Questions: Cardiac Output and Stroke Volume Regulation

Question 1

An athlete's cardiac output increases fourfold from 5 L/min at rest to 20 L/min during intense exercise. Their heart rate doubles from 70 to 140 bpm. What must be true about stroke volume?

• A. Stroke volume stayed the same at about 71 mL — doubling heart rate alone explains the fourfold increase in CO
• B. Stroke volume decreased to about 35 mL, because faster heart rates reduce ventricular filling time
• C. Stroke volume approximately doubled to about 143 mL, contributing equally with heart rate to the fourfold increase
• D. Stroke volume quadrupled to about 285 mL, since cardiac output quadrupled

Answer: (C)

CO = HR × SV. At rest: 5,000 mL/min = 70 bpm × ~71 mL. During exercise: 20,000 mL/min = 140 bpm × SV, so SV ≈ 143 mL — roughly double. Both HR and SV approximately doubled. This illustrates the core misconception identified in the file: CO is not 'just heart rate.' A student who ignores SV would predict CO should only double (to ~10 L/min). During exercise, enhanced venous return (Frank-Starling), sympathetic contractility, and reduced afterload all simultaneously increase SV, contributing equally with heart rate to the fourfold rise.

Question 2

A drug increases venous return by promoting venoconstriction. According to the Frank-Starling mechanism, what is the direct effect on stroke volume?

• A. Stroke volume decreases, because higher venous pressure makes it harder for the ventricle to fill
• B. Stroke volume increases, because greater ventricular stretch at end-diastole produces more forceful contraction
• C. Stroke volume is unchanged; the heart compensates by slowing its rate to handle the extra volume
• D. Stroke volume increases only if the sympathetic nervous system is simultaneously activated

Answer: (B)

The Frank-Starling mechanism is intrinsic to cardiac muscle: within physiological ranges, greater ventricular stretch at end-diastole (more preload) increases force of contraction and therefore stroke volume. More blood returning to the heart stretches the ventricular wall to a more optimal sarcomere length, enabling more actin-myosin cross-bridge formation. This is a built-in feedback loop — the heart automatically adjusts output to match venous return without any neural input. Option D is wrong: Frank-Starling operates intrinsically, independent of sympathetic activation (though sympathetic input can shift the Frank-Starling curve upward).

Question 3

The Frank-Starling mechanism operates intrinsically within cardiac muscle, allowing the heart to automatically match its output to venous return without neural or hormonal signals.

• T. True
• F. False

Answer: True

Yes — this intrinsic property distinguishes Frank-Starling from contractility changes driven by sympathetic stimulation. A completely denervated (transplanted) heart still exhibits Frank-Starling behavior: greater end-diastolic stretch produces stronger contraction. The mechanism works at the sarcomere level: more stretch means more optimal overlap between actin and myosin filaments, producing more force per contraction. This automatic matching of output to input is also critical for equalizing left and right ventricular output — if one side pumps more than the other, the other side's preload rises and Frank-Starling brings it back into balance.

Question 4

Because cardiac output equals heart rate multiplied by stroke volume, any increase in heart rate will necessarily increase cardiac output proportionally.

• T. True
• F. False

Answer: False

This ignores the effect of heart rate on stroke volume. At very high heart rates (e.g., >180 bpm in certain arrhythmias), diastolic filling time is so shortened that the ventricle cannot fill adequately, and preload drops. By the Frank-Starling mechanism, reduced preload means reduced stroke volume. At extreme rates, the fall in SV can outweigh the HR increase, and CO actually declines despite the higher rate. In clinical practice, tachydysrhythmias can cause hemodynamic collapse precisely because CO falls when the heart rate is too high for adequate filling — the heart 'spins its wheels' without pumping effectively.

Question 5

A patient with uncontrolled hypertension develops progressive heart failure over several years. Using preload, afterload, and contractility, explain the chain of events linking chronic hypertension to heart failure.

Model answer: Elevated blood pressure means chronically elevated afterload — the left ventricle must overcome higher aortic pressure with every beat. Initially the heart compensates by hypertrophy (thickening of the ventricular wall), which maintains stroke volume despite high afterload. However, hypertrophy becomes maladaptive: the thickened, stiff ventricle fills poorly during diastole (impaired relaxation reduces end-diastolic volume, decreasing preload and therefore SV via Frank-Starling). Meanwhile, myocardial remodeling impairs contractility — fewer functional sarcomeres, abnormal calcium handling, and interstitial fibrosis reduce intrinsic force of contraction. The result is a heart facing high afterload with diminished contractility and reduced preload-based reserve, failing to maintain adequate CO.

This sequence — pressure overload → hypertrophy → diastolic dysfunction → systolic dysfunction — is one of the most important disease mechanisms in cardiology. Treatment targets all three SV determinants: afterload reduction (ACE inhibitors, ARBs), preload optimization (diuretics), and contractility modulation (beta-blockers paradoxically, or digoxin). Each intervention corresponds directly to one leg of the preload-afterload-contractility triad.