volume represents the remaining blood in the ventricle after ejection.
    • The ejection fraction is calculated as the ratio of stroke volume to end-diastolic volume, typically around 60%.

    Mitral Valve

    • The mitral valve consists of two cusps connected to papillary muscles by chordae tendineae.
    • These structures prevent valve prolapse during ventricular contraction.

    Aortic and Pulmonary Artery Valves

    • These semilunar valves open with ventricular contraction and snap closed at the end of systole.
    • They have smaller openings than the atrioventricular valves, resulting in higher blood velocity during ejection.

    Control of Cardiac Pumping

    • Sympathetic nerve stimulation increases heart rate and contractility, enhancing cardiac pumping.
    • Parasympathetic (vagal) stimulation decreases heart rate and contractility, reducing cardiac pumping.
    • Under normal conditions, sympathetic nerve activity maintains a baseline pumping capacity approximately 30% above resting levels.
    • Parasympathetic stimulation can completely stop the heartbeat temporarily before the heart "escapes" and resumes beating at a slower rate.

    Frank-Starling Mechanism

    • The Frank-Starling mechanism describes the relationship between ventricular filling and contractile force.
    • As ventricular filling increases, the strength of contraction also increases.
    • This inherent ability of the heart to regulate its output is crucial for maintaining blood flow.

    Cardiac Muscle Structure

    • Cardiac muscle is composed of two functional syncytia: the atrial syncytium and ventricular syncytium
    • The atria are separated from the ventricles by fibrous tissue surrounding the atrioventricular (A-V) valves
    • The fibrous tissue prevents the direct conduction of electrical signals from the atria to the ventricles
    • The atrioventricular (A-V) bundle is a specialized conductive system that allows for the conduction of action potentials from the atrial syncytium to the ventricular syncytium.

    Cardiac Muscle Action Potential

    • The action potential in ventricular muscle fibers averages 105 millivolts.
    • The resting membrane potential is about -85 millivolts
    • The action potential has a plateau phase which lasts about 0.2 seconds, followed by an abrupt repolarization.
    • The plateau phase in the action potential causes ventricular contraction to last longer than skeletal muscle contractions.

    Mechanisms of Excitation-Contraction Coupling

    • Calcium ions are released from the sarcoplasmic reticulum and diffuse into the sarcoplasm from the T tubules.
    • The release of calcium ions is triggered by the action potential, which opens voltage-dependent calcium channels in the T tubule membrane.
    • The calcium ions entering the cell activate calcium release channels, also called ryanodine receptor channels, in the sarcoplasmic reticulum membrane.
    • Released calcium interacts with troponin to initiate cross-bridge formation and contraction, similar to skeletal muscle.
    • The T tubules of cardiac muscle have a larger diameter and volume than in skeletal muscle, allowing for a greater store of calcium.
    • Mucopolysaccharides in the T tubules bind calcium ions for diffusion into the sarcoplasm when an action potential occurs.

    Refractory Period

    • Cardiac muscle is refractory to restimulation during the action potential.
    • The refractory period helps to ensure that the heart can relax between contractions.
    • The refractory period is about 0.25 to 0.30 seconds for ventricles and shorter for atria (0.15 seconds).
    • A relative refractory period follows the absolute refractory period during which the muscle is more difficult to excite but can be stimulated by a strong signal.

    Strength of Cardiac Muscle Contraction

    • The strength of contraction depends on the concentration of calcium ions in the extracellular fluids
    • A heart in a calcium-free solution will quickly stop beating.

    Cardiac Cycle

    • Phase I: Period of Ventricular Filling & Atrial Contraction:
    • The ventricular volume increases to about 120 ml (end-diastolic volume) during ventricular filling.
    • The diastolic pressure rises to about 5 to 7 mm Hg.
    • Phase II: Period of Isovolumic Contraction:
    • All valves are closed and the ventricular volume does not change.
    • Pressure begins to rise but does not reach the aortic pressure.
    • Phase III: Period of Ventricular Ejection
    • The aortic valve opens, blood is ejected from the ventricle.
    • The ventricular volume decreases.
    • Phase IV: Period of Isovolumic Relaxation
    • The aortic valve closes.
    • The ventricular volume remains constant.

    Cardiac Work Output

    • The volume-pressure diagram, also known as the stroke work output curve, demonstrates the relationship between intraventricular volume and pressure during a single cardiac cycle.
    • The shaded area in the volume-pressure diagram represents the net external work (EW) output by the ventricle.
    • The stroke work output curve shows that as atrial pressure increases, stroke work output increases until it reaches the ventricle’s pumping limit.

    Nervous System Regulation

    • The vagus nerves (parasympathetic) can decrease heart rate and output.
    • Sympathetic stimulation can increase heart rate and the force of contraction, leading to a greater stroke volume and ejection pressure.
    • Sympathetic stimulation can increase cardiac output up to two to threefold.
    • The Frank-Starling mechanism, which involves the relationship between preload (end-diastolic volume) and stroke volume, also plays a role in regulating cardiac output.

    Studying That Suits You

    Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

    Quiz Team

    Related Documents

    Guyton and Hall Physiology Chapter 9 Cardiac Muscle PDF

    Description

    Explore the phases of cardiac muscle contraction and ventricular filling. Understand the periods of isovolumic contraction, ejection, and relaxation, as well as the mechanisms of blood accumulation during diastole. This quiz will test your knowledge of cardiac physiology and mechanics.

    More Like This

    Use Quizgecko on...
    Browser
    Open
    Browser
    Browser