Podcast
Questions and Answers
Which of the following factors does NOT affect stroke volume?
Which of the following factors does NOT affect stroke volume?
Frank Starling's Law states that the strength of the heart's contraction is determined by the amount of blood filling the heart.
Frank Starling's Law states that the strength of the heart's contraction is determined by the amount of blood filling the heart.
True
What is the significance of preload in cardiovascular physiology?
What is the significance of preload in cardiovascular physiology?
Preload influences the volume of blood the ventricles receive, affecting stroke volume and cardiac output.
The hormone ______________ is released from the adrenal medulla and enhances heart rate and contractility.
The hormone ______________ is released from the adrenal medulla and enhances heart rate and contractility.
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Match the following terms with their correct descriptions:
Match the following terms with their correct descriptions:
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Which phase of the cardiac cycle involves the heart muscles contracting to eject blood into the arteries?
Which phase of the cardiac cycle involves the heart muscles contracting to eject blood into the arteries?
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Diastole refers to the phase of the cardiac cycle when the heart is contracting.
Diastole refers to the phase of the cardiac cycle when the heart is contracting.
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What is the relationship between atrial systole and ventricular filling?
What is the relationship between atrial systole and ventricular filling?
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The units of measurement for cardiac output (CO) are _____ per minute.
The units of measurement for cardiac output (CO) are _____ per minute.
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Which statement correctly relates stroke volume (SV) to its components?
Which statement correctly relates stroke volume (SV) to its components?
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The Frank-Starling Law states that the heart pumps more blood when it is filled with more blood during diastole.
The Frank-Starling Law states that the heart pumps more blood when it is filled with more blood during diastole.
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Define preload in the context of cardiac mechanics.
Define preload in the context of cardiac mechanics.
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Match the following cardiac terms with their definitions:
Match the following cardiac terms with their definitions:
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What phase of the cardiac cycle represents the heart's contraction?
What phase of the cardiac cycle represents the heart's contraction?
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Atrial depolarization is represented by the QRS complex.
Atrial depolarization is represented by the QRS complex.
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What is the significance of the AV node during the cardiac cycle?
What is the significance of the AV node during the cardiac cycle?
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The __________ phase comes after ventricular depolarization in the cardiac cycle.
The __________ phase comes after ventricular depolarization in the cardiac cycle.
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Match the cardiac events with their corresponding components:
Match the cardiac events with their corresponding components:
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Which sound is associated with the completion of ventricular repolarization?
Which sound is associated with the completion of ventricular repolarization?
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Mechanical events of the heart occur simultaneously with electrical events.
Mechanical events of the heart occur simultaneously with electrical events.
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What occurs during the isovolumetric relaxation phase?
What occurs during the isovolumetric relaxation phase?
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During __________, the heart is known to be at rest and filling with blood.
During __________, the heart is known to be at rest and filling with blood.
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Which of the following best defines the cardiac cycle?
Which of the following best defines the cardiac cycle?
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What does the stroke volume (SV) represent?
What does the stroke volume (SV) represent?
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Increased preload will decrease stroke volume.
Increased preload will decrease stroke volume.
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What formula is used to calculate cardiac output (CO)?
What formula is used to calculate cardiac output (CO)?
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The volume of blood in each ventricle at the end of diastole is known as ________.
The volume of blood in each ventricle at the end of diastole is known as ________.
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Match each term with its correct definition:
Match each term with its correct definition:
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Which factor does NOT directly affect stroke volume?
Which factor does NOT directly affect stroke volume?
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Cardiac output is the total volume of blood pumped out in one minute.
Cardiac output is the total volume of blood pumped out in one minute.
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What is the average stroke volume in milliliters?
What is the average stroke volume in milliliters?
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According to the Frank-Starling law, increased blood return to the heart stretches the ventricles and increases ________.
According to the Frank-Starling law, increased blood return to the heart stretches the ventricles and increases ________.
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What is the volume of blood remaining in each ventricle after contraction known as?
What is the volume of blood remaining in each ventricle after contraction known as?
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What is the primary factor that influences stroke volume according to Starling's law?
What is the primary factor that influences stroke volume according to Starling's law?
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Increased stroke volume is achieved by decreasing venous return.
Increased stroke volume is achieved by decreasing venous return.
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What effect does exercise have on venous return?
What effect does exercise have on venous return?
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The distension of ventricular walls increases _____ and consequently the stroke volume.
The distension of ventricular walls increases _____ and consequently the stroke volume.
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Match the following mechanisms with their role in aiding venous return:
Match the following mechanisms with their role in aiding venous return:
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Which of the following is NOT a method aiding venous return?
Which of the following is NOT a method aiding venous return?
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A slow heartbeat decreases venous return.
A slow heartbeat decreases venous return.
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What are the two main factors that increase venous return?
What are the two main factors that increase venous return?
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The Frank-Starling law explains the relationship between venous return, ____ and stroke volume.
The Frank-Starling law explains the relationship between venous return, ____ and stroke volume.
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What happens to stroke volume when venous return is increased?
What happens to stroke volume when venous return is increased?
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What effect does increased afterload have on stroke volume?
What effect does increased afterload have on stroke volume?
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Which of the following would be classified as a positive ionotropic factor?
Which of the following would be classified as a positive ionotropic factor?
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What occurs in the heart due to sustained increases in calcium levels in congestive heart failure?
What occurs in the heart due to sustained increases in calcium levels in congestive heart failure?
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Which chemical would likely decrease contractility in cardiac muscle?
Which chemical would likely decrease contractility in cardiac muscle?
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How do sympathetic stimulation and calcium influence stroke volume?
How do sympathetic stimulation and calcium influence stroke volume?
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During which phase does the majority of blood enter the ventricles passively?
During which phase does the majority of blood enter the ventricles passively?
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What initiates atrial systole in the cardiac cycle?
What initiates atrial systole in the cardiac cycle?
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What happens during the isovolumetric contraction phase of the cardiac cycle?
What happens during the isovolumetric contraction phase of the cardiac cycle?
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What describes the end diastolic volume (EDV) in the cardiac cycle?
What describes the end diastolic volume (EDV) in the cardiac cycle?
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Which heart sound is associated with the closure of the AV valves during the cardiac cycle?
Which heart sound is associated with the closure of the AV valves during the cardiac cycle?
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What occurs when the pressure in the ventricles exceeds the pressure in the large arteries?
What occurs when the pressure in the ventricles exceeds the pressure in the large arteries?
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Which component is represented by the end systolic volume (ESV) in the cardiac cycle?
Which component is represented by the end systolic volume (ESV) in the cardiac cycle?
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Which electrical event marks the completion of the ventricular filling phase?
Which electrical event marks the completion of the ventricular filling phase?
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During which phase of the cardiac cycle does isovolumetric relaxation occur?
During which phase of the cardiac cycle does isovolumetric relaxation occur?
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What occurs following the closure of the SL valves during isovolumetric relaxation?
What occurs following the closure of the SL valves during isovolumetric relaxation?
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What is the term for the brief rise in aortic pressure caused by blood rebounding off the closed SL valves?
What is the term for the brief rise in aortic pressure caused by blood rebounding off the closed SL valves?
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What sound is associated with the closure of the SL valves during the cardiac cycle?
What sound is associated with the closure of the SL valves during the cardiac cycle?
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What is the mathematical relationship represented by the formula $CO = HR \times SV$?
What is the mathematical relationship represented by the formula $CO = HR \times SV$?
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In isovolumetric relaxation, what is happening to the pressure in the ventricles compared to the aorta and pulmonary arteries?
In isovolumetric relaxation, what is happening to the pressure in the ventricles compared to the aorta and pulmonary arteries?
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Which factor primarily determines the stroke volume (SV) according to the relationship $SV = EDV - ESV$?
Which factor primarily determines the stroke volume (SV) according to the relationship $SV = EDV - ESV$?
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Which of the following is NOT a characteristic of isovolumetric relaxation?
Which of the following is NOT a characteristic of isovolumetric relaxation?
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How does the Frank-Starling law explain cardiac output in relation to venous return?
How does the Frank-Starling law explain cardiac output in relation to venous return?
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What causes the pulmonary arteries and aorta to experience a rapid increase in pressure during this phase?
What causes the pulmonary arteries and aorta to experience a rapid increase in pressure during this phase?
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Which of the following correctly describes preload?
Which of the following correctly describes preload?
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Which statement about the events in early diastole is correct?
Which statement about the events in early diastole is correct?
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During which electrical event on an ECG does isovolumetric relaxation begin?
During which electrical event on an ECG does isovolumetric relaxation begin?
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During which phase of the cardiac cycle does ventricular filling occur?
During which phase of the cardiac cycle does ventricular filling occur?
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What is the average stroke volume (SV) during a typical cardiac cycle?
What is the average stroke volume (SV) during a typical cardiac cycle?
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What describes the pressure in the ventricles during the isovolumetric relaxation phase?
What describes the pressure in the ventricles during the isovolumetric relaxation phase?
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Which factor has the greatest influence on the preload of cardiac muscles?
Which factor has the greatest influence on the preload of cardiac muscles?
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What main physiological characteristic affects the contractility of the heart?
What main physiological characteristic affects the contractility of the heart?
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What happens to stroke volume when preload is increased?
What happens to stroke volume when preload is increased?
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How do skeletal muscle contractions affect venous return?
How do skeletal muscle contractions affect venous return?
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Which of the following best defines the role of contractility in the cardiac cycle?
Which of the following best defines the role of contractility in the cardiac cycle?
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In what way does a slow heartbeat affect stroke volume?
In what way does a slow heartbeat affect stroke volume?
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What physiological mechanism helps blood return to the heart during inhalation?
What physiological mechanism helps blood return to the heart during inhalation?
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How does a slow heartbeat influence cardiac output?
How does a slow heartbeat influence cardiac output?
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According to Starling's law, which of the following statements is true?
According to Starling's law, which of the following statements is true?
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Which of the following is NOT a factor aiding in venous return?
Which of the following is NOT a factor aiding in venous return?
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What effect does an increase in end-diastolic volume (EDV) have on stroke volume (SV)?
What effect does an increase in end-diastolic volume (EDV) have on stroke volume (SV)?
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Which of the following statements best describes the relationship between venous return and stroke volume?
Which of the following statements best describes the relationship between venous return and stroke volume?
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What role do heart valves play in venous return?
What role do heart valves play in venous return?
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Which of the following actions would likely reduce venous return?
Which of the following actions would likely reduce venous return?
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What is the primary event that occurs during the T wave in the cardiac cycle?
What is the primary event that occurs during the T wave in the cardiac cycle?
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Which phase of the cardiac cycle directly follows ventricular systole?
Which phase of the cardiac cycle directly follows ventricular systole?
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What part of the heart's electrical conduction system ensures a delay before ventricular contraction?
What part of the heart's electrical conduction system ensures a delay before ventricular contraction?
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During which part of the cardiac cycle does the first heart sound (S1) occur?
During which part of the cardiac cycle does the first heart sound (S1) occur?
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Which event is characterized by a series of pressure changes and the efficient pumping of blood by the heart?
Which event is characterized by a series of pressure changes and the efficient pumping of blood by the heart?
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What marks the transition from atrial systole to ventricular systole?
What marks the transition from atrial systole to ventricular systole?
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Which of the following describes the relationship between electrical and mechanical events in the heart?
Which of the following describes the relationship between electrical and mechanical events in the heart?
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During the cardiac cycle, when does the majority of blood enter the ventricles?
During the cardiac cycle, when does the majority of blood enter the ventricles?
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Which factor is primarily responsible for increasing stroke volume through enhanced contractility?
Which factor is primarily responsible for increasing stroke volume through enhanced contractility?
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What is the primary effect of hypertension on afterload and stroke volume?
What is the primary effect of hypertension on afterload and stroke volume?
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Which of the following substances is classified as a negative ionotropic factor?
Which of the following substances is classified as a negative ionotropic factor?
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What physiological change occurs in cardiac myocytes due to chronic congestive heart failure?
What physiological change occurs in cardiac myocytes due to chronic congestive heart failure?
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Which statement correctly describes the role of adenylate cyclase in cardiac muscle contraction?
Which statement correctly describes the role of adenylate cyclase in cardiac muscle contraction?
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What primarily causes the rapid drop in pressure in the ventricles during isovolumetric relaxation?
What primarily causes the rapid drop in pressure in the ventricles during isovolumetric relaxation?
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Which event corresponds to the closure of the semilunar valves in the cardiac cycle?
Which event corresponds to the closure of the semilunar valves in the cardiac cycle?
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What is the physiological significance of the dicrotic notch observed in the pressure tracing of the aorta?
What is the physiological significance of the dicrotic notch observed in the pressure tracing of the aorta?
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During early diastole, what happens immediately after the T wave in the electrocardiogram?
During early diastole, what happens immediately after the T wave in the electrocardiogram?
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Which phase of the cardiac cycle directly follows the ventricular ejection phase?
Which phase of the cardiac cycle directly follows the ventricular ejection phase?
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What characterizes the atrioventricular and semilunar valves during isovolumetric relaxation?
What characterizes the atrioventricular and semilunar valves during isovolumetric relaxation?
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What leads to the second heart sound during the cardiac cycle?
What leads to the second heart sound during the cardiac cycle?
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Which occurrence marks the transition from ventricular systole to diastole?
Which occurrence marks the transition from ventricular systole to diastole?
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What is the primary consequence of the rapid increase of pressure in the pulmonary arteries during isovolumetric relaxation?
What is the primary consequence of the rapid increase of pressure in the pulmonary arteries during isovolumetric relaxation?
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What is the primary regulator of stroke volume according to the relationship described?
What is the primary regulator of stroke volume according to the relationship described?
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Which of the following formulas correctly represents the calculation for stroke volume?
Which of the following formulas correctly represents the calculation for stroke volume?
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During what condition does increased preload result in an augmented stroke volume?
During what condition does increased preload result in an augmented stroke volume?
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How is cardiac output calculated from stroke volume and heart rate?
How is cardiac output calculated from stroke volume and heart rate?
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Which statement about end-diastolic volume (EDV) is accurate?
Which statement about end-diastolic volume (EDV) is accurate?
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Which component is NOT directly affecting stroke volume?
Which component is NOT directly affecting stroke volume?
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What effect does increased afterload have on stroke volume?
What effect does increased afterload have on stroke volume?
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Which factor contributes to the increase in stroke volume under normal physiological conditions?
Which factor contributes to the increase in stroke volume under normal physiological conditions?
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According to Frank-Starling's law, increased blood return to the heart leads to what result?
According to Frank-Starling's law, increased blood return to the heart leads to what result?
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Which of the following best defines cardiac output (CO)?
Which of the following best defines cardiac output (CO)?
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What is the primary mechanism by which the muscular pump aids venous return?
What is the primary mechanism by which the muscular pump aids venous return?
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Which condition would most likely lead to a decrease in stroke volume?
Which condition would most likely lead to a decrease in stroke volume?
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Which of the following factors is least likely to influence venous return?
Which of the following factors is least likely to influence venous return?
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What role does the respiratory pump play in venous return?
What role does the respiratory pump play in venous return?
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How would a reduced heart rate influence stroke volume?
How would a reduced heart rate influence stroke volume?
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According to Starling’s law, what happens when the cardiac muscle is stretched further?
According to Starling’s law, what happens when the cardiac muscle is stretched further?
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What is the effect of exercise on venous return?
What is the effect of exercise on venous return?
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What is the overall relationship described by the Frank-Starling law?
What is the overall relationship described by the Frank-Starling law?
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Which factor most directly affects preload in the heart?
Which factor most directly affects preload in the heart?
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What physiological change occurs during inhalation that aids in venous return?
What physiological change occurs during inhalation that aids in venous return?
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Cardiac output is defined as the total volume of blood pumped out in one minute and is measured in liters.
Cardiac output is defined as the total volume of blood pumped out in one minute and is measured in liters.
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End diastolic volume (EDV) is the volume of blood remaining in each ventricle after contraction.
End diastolic volume (EDV) is the volume of blood remaining in each ventricle after contraction.
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The phases of the cardiac cycle include atrial filling, isovolumetric contraction, atrial ejection, and isovolumetric relaxation.
The phases of the cardiac cycle include atrial filling, isovolumetric contraction, atrial ejection, and isovolumetric relaxation.
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During isovolumetric relaxation, all heart valves are closed and the heart is at rest.
During isovolumetric relaxation, all heart valves are closed and the heart is at rest.
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The Frank-Starling Law states that increased venous return directly decreases stroke volume.
The Frank-Starling Law states that increased venous return directly decreases stroke volume.
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Ventricular depolarization is represented by the P wave.
Ventricular depolarization is represented by the P wave.
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During isovolumetric relaxation, all heart valves are open allowing blood to flow freely.
During isovolumetric relaxation, all heart valves are open allowing blood to flow freely.
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The first heart sound occurs during the completion of ventricular repolarization.
The first heart sound occurs during the completion of ventricular repolarization.
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Atrial systole delivers 80% of the blood volume to the ventricles.
Atrial systole delivers 80% of the blood volume to the ventricles.
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The end diastolic volume (EDV) represents the volume of blood remaining in each ventricle after contraction.
The end diastolic volume (EDV) represents the volume of blood remaining in each ventricle after contraction.
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Atrial systole involves the contraction of the atria to fill the ventricles with blood.
Atrial systole involves the contraction of the atria to fill the ventricles with blood.
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Systole refers to the relaxation phase of the cardiac cycle.
Systole refers to the relaxation phase of the cardiac cycle.
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The isovolumetric contraction phase occurs when all heart valves are open.
The isovolumetric contraction phase occurs when all heart valves are open.
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The first heart sound is associated with the closure of the semilunar valves.
The first heart sound is associated with the closure of the semilunar valves.
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Electrically, atrial depolarization is represented by the QRS wave on the ECG.
Electrically, atrial depolarization is represented by the QRS wave on the ECG.
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Study Notes
Cardiac Cycle
- The cardiac cycle encompasses all events associated with blood flow during one heartbeat.
- Systole is the contraction phase, pushing blood out.
- Diastole is the relaxation phase, allowing chambers to refill.
Phases of the Cardiac Cycle
- Ventricular Filling:
- Passive filling occurs first as pressure in atria exceeds that in ventricles, filling the ventricles by 70%.
- Atrial systole, atrial contraction, contributes the remaining 30%, actively filling the ventricles.
- This phase corresponds to the P wave on an ECG.
- Isovolumetric Contraction:
- Ventricles start contracting, but all valves remain closed, increasing pressure inside.
- This phase marks the beginning of ventricular systole.
- It corresponds to the QRS complex on an ECG.
- Ventricular Ejection:
- Pressure inside ventricles exceeds that in aorta and pulmonary trunk, opening the semilunar valves.
- Blood is ejected into the aorta and pulmonary trunk.
- This phase corresponds to the plateau of the T wave on an ECG.
- Isovolumetric Relaxation:
- Ventricles start to relax, pressure drops.
- All valves remain closed until ventricular pressure drops below that in atria.
- Corresponding with the end of the T wave, the heart is preparing for the next cycle.
Cardiac Output (CO)
- The volume of blood pumped by each ventricle in one minute.
- CO = Heart rate (HR) x Stroke volume (SV)
- HR: number of beats per minute.
- SV: volume of blood pumped out by one ventricle with each beat.
Stroke Volume (SV)
- The volume of blood ejected from the ventricle with each beat: SV = EDV – ESV.
- EDV: End Diastolic Volume, the volume of blood in the ventricle at the end of diastole (ventricular relaxation).
- ESV: End Systolic Volume, the volume of blood remaining in the ventricle after systole (ventricular contraction).
- Factors affecting SV:
- Preload: The degree of stretch of cardiac muscle cells before contraction.
- Increased preload generally leads to increased SV.
- Venous return is the most important factor influencing preload.
- Contractility: The inherent strength of the heart muscle.
- Positive inotropic agents increase contractility and SV.
- Negative inotropic agents decrease contractility and SV.
- Afterload: The resistance the ventricle must overcome to eject blood.
- Increased afterload generally leads to decreased SV.
- Preload: The degree of stretch of cardiac muscle cells before contraction.
Frank-Starling Law of the Heart
- Increased venous return stretches the ventricle, increasing contraction force.
- This leads to an increased SV, enhancing circulatory efficiency.
- This mechanism is crucial for adapting to increased demands, such as exercise.
Regulation of Stroke Volume
- Venous return (the volume of blood returning to the heart) is the primary factor influencing preload and SV.
- Factors aiding venous return:
- Muscular pump: Contraction of skeletal muscles "milks" blood toward the heart.
- Respiratory pump: Pressure changes during breathing aid blood flow towards the heart.
Autonomic Nervous System Regulation
- The autonomic nervous system regulates HR and hence CO.
- Sympathetic stimulation increases HR and contractility; parasympathetic stimulation decreases HR.
- Atrial (Bainbridge) Reflex: Increased venous return stretches atrial walls, stimulating the SA node to increase HR.
Chemical Regulation of Heart Rate
- Hormones
- Epinephrine (from adrenal medulla) increases HR and contractility.
- Thyroxine (from thyroid gland) enhances HR and the effects of epinephrine.
- Ion concentrations: Maintaining proper intracellular and extracellular ion concentrations (like Ca2+ and K+) is essential for normal heart function.
Ventricular Filling
- Occurs in mid-to-late diastole
- AV valves open, SL valves closed
- 80% of blood passively flows into ventricles
- Transition into atrial systole begins after depolarization
- Atrial systole delivers the remaining 20% blood volume
- Ventricles are at end diastole, having received nearly 100% of their blood volume
- End diastolic volume(EDV): volume of blood in each ventricle at the end of diastole
Ventricular Systole
- Atria are relaxed (diastole)
- Ventricles begin to contract (wave of depolarization just occurred), raising ventricular pressure and closing the AV valves (first heart sound)
- Isovolumetric contraction phase occurs, all valves closed
- Ventricles contract (full systole, end of QRS wave)
- With ventricular contraction, pressure exceeds pressure in the large arteries, forcing the SL valves open
- Blood flows into the pulmonary arteries and aorta (ejection phase)
- End systolic volume (ESV): volume of blood remaining in each ventricle after contraction.
- Ventricular relaxation begins after ESV
Isovolumetric Relaxation
- Occurs in early diastole (following T wave on ECG)
- Ventricles relax
- Rapid drop in ventricular pressure, high pressure in the pulmonary arteries and aorta
- Backflow of blood in the aorta and pulmonary trunk closes the SL valves
- Closure of the SL valves leads to a rapid increase in pulmonary artery and aortic pressure, causing the dicrotic notch (brief rise in aortic pressure)
- This is the second heart sound
Phases of the Cardiac Cycle
- Ventricular filling
- Mid-to-late diastole
- Atrial systole
- Ventricular Systole
- Isovolumetric contraction
- Ejection phase
- Early diastole
- Isovolumetric relaxation
Useful Terms
- EDV (end diastolic volume)
- ESV (end systolic volume)
- SV (stroke volume)
- CO (cardiac output)
- CO = HR * SV
- SV = EDV - ESV
Cardiac Output
- Volume of blood pumped by each ventricle in one minute
- CO = HR * SV
- HR = number of beats per minute
- SV = volume of blood pumped by a ventricle with each beat
Regulation of Stroke Volume
- SV = EDV - ESV
- Avg SV = 70 ml/beat (60% of blood in chambers)
- Preload, Contractibility, and Afterload affect SV
- Preload: degree of stretch of cardiac muscle cells before contraction
- Length-tension relationship
- INCREASED PRELOAD INCREASED SV
- Slow heartbeat and exercise increase venous return, increase EDV, SV and force of contraction
- Frank-Starling law of heart: increased blood return stretches ventricles and increases contraction force so more is propelled out.
- VENOUS RETURN is the most important factor
- Anything increasing venous return (volume or speed) increases EDV, SV, contraction and force.
- Contractility: contractile strength
- Independent of muscle stretch and EDV
- Increased SV, decreased ESV
- Increased Ca2+ from sympathetic stimulation
-
Positive Ionotropic Factors
- Epinephrine
- Thyroxine
- Glucagon
- Digitalis
- High extracellular Ca++
-
Negative Ionotropic Factors
- Acidosis
- Increased extracellular K+
- Calcium channel blockers
- Afterload: pressure ventricles must overcome to eject blood
- Backpressure from blood in aorta and pulmonary vessels
- Hypertension increases afterload, resulting in increased ESV and reduced SV
Factors Aiding Venous Return
- Muscular pump
- Respiratory pump
- Venoconstriction
Congestive Heart Failure (CHF)
- CO is so low that blood circulation cannot meet tissue needs
- Heart attempts to work harder, increasing Ca2+ levels in cardiac cells
- Sustained increase in Ca2+ leads to altered heart architecture
- Caused by:
- Coronary atherosclerosis
- Persistent high blood pressure
- Dead cardiac cells from heart attacks
- Dilated cardiomyopathy (DCM)
Cardiac Anatomy & Heart Sounds
- The SA node depolarizes, represented by the P wave on the ECG.
- The impulse is delayed at the AV node, allowing for atrial systole (contraction).
- Ventricular depolarization creates the QRS complex on the ECG, while atrial repolarization also occurs. Ventricular systole begins.
- Ventricular depolarization is complete, and ventricular systole continues. The first heart sound occurs as the AV valves close.
- Ventricular repolarization is represented by the T wave on the ECG.
- Ventricular repolarization is complete. The second heart sound occurs as the semilunar valves close.
Mechanical Events: The Cardiac Cycle
- The heart functions as an efficient pump, driven by pressure and volume changes.
- Mechanical events follow electrical events.
- The cardiac cycle encompasses all events associated with blood flow through the heart during one heartbeat.
- Systole refers to contraction, while diastole refers to relaxation.
Phases of the Cardiac Cycle
-
Isovolumetric relaxation occurs at the beginning of diastole (after the T wave).
- Ventricles relax, pressure decreases, and the majority of blood has been ejected into the pulmonary artery and aorta.
- The decrease in ventricular pressure, combined with higher pressure in the pulmonary artery and aorta, causes a backflow of blood. This closes the semilunar valves, contributing to the second heart sound.
-
Ventricular filling occurs during diastole.
- The AV valves are open.
- Blood passively flows from atria to ventricles.
- Atrial contraction occurs, contributing to ventricular filling.
-
Isovolumetric contraction occurs at the beginning of systole.
- The AV valves close before the semilunar valves open.
- Ventricular pressure increases, but no blood is ejected yet.
-
Ventricular ejection occurs during systole.
- Semilunar valves open and blood is ejected into the aorta and pulmonary artery.
- The pressure in the ventricles is higher than the pressure in the aorta and pulmonary artery.
Useful Terms
- EDV (End Diastolic Volume): The volume of blood in each ventricle at the end of diastole (relaxation).
- ESV (End Systolic Volume): The volume of blood remaining in each ventricle after contraction.
- SV (Stroke Volume): The volume of blood pumped out by the ventricle with each beat (EDV - ESV).
- CO (Cardiac Output): The amount of blood pumped out by each ventricle in 1 minute (HR x SV).
Regulation of Stroke Volume
- SV = EDV - ESV, meaning the volume pumped out is the total volume filled minus the remaining volume after contraction.
- Three main factors influence SV: preload, contractility, and afterload.
Preload
- Preload: degree of stretch of cardiac muscle cells before contraction.
- Increased preload leads to increased SV.
- Slow heartbeat and exercise increase venous return, leading to increased EDV and SV.
- Frank-Starling law of the heart: Increased venous return stretches ventricles and increases contraction force, resulting in more blood being propelled out.
Factors Aiding Venous Return
- Muscular pump: Contraction of skeletal muscles "milks" blood toward the heart. Valves prevent backflow.
- Respiratory pump: Pressure changes during breathing move blood toward the heart. This occurs by squeezing abdominal veins as thoracic veins expand.
- Venoconstriction: Under sympathetic control, venoconstriction pushes blood toward the heart.
Contractility
- Contractility: Contractile strength independent of muscle stretch and EDV.
- Increased contractility results in increased SV and decreased ESV.
- Increased Ca2+ from sympathetic stimulation (extrinsic factor), such as epinephrine and norepinephrine, enhance contractility.
Chemicals Influencing Contractility
-
Positive Inotropic Agents: Increase contractility.
- Epinephrine
- Thyroxine
- Glucagon
- Digitalis (medication)
- High extracellular Ca++
-
Negative Inotropic Agents: Decrease contractility.
- Acidosis
- Increased extracellular K+
- Calcium channel blockers
Afterload
- Afterload: The pressure that must be overcome for the ventricles to eject blood.
- This includes backpressure from blood in the aorta and pulmonary vessels on the valves.
- Hypertension increases afterload, leading to increased ESV and reduced SV.
Congestive Heart Failure (CHF)
- Cardiac output is so low that blood circulation is inadequate to meet tissue needs.
- The heart attempts to work harder, leading to increased Ca++ levels in cardiac cells. Sustained elevation in Ca++ can lead to increased calcineurin, which alters the structure of cardiac myocytes to adapt to increased demands.
- Causes include:
- Coronary atherosclerosis (clogging of arteries)
- Persistent high blood pressure
- Damaged cardiac cells from heart attacks
- Dilated cardiomyopathy (DCM)
Cardiac Mechanics
- The Cardiac cycle is a complete heartbeat, including all events associated with blood flow through the heart.
- Systole is the contraction phase.
- Diastole is the relaxation phase.
- Mechanical events follow Electrical events in the heart.
Phases of the Cardiac Cycle
-
Ventricular Filling:
- Takes place in mid-to-late diastole.
- AV valves are open, SL valves are closed.
- 80% of blood passively flows into ventricles.
- Atrial systole delivers the remaining blood volume (20%).
- End diastolic volume (EDV) is the volume of blood in each ventricle at the end of diastole.
-
Ventricular Systole:
- Atria are relaxed.
- Ventricles contract, increasing pressure and closing the AV valves (creating the first heart sound).
- Isovolumetric contraction phase: All valves are closed, pressure builds.
- Ventricles contract, and pressure exceeds the pressure in the aorta and pulmonary artery, forcing the SL valves open (ejection phase), allowing blood flow into the Pulmonary Artery and Aorta.
- End systolic volume (ESV) is the volume of blood remaining in each ventricle after contraction.
-
Isovolumetric Relaxation:
- Ventricular diastole begins.
- Ventricles relax, pressure drops.
- SL valves close (creating the second heart sound).
- AV valves remain closed as pressure in the ventricles is still higher than that in the atria.
Cardiac Output
- Cardiac Output(CO) is the volume of blood pumped by each ventricle per minute.
- CO = Heart Rate (HR) x Stroke Volume (SV)
- HR: The number of heart beats per minute.
- SV: The volume of blood pumped out by one ventricle per beat.
Stroke Volume
- Stroke Volume (SV) = EDV - ESV
- The volume of blood pumped out by the ventricle is the total volume it fills with, minus the amount left in the heart after contraction.
- Three main factors affect SV:
- Preload: the degree of stretch of cardiac muscle cells before they contract (length-tension relationship).
- Contractility: the contractile strength of the heart muscle.
- Afterload: the pressure that must be overcome for ventricles to eject blood.
Preload
- Increased Preload leads to increased Stroke Volume.
- Slow heartbeat and exercise increase venous return, which increases EDV, SV, and force of contraction.
- The Frank-Starling Law of the heart: Increased blood return stretches ventricles and increases contraction force, propelling more blood out.
Contractility
- Contractility is independent of muscle stretch and EDV (intrinsic factors).
- Increased Ca2+ from sympathetic stimulation (extrinsic factor) increases Contractility and decreases ESV.
- Positive Ionotropic Agents: Chemicals that increase Contractility, such as Epinephrine, Thyroxine, Glucagon, Digitalis, and high extracellular Ca++.
- Negative Ionotropic Agents: Chemicals that decrease Contractility, such as Acidosis, increased extracellular K+, and calcium channel blockers.
Afterload
- Afterload is the backpressure from blood in the aorta and pulmonary vessels on the valves.
- Hypertension increases Afterload, resulting in increased ESV and reduced SV.
Congestive Heart Failure (CHF)
- Occurs when CO is so low that blood circulation is inadequate to meet tissue needs.
- Caused by: coronary atherosclerosis, persistent high blood pressure, dead cardiac cells from heart attacks, dilated cardiomyopathy.
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Description
This quiz explores the cardiac cycle, detailing the phases involved in one heartbeat. Understand the systole and diastole events and how they correspond to the ECG waves. Test your knowledge on the mechanisms of ventricular filling, isovolumetric contraction, and ventricular ejection.