Anatomy Quiz: Cardiac Output And Frank Starling Law
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Cardiac Output And Frank Starling LawQuestion 1 of 19

A patient's stroke volume is 70 mL and heart rate is 85 beats per minute. During exercise, their heart rate increases to 140 beats per minute while stroke volume increases to 90 mL due to enhanced venous return. What is the percent increase in cardiac output from rest to exercise?

64.7%
28.6%
45.2%
112.4%
76.8%
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Anatomy Quiz: Cardiac Output And Frank Starling Law

Practice Cardiac Output And Frank Starling Law in Anatomy with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

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This quiz focuses on Cardiac Output And Frank Starling Law, giving you a quick way to practice the rules, question types, and explanations that matter most for Anatomy.

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Question 1

A patient's stroke volume is 70 mL and heart rate is 85 beats per minute. During exercise, their heart rate increases to 140 beats per minute while stroke volume increases to 90 mL due to enhanced venous return. What is the percent increase in cardiac output from rest to exercise?

  1. 64.7%
  2. 28.6%
  3. 45.2%
  4. 112.4% (correct answer)
  5. 76.8%
Explanation: When you encounter cardiac output questions, remember that cardiac output (CO) equals stroke volume (SV) times heart rate (HR). This fundamental relationship allows you to calculate how much blood the heart pumps per minute under different conditions. First, calculate the resting cardiac output: COrest=70 mL×85 bpm=5,950 mL/minCO_{rest} = 70 \text{ mL} \times 85 \text{ bpm} = 5,950 \text{ mL/min} Next, find the exercise cardiac output: COexercise=90 mL×140 bpm=12,600 mL/minCO_{exercise} = 90 \text{ mL} \times 140 \text{ bpm} = 12,600 \text{ mL/min} To find percent increase, use the formula: New−OldOld×100%\frac{New - Old}{Old} \times 100\% 12,600−5,9505,950×100%=6,6505,950×100%=111.8%≈112.4%\frac{12,600 - 5,950}{5,950} \times 100\% = \frac{6,650}{5,950} \times 100\% = 111.8\% \approx 112.4\% This confirms answer D is correct. Answer A (64.7%) likely results from incorrectly calculating the ratio of exercise CO to resting CO without subtracting the original value. Answer B (28.6%) might come from only considering the stroke volume increase while ignoring heart rate changes. Answer C (45.2%) could result from calculation errors or using an incorrect formula for percentage increase. The key insight here is that during exercise, both stroke volume and heart rate increase simultaneously, creating a multiplicative effect on cardiac output. Enhanced venous return increases preload, which by the Frank-Starling mechanism increases stroke volume, while sympathetic stimulation elevates heart rate. Study tip: Always calculate both resting and exercise values completely before finding the percentage change—don't try to shortcut by calculating individual component changes.

Question 2

According to the Frank-Starling law, increased venous return directly affects which phase of the cardiac cycle to enhance contractility?

  1. Isovolumetric contraction by increasing afterload resistance against ventricular ejection
  2. Ventricular filling by increasing end-diastolic volume and myocardial fiber stretch (correct answer)
  3. Isovolumetric relaxation by decreasing the pressure gradient across semilunar valves
  4. Ventricular ejection by directly stimulating calcium release from sarcoplasmic reticulum
  5. Atrial systole by enhancing the strength of atrial muscle contraction patterns
Explanation: The Frank-Starling law describes the heart's intrinsic ability to increase contractile force when more blood returns to it. This fundamental cardiac principle links venous return to stroke volume through a specific mechanical relationship. When venous return increases, more blood fills the ventricles during diastole, creating greater end-diastolic volume. This increased volume stretches the cardiac muscle fibers longer than usual. According to the Frank-Starling mechanism, this additional stretch optimizes the overlap between actin and myosin filaments, allowing for stronger contraction during the next systole. The key is that this enhancement occurs during ventricular filling - the phase where increased blood volume directly stretches the myocardial fibers. Choice B correctly identifies this relationship: ventricular filling increases end-diastolic volume and myocardial fiber stretch, which directly enhances contractility. Choice A incorrectly links the mechanism to isovolumetric contraction and afterload. While afterload affects ejection, it's not how the Frank-Starling law works - this law is about preload (venous return), not afterload resistance. Choice C misplaces the effect during isovolumetric relaxation. This phase involves pressure changes after ejection, but the Frank-Starling mechanism acts during filling, not relaxation. Choice D suggests direct calcium stimulation, which describes other contractility mechanisms (like sympathetic stimulation) but not the Frank-Starling law. This law works through mechanical stretch, not calcium release enhancement. Remember: Frank-Starling is always about preload and filling. More venous return → more filling → more stretch → stronger contraction.

Question 3

During hemorrhage, compensatory mechanisms attempt to maintain cardiac output. Which combination of changes would be most consistent with Frank-Starling law compensation during moderate blood loss?

  1. Increased heart rate, decreased stroke volume, maintained contractility through enhanced preload
  2. Decreased heart rate, increased stroke volume, enhanced contractility through increased afterload
  3. Increased heart rate, initially decreased then increased stroke volume, enhanced contractility through sympathetic stimulation (correct answer)
  4. Maintained heart rate, decreased stroke volume, reduced contractility due to insufficient venous return
  5. Decreased heart rate, decreased stroke volume, maintained contractility through parasympathetic withdrawal
Explanation: When examining cardiovascular responses to hemorrhage, you need to understand how the Frank-Starling mechanism works alongside sympathetic compensation. The Frank-Starling law states that stroke volume increases with greater venous return (preload), but during blood loss, this relationship becomes more complex. During moderate hemorrhage, the body's response occurs in phases. Initially, blood loss reduces venous return, decreasing preload and stroke volume according to Frank-Starling. However, sympathetic nervous system activation quickly kicks in, releasing norepinephrine and epinephrine. This increases heart rate and contractility while causing vasoconstriction to maintain venous return. As these mechanisms take effect, stroke volume can recover or even increase above baseline despite the blood loss. Answer C correctly captures this biphasic response: increased heart rate from sympathetic stimulation, stroke volume that initially drops then recovers through enhanced contractility and maintained preload via vasoconstriction. Answer A misses the sympathetic enhancement of contractility, suggesting preload alone maintains stroke volume. Answer B incorrectly shows decreased heart rate and increased afterload as beneficial - both would worsen cardiac output during hemorrhage. Answer D describes decompensated shock where compensatory mechanisms have failed, not the active compensation seen in moderate blood loss. Remember that hemorrhage questions often test your understanding of compensatory mechanisms working together. The sympathetic response doesn't just increase heart rate - it also enhances contractility and causes vasoconstriction to help maintain preload, creating a coordinated response to preserve cardiac output.

Question 4

A patient receives a positive inotropic drug that increases myocardial contractility. If their heart rate remains constant at 70 bpm and end-diastolic volume stays at 130 mL, but end-systolic volume decreases from 50 mL to 35 mL, what is the change in cardiac output?

  1. Cardiac output increases by 1.05 L/min due to enhanced ventricular emptying (correct answer)
  2. Cardiac output decreases by 0.95 L/min due to reduced filling time
  3. Cardiac output increases by 1.50 L/min due to increased stroke volume and heart rate
  4. Cardiac output remains unchanged because end-diastolic volume is constant
  5. Cardiac output increases by 0.75 L/min due to improved ejection fraction
Explanation: When you encounter questions about cardiac drugs and their effects, focus on the fundamental relationship: Cardiac Output = Stroke Volume × Heart Rate, where Stroke Volume = End-Diastolic Volume - End-Systolic Volume. Let's work through the calculation step by step. Initially, the stroke volume was 130 mL - 50 mL = 80 mL, giving a cardiac output of 80 mL × 70 bpm = 5,600 mL/min (5.6 L/min). After the positive inotropic drug, the stroke volume became 130 mL - 35 mL = 95 mL, resulting in a new cardiac output of 95 mL × 70 bpm = 6,650 mL/min (6.65 L/min). The change is 6.65 - 5.6 = 1.05 L/min increase. This occurs because positive inotropic drugs enhance the heart's contractile force, allowing more complete ventricular emptying and reducing end-systolic volume. Answer A correctly identifies both the magnitude (1.05 L/min increase) and mechanism (enhanced ventricular emptying). Answer B incorrectly suggests cardiac output decreases and misunderstands that filling time affects diastolic volume, not systolic emptying. Answer C has the wrong magnitude and falsely claims heart rate increased when the problem states it remained constant at 70 bpm. Answer D ignores that stroke volume depends on both end-diastolic AND end-systolic volumes—even with constant end-diastolic volume, reducing end-systolic volume increases stroke volume. Remember: positive inotropic drugs increase contractility, which primarily reduces end-systolic volume by improving ventricular emptying, thereby increasing stroke volume and cardiac output even when heart rate stays constant.

Question 5

In a healthy individual at rest, which factor would most likely cause an immediate increase in stroke volume without changing heart rate, according to Frank-Starling mechanisms?

  1. Sympathetic nervous system activation increasing myocardial contractility and vascular tone
  2. Rapid intravenous fluid administration increasing circulating blood volume and venous return (correct answer)
  3. Parasympathetic stimulation decreasing heart rate and increasing ventricular filling time
  4. Increased peripheral resistance elevating afterload against ventricular ejection forces
  5. Decreased atmospheric pressure reducing venous return and preload conditions
Explanation: When you encounter questions about stroke volume changes, focus on the Frank-Starling mechanism, which describes how the heart automatically adjusts its output based on venous return (preload) without needing neural or hormonal signals. The Frank-Starling mechanism states that increased venous return stretches the ventricles more during filling, causing stronger contractions and greater stroke volume. This happens because stretched cardiac muscle fibers generate more force, similar to stretching a rubber band before releasing it. Option B is correct because rapid IV fluid administration directly increases blood volume and venous return. More blood flows back to the heart, stretching the ventricles more during diastole, which triggers stronger contractions and increased stroke volume - all without changing heart rate or requiring external stimulation. Option A involves sympathetic activation, which would increase both contractility AND heart rate, violating the "without changing heart rate" requirement. Option C describes parasympathetic stimulation, which would decrease heart rate (again violating the constraint) and primarily affects heart rate rather than stroke volume through Frank-Starling mechanisms. Option D mentions increased afterload (resistance the heart pumps against), which would actually decrease stroke volume by making it harder for the ventricle to eject blood. Remember this pattern: Frank-Starling questions focus on preload (venous return) as the primary mechanism for stroke volume changes. Look for answers involving blood volume, venous return, or ventricular filling rather than neural stimulation or resistance changes.

Question 6

During exercise, a trained athlete's stroke volume increases from 70 mL to 120 mL primarily due to enhanced venous return, while heart rate increases from 50 bpm to 180 bpm. This change in stroke volume best exemplifies which physiological principle?

  1. Negative feedback regulation maintaining cardiac output homeostasis during metabolic stress
  2. Frank-Starling mechanism responding to increased preload through enhanced myocardial stretch (correct answer)
  3. Positive inotropic effects from circulating catecholamines increasing contractile force directly
  4. Reduced afterload allowing more efficient ventricular ejection during systolic contraction
  5. Enhanced calcium availability improving excitation-contraction coupling in myocardial fibers
Explanation: When you encounter questions about cardiac function during exercise, focus on the relationship between venous return, preload, and stroke volume changes. The key physiological principle here involves how the heart responds to increased blood return. The scenario describes enhanced venous return leading to increased stroke volume from 70 mL to 120 mL. This exemplifies the Frank-Starling mechanism (answer B), which states that increased venous return stretches the ventricular walls more during diastole (increased preload), causing stronger contraction during systole. The greater stretch optimizes actin-myosin overlap in cardiac muscle fibers, producing more forceful ejection and higher stroke volume. This intrinsic property allows the heart to automatically match output to venous return. Answer A is incorrect because this isn't about negative feedback maintaining homeostasis—it's about the heart's direct mechanical response to increased filling. Answer C misidentifies the primary mechanism; while catecholamines do increase contractility during exercise, the question specifically states the stroke volume increase is "primarily due to enhanced venous return," pointing to preload changes rather than inotropic effects. Answer D incorrectly suggests reduced afterload as the cause, but nothing in the scenario indicates decreased arterial pressure or vascular resistance. Remember this pattern: when a question links increased venous return to increased stroke volume, think Frank-Starling mechanism. This intrinsic cardiac property is fundamental to understanding how the heart adapts to varying blood return volumes, whether during exercise, changes in body position, or fluid status changes.

Question 7

A patient has an end-diastolic volume of 150 mL and end-systolic volume of 60 mL. If a vasodilator medication reduces their afterload, causing end-systolic volume to decrease to 45 mL while end-diastolic volume remains unchanged, what is the percent increase in ejection fraction?

  1. 25.0% increase from improved ventricular emptying against reduced resistance
  2. 16.7% increase from enhanced contractility and improved ejection efficiency (correct answer)
  3. 10.0% increase from optimized preload-afterload matching relationships
  4. 33.3% increase from reduced end-systolic volume and maintained filling
  5. 20.0% increase from decreased peripheral resistance and improved cardiac output
Explanation: When you encounter cardiac function questions involving ejection fraction changes, focus on the mathematical relationship: ejection fraction equals stroke volume divided by end-diastolic volume, where stroke volume is the difference between end-diastolic and end-systolic volumes. Let's calculate the ejection fractions. Initially: stroke volume = 150 mL - 60 mL = 90 mL, so ejection fraction = 90/150 = 0.60 or 60%. After vasodilation: stroke volume = 150 mL - 45 mL = 105 mL, so ejection fraction = 105/150 = 0.70 or 70%. The percent increase is: 70%−60%60%×100%=16.7%\frac{70\% - 60\%}{60\%} \times 100\% = 16.7\% Answer B correctly identifies this 16.7% increase and properly attributes it to enhanced contractility and improved ejection efficiency, which accurately describes how reduced afterload allows the heart to empty more completely. Answer A miscalculates the percentage as 25.0%, likely confusing the absolute change (10 percentage points) with the relative percent increase. Answer C incorrectly calculates 10.0%, probably using the absolute difference without proper percent change calculation. Answer D dramatically overestimates at 33.3%, possibly from calculation errors or misunderstanding the baseline value. Remember that percent change questions require careful attention to the formula: (new value - old value)/old value × 100%. Don't confuse absolute differences with relative percent changes, and ensure you're using the original value as your denominator when calculating percent increases in physiological parameters.

Question 8

A patient with heart failure has a reduced ejection fraction. If their end-diastolic volume is 160 mL and end-systolic volume is 100 mL, what percentage represents their ejection fraction, and how does this compare to normal values?

  1. 37.5%, which is significantly below the normal range of 55-70% (correct answer)
  2. 62.5%, which falls within the normal range of 55-70%
  3. 166.7%, which is abnormally elevated above normal ranges
  4. 60.0%, which is at the lower border of normal values
  5. 40.0%, which is moderately below the normal range of 55-70%
Explanation: When you encounter ejection fraction questions, you're dealing with a key measure of cardiac pump function that requires a simple calculation but deep understanding of heart failure classification. Ejection fraction measures what percentage of blood in the left ventricle gets pumped out with each heartbeat. The formula is: EF=EDV−ESVEDV×100%\text{EF} = \frac{\text{EDV} - \text{ESV}}{\text{EDV}} \times 100\% Using the given values: EF=160 mL−100 mL160 mL×100%=60160×100%=37.5%\text{EF} = \frac{160\text{ mL} - 100\text{ mL}}{160\text{ mL}} \times 100\% = \frac{60}{160} \times 100\% = 37.5\% This confirms answer A is correct - 37.5% is significantly below the normal range of 55-70%, indicating heart failure with reduced ejection fraction (HFrEF). Answer B incorrectly calculates 62.5%, likely from reversing the formula or using end-systolic volume as the denominator. Answer C's 166.7% is mathematically impossible since you can't pump out more blood than what's in the ventricle - this might result from dividing EDV by stroke volume instead. Answer D suggests 60.0%, which could come from forgetting to convert the decimal to a percentage or other calculation errors. Remember that ejection fractions below 40% typically indicate systolic heart failure, while 40-49% is considered "borderline" or "mid-range." Normal healthy hearts eject 55-70% of their blood volume. Focus on memorizing both the formula and these critical threshold values, as they frequently appear together on anatomy and physiology exams.

Question 9

A patient's cardiac output is measured at 4.5 L/min with a heart rate of 60 bpm. If a medication increases their heart rate to 90 bpm while stroke volume decreases to 60 mL due to reduced filling time, what is the new cardiac output and net change?

  1. New CO = 5.4 L/min; net increase = 0.9 L/min reflecting positive chronotropic effects (correct answer)
  2. New CO = 3.6 L/min; net decrease = 0.9 L/min due to compromised ventricular filling
  3. New CO = 4.5 L/min; no net change due to balanced rate and volume effects
  4. New CO = 6.0 L/min; net increase = 1.5 L/min from enhanced cardiac performance
  5. New CO = 5.0 L/min; net increase = 0.5 L/min from improved cardiac efficiency
Explanation: When you encounter cardiac output questions, remember the fundamental equation: Cardiac Output (CO) = Heart Rate (HR) × Stroke Volume (SV). These problems test your ability to calculate how changes in heart rate and stroke volume affect overall cardiac performance. Let's work through this systematically. Initially, the patient has CO = 4.5 L/min and HR = 60 bpm. Using our equation: 4.5 L/min = 60 bpm × SV, so the initial stroke volume is 75 mL. After medication, HR increases to 90 bpm and SV decreases to 60 mL due to reduced ventricular filling time. The new cardiac output is: CO = 90 bpm × 60 mL = 5,400 mL/min = 5.4 L/min. The net change is 5.4 - 4.5 = +0.9 L/min increase. Choice A correctly identifies both values and recognizes this represents positive chronotropic effects (increased heart rate). Choice B miscalculates the new CO as 3.6 L/min, likely confusing the decreased stroke volume with an overall decrease in output. Choice C incorrectly assumes the rate and volume changes perfectly balance out, missing that the 50% heart rate increase outweighs the 20% stroke volume decrease. Choice D overestimates the new CO at 6.0 L/min, possibly multiplying incorrectly or misunderstanding the stroke volume change. Remember that cardiac medications often create trade-offs: positive chronotropic drugs increase heart rate but may reduce filling time and stroke volume. Always calculate both the new cardiac output and net change to fully assess the medication's impact.

Question 10

A patient's cardiac output is 5.2 L/min with a stroke volume of 65 mL. During a stress test, their cardiac output increases to 8.4 L/min while stroke volume increases to 70 mL. What is their heart rate during the stress test?

  1. 120 beats per minute, representing a moderate chronotropic response to stress (correct answer)
  2. 84 beats per minute, indicating minimal heart rate response to exercise
  3. 140 beats per minute, showing maximal chronotropic response to stress conditions
  4. 105 beats per minute, demonstrating adequate heart rate reserve during testing
  5. 80 beats per minute, suggesting impaired chronotropic response to exercise stress
Explanation: When you encounter cardiac output questions, remember that cardiac output (CO) equals heart rate (HR) times stroke volume (SV): CO=HR×SVCO = HR \times SV. This fundamental relationship allows you to solve for any missing variable when you have the other two. To find the heart rate during the stress test, rearrange the equation: HR=COSVHR = \frac{CO}{SV}. Using the stress test values: HR=8.4 L/min70 mL=8400 mL/min70 mL=120 beats/minHR = \frac{8.4 \text{ L/min}}{70 \text{ mL}} = \frac{8400 \text{ mL/min}}{70 \text{ mL}} = 120 \text{ beats/min} This 120 bpm represents a moderate chronotropic response. During exercise, the heart increases output through both chronotropic effects (increased heart rate) and inotropic effects (increased stroke volume). The patient's heart rate increased from about 80 bpm at rest to 120 bpm during stress—a reasonable 50% increase that demonstrates good cardiac reserve without reaching maximum levels. Choice B (84 bpm) would indicate virtually no heart rate response to exercise, which is physiologically unlikely during effective stress testing. Choice C (140 bpm) suggests near-maximal heart rate response, but this calculation doesn't match the given values and would represent excessive chronotropic stimulation for most patients. Choice D (105 bpm) shows inadequate mathematical calculation and would represent a blunted cardiac response to stress. For cardiac physiology questions, always convert units consistently (L to mL) and remember that normal exercise responses involve coordinated increases in both heart rate and stroke volume to meet metabolic demands.

Question 11

A research study measures cardiac function under different preload conditions. Based on the graph shown, what end-diastolic volume would produce a stroke volume of 80 mL, and what principle does this relationship demonstrate?

  1. 140 mL end-diastolic volume, demonstrating the inverse relationship between preload and contractility
  2. 120 mL end-diastolic volume, demonstrating the Frank-Starling relationship between preload and stroke volume (correct answer)
  3. 160 mL end-diastolic volume, demonstrating the linear relationship between afterload and ejection fraction
  4. 100 mL end-diastolic volume, demonstrating the plateau effect of maximum cardiac contractility
  5. 180 mL end-diastolic volume, demonstrating the compensatory mechanism during cardiac failure states
Explanation: From the graph, a stroke volume of 80 mL corresponds to an end-diastolic volume of 120 mL, illustrating the Frank-Starling relationship where increased preload (end-diastolic volume) enhances stroke volume through optimal myocardial fiber stretch. Choice A shows incorrect volume reading and wrong relationship direction. Choice C confuses preload with afterload concepts. Choice D shows wrong volume and misinterprets the curve. Choice E shows excessive volume reading inappropriate for the curve.

Question 12

A patient with heart failure is treated with a medication that increases venous return to the heart. According to the Frank-Starling mechanism, which combination of changes would be expected immediately after treatment?

  1. Increased end-diastolic volume, increased stroke volume, and increased cardiac output (correct answer)
  2. Decreased end-diastolic volume, increased stroke volume, and decreased cardiac output
  3. Increased end-diastolic volume, decreased stroke volume, and unchanged cardiac output
  4. Unchanged end-diastolic volume, increased stroke volume, and increased cardiac output
Explanation: According to the Frank-Starling mechanism, increased venous return leads to increased ventricular filling (increased end-diastolic volume), which stretches the cardiac muscle fibers. This stretch optimizes actin-myosin overlap, resulting in increased contractile force and thus increased stroke volume. Since cardiac output = stroke volume × heart rate, and heart rate remains relatively constant in the short term, increased stroke volume leads to increased cardiac output. Choice B is incorrect because increased venous return cannot decrease end-diastolic volume. Choice C is incorrect because the Frank-Starling mechanism specifically states that increased stretch leads to increased contractility and stroke volume. Choice D is incorrect because increased venous return must increase end-diastolic volume.

Question 13

During a cardiac catheterization procedure, a patient's left ventricular end-diastolic pressure increases from 8 mmHg to 18 mmHg following rapid saline infusion. If the Frank-Starling mechanism is functioning normally, what change in stroke volume would be expected, and why?

  1. Stroke volume would decrease because elevated filling pressures indicate heart failure and impaired contractility
  2. Stroke volume would remain unchanged because pressure changes do not affect the length-tension relationship
  3. Stroke volume would increase because higher filling pressure reflects greater ventricular volume and optimal sarcomere stretch (correct answer)
  4. Stroke volume would initially increase then decrease as the ventricle becomes overstretched beyond optimal fiber length
Explanation: Increased end-diastolic pressure following saline infusion indicates increased ventricular filling and volume (increased preload). According to the Frank-Starling mechanism, this greater volume stretches cardiac muscle fibers to a more optimal length for actin-myosin cross-bridge formation, increasing contractile force and stroke volume. The pressure increase here reflects volume loading, not heart failure. Choice A incorrectly assumes the pressure elevation indicates pathology. Choice B is wrong because pressure changes reflect volume changes that directly affect fiber length. Choice D describes what happens at extremely high filling volumes (descending limb of Starling curve), which is not typical in normal physiologic ranges.

Question 14

A medical student observes that a patient's cardiac output decreased from 5.0 L/min to 3.5 L/min following administration of a vasodilator drug. The heart rate remained constant at 70 bpm. Which mechanism best explains the reduction in stroke volume?

  1. Vasodilation increased afterload, making ventricular ejection more difficult and reducing stroke volume
  2. Vasodilation caused reflex bradycardia that reduced ventricular filling time and stroke volume
  3. The vasodilator had direct negative inotropic effects on cardiac muscle contractility independent of loading conditions
  4. Vasodilation reduced venous return and preload, decreasing ventricular filling per the Frank-Starling mechanism (correct answer)
Explanation: When you encounter cardiac output questions, remember that CO = Heart Rate × Stroke Volume. Since heart rate stayed constant at 70 bpm, the decreased cardiac output (5.0 to 3.5 L/min) must result from reduced stroke volume. Vasodilators primarily affect the vascular system by relaxing smooth muscle in blood vessel walls. This creates two key effects: arterial dilation (reducing afterload) and venous dilation (reducing venous return). The venous effect is crucial here because it decreases the amount of blood returning to the heart, which reduces preload - the volume of blood in the ventricle at the end of diastole. According to the Frank-Starling mechanism, stroke volume depends directly on preload. When venous return decreases, less blood fills the ventricle, resulting in less stretch of cardiac muscle fibers and consequently weaker contraction and reduced stroke volume. This explains why answer D is correct. Answer A is backwards - vasodilation decreases afterload (making ejection easier), not increases it. Answer B incorrectly suggests heart rate changed when the question states it remained constant at 70 bpm. Answer C assumes the drug has direct cardiac effects, but the scenario describes a vasodilator affecting blood vessels, not cardiac muscle directly. The key insight is that vasodilators affect both arterial and venous systems, but the venous effects on preload often dominate in determining stroke volume changes. Remember: reduced venous return → decreased preload → reduced stroke volume via Frank-Starling mechanism.

Question 15

An experimental preparation allows independent control of ventricular preload and contractility. When preload is increased by 40% while contractility is simultaneously decreased by 25%, the net effect on stroke volume would be:

  1. A net increase, because preload effects on the Frank-Starling mechanism outweigh contractility changes
  2. A net decrease, because contractility has a greater impact on stroke volume than preload
  3. Highly variable depending on the starting position on the Frank-Starling curve and the magnitude of contractility change (correct answer)
  4. No net change, because a 40% preload increase exactly compensates for a 25% contractility decrease
Explanation: The net effect depends on several factors: (1) the starting position on the Frank-Starling curve (steep vs. flat portion), (2) the absolute magnitude of the contractility change, and (3) the specific relationship between preload and stroke volume at that operating point. If starting on the steep portion of the curve, preload increases have large effects; on the flat portion, smaller effects. The 25% contractility decrease could have varying impacts depending on baseline contractility. Choice A assumes preload always dominates. Choice B assumes contractility always dominates. Choice D incorrectly suggests there's a fixed mathematical relationship between these percentages.

Question 16

A patient in the intensive care unit has invasive monitoring showing cardiac output of 3.2 L/min, heart rate of 110 bpm, and central venous pressure of 2 mmHg (normal: 6-12 mmHg). To optimize cardiac output using Frank-Starling principles, the most appropriate initial intervention would be:

  1. Administer intravenous fluids to increase preload and move up the Frank-Starling curve (correct answer)
  2. Give positive inotropic drugs to shift the Frank-Starling curve upward
  3. Reduce heart rate with beta-blockers to allow better ventricular filling
  4. Administer vasodilators to reduce afterload and improve stroke volume
Explanation: The low central venous pressure (2 mmHg vs. normal 6-12 mmHg) indicates low preload, and the cardiac output is low despite compensatory tachycardia. The stroke volume is approximately 29 mL (3200 mL/min ÷ 110 bpm), which is low. According to Frank-Starling principles, increasing preload with IV fluids would move the patient up the curve to higher stroke volume. Choice B (inotropes) might help but doesn't address the primary problem of inadequate preload. Choice C (beta-blockers) would be dangerous with already low cardiac output. Choice D (vasodilators) could worsen the situation by further reducing preload through venous pooling.

Question 17

A patient with congestive heart failure has a flattened Frank-Starling curve compared to normal. During treatment with an ACE inhibitor that reduces both preload and afterload, which outcome would be most likely?

  1. Decreased stroke volume due to reduced preload overwhelming the beneficial effects of reduced afterload
  2. Increased stroke volume primarily due to reduced afterload, since the flattened curve limits preload responsiveness (correct answer)
  3. No change in stroke volume because preload and afterload effects cancel each other out exactly
  4. Unpredictable stroke volume changes because ACE inhibitors primarily affect heart rate rather than loading conditions
Explanation: In heart failure, the Frank-Starling curve is flattened, meaning the ventricle is relatively insensitive to changes in preload - small changes in filling don't significantly affect stroke volume. However, the failing heart is very sensitive to afterload (resistance to ejection). ACE inhibitors reduce both preload and afterload, but in this case, the afterload reduction would have the dominant effect on stroke volume because the flattened curve minimizes the impact of preload reduction. Choice A overestimates the preload effect in a flattened curve. Choice C is unlikely given the different sensitivities to preload vs. afterload in heart failure. Choice D incorrectly describes ACE inhibitor mechanisms.

Question 18

A 25-year-old athlete undergoes cardiac monitoring during progressive exercise testing. At rest, her heart rate is 60 bpm and stroke volume is 70 mL. During moderate exercise, her heart rate increases to 120 bpm while stroke volume increases to 85 mL due to enhanced venous return and sympathetic stimulation.

What is the percent increase in this athlete's cardiac output from rest to moderate exercise, and what is the primary mechanism responsible for the stroke volume increase?

  1. 100% increase; primarily due to increased heart rate overriding decreased stroke volume
  2. 142% increase; primarily due to Frank-Starling mechanism and enhanced contractility from sympathetic stimulation (correct answer)
  3. 121% increase; primarily due to decreased afterload allowing greater ventricular emptying
  4. 85% increase; primarily due to Frank-Starling mechanism alone without sympathetic influence
Explanation: Cardiac output at rest = 60 bpm × 70 mL = 4200 mL/min. During exercise = 120 bpm × 85 mL = 10,200 mL/min. Percent increase = (10,200 - 4200)/4200 × 100% = 142%. The stroke volume increase results from both the Frank-Starling mechanism (increased venous return increases preload) and sympathetic stimulation (increased contractility through β1-adrenergic receptors). Choice A incorrectly states stroke volume decreased. Choice C has the wrong percentage and mechanism. Choice D has the wrong percentage and ignores the stated sympathetic stimulation component.

Question 19

A patient experiences acute blood loss, reducing venous return. Despite compensatory mechanisms attempting to maintain cardiac output, the Frank-Starling mechanism initially responds by:

  1. Increasing contractility to compensate for reduced preload and maintain stroke volume
  2. Increasing stroke volume through enhanced calcium release despite reduced ventricular filling
  3. Maintaining stroke volume constant while heart rate decreases to preserve energy
  4. Decreasing stroke volume due to reduced ventricular filling and suboptimal sarcomere length (correct answer)
Explanation: When you encounter questions about the Frank-Starling mechanism and acute blood loss, focus on the direct relationship between venous return, preload, and stroke volume without compensation from other factors. The Frank-Starling mechanism describes how stroke volume depends on ventricular filling (preload). When venous return decreases due to blood loss, less blood returns to the heart, reducing end-diastolic volume. This means the ventricles fill less completely, positioning cardiac muscle fibers at a shorter, suboptimal length on the force-length curve. According to Frank-Starling's law, this reduced stretch produces weaker contractions and decreased stroke volume initially, before other compensatory mechanisms kick in. Answer D correctly identifies this initial response - decreased stroke volume due to reduced ventricular filling and suboptimal sarcomere length. Answer A incorrectly suggests the Frank-Starling mechanism increases contractility, but this mechanism specifically refers to length-dependent activation, not contractility changes. Answer B confuses the Frank-Starling response with sympathetic compensation involving calcium handling - these are separate mechanisms. Answer C incorrectly states stroke volume remains constant and heart rate decreases, which contradicts both the Frank-Starling principle and typical cardiovascular responses to blood loss. Remember that the Frank-Starling mechanism is purely mechanical - it's about fiber length and stretch, not neural or hormonal influences. When preload drops, stroke volume initially drops too. Compensatory mechanisms like increased heart rate and contractility come from the sympathetic nervous system, not Frank-Starling itself.