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Questions and Answers
What is the volume of blood remaining in the ventricle at the end of systole known as?
What is the volume of blood remaining in the ventricle at the end of systole known as?
Which term is used to describe the fraction of the end-diastolic volume that is ejected during ventricular contraction?
Which term is used to describe the fraction of the end-diastolic volume that is ejected during ventricular contraction?
How do semilunar valves differ from A-V valves during the ventricular contraction phase?
How do semilunar valves differ from A-V valves during the ventricular contraction phase?
What occurs during the isovolumic contraction phase?
What occurs during the isovolumic contraction phase?
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What is a potential consequence of severe mitral valve dysfunction?
What is a potential consequence of severe mitral valve dysfunction?
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What primarily causes the c wave during ventricular contraction?
What primarily causes the c wave during ventricular contraction?
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What happens to ventricular pressure at the onset of the period of ejection?
What happens to ventricular pressure at the onset of the period of ejection?
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During which phase is 60% of the blood in the ventricles ejected?
During which phase is 60% of the blood in the ventricles ejected?
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What is the primary function of the A-V valves during ventricular contraction?
What is the primary function of the A-V valves during ventricular contraction?
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What happens at the end of systole during the period of isovolumic relaxation?
What happens at the end of systole during the period of isovolumic relaxation?
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How much blood is typically ejected during the first third of the ejection period?
How much blood is typically ejected during the first third of the ejection period?
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What phase follows the closing of the A-V valves during ventricular systole?
What phase follows the closing of the A-V valves during ventricular systole?
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What characterizes the period of slow ejection?
What characterizes the period of slow ejection?
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What occurs immediately after the aortic valve closes during the cardiac cycle?
What occurs immediately after the aortic valve closes during the cardiac cycle?
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Which phase of the cardiac cycle is characterized by a gradual increase in ventricular pressure and no change in volume?
Which phase of the cardiac cycle is characterized by a gradual increase in ventricular pressure and no change in volume?
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During which phase does the stroke volume get ejected from the ventricle?
During which phase does the stroke volume get ejected from the ventricle?
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What happens to the aortic valve when the left ventricular pressure exceeds aortic pressure?
What happens to the aortic valve when the left ventricular pressure exceeds aortic pressure?
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What is the primary reason the aortic valve closes at the end of the ejection phase?
What is the primary reason the aortic valve closes at the end of the ejection phase?
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Which waveform represents the pressure changes in the left ventricle during the ejection phase?
Which waveform represents the pressure changes in the left ventricle during the ejection phase?
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During which phase does the volume of blood in the ventricle remain constant despite changes in pressure?
During which phase does the volume of blood in the ventricle remain constant despite changes in pressure?
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What characterizes the end-diastolic volume in the left ventricle?
What characterizes the end-diastolic volume in the left ventricle?
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In which phase does the left ventricular pressure drop significantly as the ventricle returns to its relaxed state?
In which phase does the left ventricular pressure drop significantly as the ventricle returns to its relaxed state?
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Which of the following is NOT a function of the heart valves during the cardiac cycle?
Which of the following is NOT a function of the heart valves during the cardiac cycle?
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What determines the preload for cardiac contraction?
What determines the preload for cardiac contraction?
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During which phase does the aortic valve open and blood flows into the aorta?
During which phase does the aortic valve open and blood flows into the aorta?
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What occurs at point D in the cardiac cycle?
What occurs at point D in the cardiac cycle?
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What describes the afterload during ventricular contraction?
What describes the afterload during ventricular contraction?
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Which phase corresponds to the period of isovolumic relaxation?
Which phase corresponds to the period of isovolumic relaxation?
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What occurs during the period of ejection in the ventricles?
What occurs during the period of ejection in the ventricles?
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Which of the following describes the changes during Phase III: Period of Ejection?
Which of the following describes the changes during Phase III: Period of Ejection?
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Which statement is true about the role of valves during the cardiac cycle?
Which statement is true about the role of valves during the cardiac cycle?
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What effect does sympathetic nervous system stimulation have on ventricular pressure?
What effect does sympathetic nervous system stimulation have on ventricular pressure?
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What happens to the volume of the ventricle at the end of the period of ejection?
What happens to the volume of the ventricle at the end of the period of ejection?
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What causes the a wave in the atrial pressure curve?
What causes the a wave in the atrial pressure curve?
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During which phase does the ventricular pressure rise abruptly?
During which phase does the ventricular pressure rise abruptly?
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What occurs immediately after the ventricular contraction begins?
What occurs immediately after the ventricular contraction begins?
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What is the main purpose of the aortic and pulmonary valves during ventricular contraction?
What is the main purpose of the aortic and pulmonary valves during ventricular contraction?
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What average increase in pressure occurs in the left atrium during atrial contraction?
What average increase in pressure occurs in the left atrium during atrial contraction?
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How much more blood can the heart typically pump compared to resting body requirements?
How much more blood can the heart typically pump compared to resting body requirements?
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What is the effect of atrial failure on the cardiac output during exercise?
What is the effect of atrial failure on the cardiac output during exercise?
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During which period does no emptying of the ventricles occur despite contraction?
During which period does no emptying of the ventricles occur despite contraction?
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When do the A-V valves close during the cardiac cycle?
When do the A-V valves close during the cardiac cycle?
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What are the pressure waves in the atrial pressure curve identified as a, c, and v waves primarily associated with?
What are the pressure waves in the atrial pressure curve identified as a, c, and v waves primarily associated with?
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What typically happens to atrial pressure during atrial contraction?
What typically happens to atrial pressure during atrial contraction?
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During which phase does the heart still manage to function effectively even if the atria fail to contract?
During which phase does the heart still manage to function effectively even if the atria fail to contract?
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What occurs during the period of isovolumic contraction?
What occurs during the period of isovolumic contraction?
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How does the heart typically perform if atrial contraction does not occur?
How does the heart typically perform if atrial contraction does not occur?
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What is the significance of the a, c, and v waves in the atrial pressure curve?
What is the significance of the a, c, and v waves in the atrial pressure curve?
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What causes the v wave during ventricular contraction?
What causes the v wave during ventricular contraction?
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During which part of the ejection period is the majority of blood (approximately 60%) expelled from the ventricles?
During which part of the ejection period is the majority of blood (approximately 60%) expelled from the ventricles?
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What primarily happens at the end of systole during the isovolumic relaxation phase?
What primarily happens at the end of systole during the isovolumic relaxation phase?
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What role do the A-V valves play during ventricular contraction?
What role do the A-V valves play during ventricular contraction?
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What occurs when the left ventricular pressure rises slightly above 80 mm Hg?
What occurs when the left ventricular pressure rises slightly above 80 mm Hg?
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What is the typical ejection fraction percentage associated with healthy cardiac function?
What is the typical ejection fraction percentage associated with healthy cardiac function?
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Which mechanism is responsible for the closure of the aortic and pulmonary valves after ventricular contraction?
Which mechanism is responsible for the closure of the aortic and pulmonary valves after ventricular contraction?
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What is the consequence of reduced end-systolic volume in the ventricles?
What is the consequence of reduced end-systolic volume in the ventricles?
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How do semilunar valves function differently from A-V valves during contraction?
How do semilunar valves function differently from A-V valves during contraction?
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What role do the chordae tendineae and papillary muscles play in cardiac function?
What role do the chordae tendineae and papillary muscles play in cardiac function?
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Which condition can severely compromise the function of the mitral valve?
Which condition can severely compromise the function of the mitral valve?
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What is the consequence of severe leakage during ventricular contraction?
What is the consequence of severe leakage during ventricular contraction?
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What is the relationship between preload and end-diastolic volume?
What is the relationship between preload and end-diastolic volume?
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What triggers the need for more blood or preload during early diastole?
What triggers the need for more blood or preload during early diastole?
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What primarily causes the higher velocity of blood ejection through the aortic and pulmonary valves?
What primarily causes the higher velocity of blood ejection through the aortic and pulmonary valves?
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The calcium ions that enter during the plateau phase of cardiac muscle action potential are derived from the extracellular fluid.
The calcium ions that enter during the plateau phase of cardiac muscle action potential are derived from the extracellular fluid.
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Fast sodium channels remain open for several seconds during the cardiac muscle action potential.
Fast sodium channels remain open for several seconds during the cardiac muscle action potential.
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The permeability of the cardiac muscle membrane for potassium ions increases immediately after the onset of the action potential.
The permeability of the cardiac muscle membrane for potassium ions increases immediately after the onset of the action potential.
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During Phase 2 of the cardiac action potential, both calcium and sodium ions flux into the cardiac muscle cell.
During Phase 2 of the cardiac action potential, both calcium and sodium ions flux into the cardiac muscle cell.
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The membrane potential of cardiac muscle cells reaches a value of approximately +20 millivolts before the sodium channels close.
The membrane potential of cardiac muscle cells reaches a value of approximately +20 millivolts before the sodium channels close.
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The pressure in the ventricle rises rapidly after the aortic valve opens.
The pressure in the ventricle rises rapidly after the aortic valve opens.
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The end-systolic volume can decrease to as little as 5 to 10 ml in a healthy heart.
The end-systolic volume can decrease to as little as 5 to 10 ml in a healthy heart.
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The aortic and pulmonary valves are constructed with particularly strong fibrous tissue to withstand mechanical stress.
The aortic and pulmonary valves are constructed with particularly strong fibrous tissue to withstand mechanical stress.
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The A-V valves are supported by chordae tendineae while the semilunar valves are not.
The A-V valves are supported by chordae tendineae while the semilunar valves are not.
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The volume of blood that enters the ventricles during diastole can range from 100 to 120 ml in a healthy heart.
The volume of blood that enters the ventricles during diastole can range from 100 to 120 ml in a healthy heart.
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The A-V valves open actively during ventricular contraction.
The A-V valves open actively during ventricular contraction.
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The aortic valve closes due to a short period of backward flow of blood.
The aortic valve closes due to a short period of backward flow of blood.
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Papillary muscles assist in the closure of the A-V valves by themselves.
Papillary muscles assist in the closure of the A-V valves by themselves.
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Diastolic pressure in the aorta can fall to around 80 mm Hg before the ventricle contracts again.
Diastolic pressure in the aorta can fall to around 80 mm Hg before the ventricle contracts again.
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The semilunar valves close passively during diastole.
The semilunar valves close passively during diastole.
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During systole, pressure in the aorta rises to about 100 mm Hg.
During systole, pressure in the aorta rises to about 100 mm Hg.
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The walls of arteries maintain low pressure throughout diastole.
The walls of arteries maintain low pressure throughout diastole.
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The opening and closing of heart valves are completely powered by muscle contractions.
The opening and closing of heart valves are completely powered by muscle contractions.
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The maximum pressure in the aorta during ventricular contraction is referred to as diastolic pressure.
The maximum pressure in the aorta during ventricular contraction is referred to as diastolic pressure.
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During the cardiac cycle, the pressure in the aorta decreases steadily during diastole.
During the cardiac cycle, the pressure in the aorta decreases steadily during diastole.
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Match the components of the cardiac muscle with their functions:
Match the components of the cardiac muscle with their functions:
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Match the terms with their corresponding characteristics in cardiac muscle action potentials:
Match the terms with their corresponding characteristics in cardiac muscle action potentials:
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Match the phases of the cardiac cycle with their descriptions:
Match the phases of the cardiac cycle with their descriptions:
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Match the effects of the cardiac muscle contraction with their significance:
Match the effects of the cardiac muscle contraction with their significance:
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Match the actions during a heartbeat with their resulting events:
Match the actions during a heartbeat with their resulting events:
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Match the following components of cardiac muscle with their functions:
Match the following components of cardiac muscle with their functions:
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Match the following phases of cardiac action potential with their descriptions:
Match the following phases of cardiac action potential with their descriptions:
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Match the following terms related to signal conduction in cardiac muscle with their characteristics:
Match the following terms related to signal conduction in cardiac muscle with their characteristics:
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Match the following structures with their roles in cardiac muscle contraction:
Match the following structures with their roles in cardiac muscle contraction:
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Match the following responses in cardiac muscle during contraction with their triggers:
Match the following responses in cardiac muscle during contraction with their triggers:
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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.
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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.
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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.
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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.
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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.
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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.
