Frank-Starling Law: Cardiac Output Explained

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-20

The Frank-Starling law states that the mechanical energy of cardiac contraction is a function of the length of the cardiac muscle fiber, so that within physiologic limits a ventricle that fills with more blood ejects more blood. In practical terms, increasing end-diastolic volume (preload) stretches the myocardium and increases stroke volume, but only up to a plateau beyond which further filling adds no output.

This principle matters because it is the heart's built-in autoregulation. Every time a dog, cat, horse, or cow changes its activity level, posture, or hydration status, venous return changes and the ventricle must adjust its output within a few beats. The Frank-Starling mechanism does that without waiting for nerves or hormones. It also explains why fluid therapy helps some patients and harms others, and why a failing heart can be congested and underperfused at the same time.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

The Core Definition and the Variables That Surround It

The law is a length-tension relationship. Stretch a strip of ventricular muscle and stimulate it, and the force it generates increases. Starling's own phrasing, quoted in later reviews of his work, was that "the energy of contraction, however measured, is a function of the length of the muscle fibre" [1]. Otto Frank had earlier shown two distinct pressure-volume relations in the isolated frog heart, one for isovolumic twitches and one for afterloaded twitches, and Starling referenced that work without testing the distinction himself [1]. The hyphenated name reflects both contributions.

Three variables govern how much blood leaves the ventricle each beat. Students confuse them constantly, so define them cleanly before going further.

Preload is the load on the ventricular wall at the end of diastole, just before contraction begins. It is measured as end-diastolic volume (EDV) or as the pressure that fills the ventricle (end-diastolic pressure). Preload is the stretch variable in the Frank-Starling relationship.

Afterload is the load the ventricle must overcome to eject. It is best expressed as wall stress during systole, and it is approximated clinically by aortic pressure or by effective arterial elastance, the combined resistance and stiffness of the arterial tree.

Contractility is the intrinsic inotropic state of the myocardium, independent of load. It is the property that changes with sympathetic stimulation, acidosis, or myocardial disease.

The Frank-Starling law is a statement about preload only. It says nothing about afterload or contractility, and a change in either of those moves the whole relationship rather than sliding along it.

Stroke Volume, Ejection Fraction, and Cardiac Output

Cardiac output (CO) is the volume of blood pumped per minute. It equals heart rate (HR) multiplied by stroke volume (SV).

Stroke volume is the difference between end-diastolic volume and end-systolic volume (ESV), the blood left in the ventricle after ejection.

Ejection fraction (EF) is stroke volume divided by end-diastolic volume, expressed as a percentage. It is the standard index of how completely the ventricle empties.

Worked Example: A Dog With EDV 60 mL and ESV 30 mL

Take a dog with an end-diastolic volume of 60 mL and an end-systolic volume of 30 mL.

• Stroke volume = EDV minus ESV = 60 mL minus 30 mL = 30 mL
• Ejection fraction = SV divided by EDV = 30 mL divided by 60 mL = 0.50 = 50%
• If heart rate is 120 beats per minute, cardiac output = 120 times 30 mL = 3,600 mL/min, or 3.6 L/min

Now apply the Frank-Starling law. Suppose venous return increases and end-diastolic volume rises from 60 mL to 80 mL while contractility stays the same. End-systolic volume falls modestly, to about 25 mL, because the stretched fibers generate more force and empty a little further. Stroke volume becomes 80 minus 25 = 55 mL, and ejection fraction becomes 55 divided by 80 = 69%. At the same heart rate of 120 beats per minute, cardiac output rises to 6.6 L/min.

That is the mechanism in one calculation. More filling, more stretch, more force, more output.

The Cellular Basis: Why Stretch Increases Force

The cellular version of the law is called length-dependent activation. The mechanism is not simply that stretched muscle has more overlap between actin and myosin. If it were, force would follow the classic sarcomere length-tension curve and peak sharply at optimal overlap. Instead, cardiac muscle shows a much steeper and more sustained increase in calcium sensitivity as sarcomeres lengthen.

Four cellular events are consistently implicated:

1. Increased myofilament sensitivity to calcium. At longer sarcomere lengths, the same intracellular calcium concentration produces more force. This is the central finding across vertebrate classes [2].
2. Decreased myofilament lattice spacing. As the cell stretches, the distance between thick and thin filaments narrows, which promotes crossbridge formation [2].
3. Increased thin filament cooperativity. Calcium binding at one regulatory site makes neighboring sites more likely to bind calcium, amplifying the response [2].
4. Altered myosin head orientation. X-ray diffraction studies on intact rat trabeculae show that systolic force tracks changes in myosin head orientation more closely than changes in lattice spacing, suggesting head position before activation sets the activation level [3].

Stretch also triggers signaling. Myocardial stretch activates NADPH oxidase 2 (NOX2) to produce reactive oxygen species, and in NOX2 knockout mice the stretch-induced acceleration of the calcium transient is lost and contractility is reduced [4]. Separate work frames the myocyte as a mechanosensor, with intracellular sensors modulating reactive oxygen species and cell-surface sensors modulating nitric oxide, both feeding back on calcium handling [5]. The cytoskeleton, including titin and its anchorage at the Z-disc and M-band, participates in sensing and transmitting the stretch signal [6].

Two structural details explain species differences. The sarcomere length at which tension peaks is longer in non-mammalian vertebrates than in mammals, and fish and amphibians vary stroke volume over a wider range than mammals do [2]. Mammalian ventricular myocytes are wider and stiffer than those of other vertebrate classes, which may account for the difference [2].

The Frank-Starling Curve and Its Plateau

Plot stroke volume (or stroke work) on the vertical axis against end-diastolic volume or filling pressure on the horizontal axis. The result is a curve that rises steeply, then flattens.

On the steep ascending limb, small increases in filling produce large increases in output. This is where a normal heart operates most of the time. On the flat portion, additional filling produces little or no additional output. Beyond that, further volume loading raises filling pressure without raising stroke volume, and the ventricle can become congested.

The plateau is not a failure of the law. It is the upper limit of the length-tension relationship. Sarcomeres cannot be stretched indefinitely without losing overlap and eventually losing force.

The position of the whole curve is set by contractility. A more contractile heart sits on a higher curve. A less contractile heart sits on a lower one. Afterload shifts the operating point along the curve rather than moving the curve itself, though sustained afterload elevation can depress contractility over time.

The curve is also the basis for a clinical test. In a pig study, model-based estimates of left ventricular end-diastolic volume and stroke volume were used to construct Frank-Starling curves and to identify fluid non-responders with an area under the ROC curve of 0.83 [7]. The gradient of the curve, termed Frank-Starling contractility, was estimated with acceptable agreement against admittance catheter measurements [7]. The same group notes that continuous non-invasive monitoring of preload changes and contractility could make circulatory failure easier to diagnose and manage [7].

Summary Table: Preload, Afterload, and Contractility

How the Law Is Observed and Tested

In the teaching laboratory, ultrasound imaging lets students measure ventricular dimensions in systole and diastole and derive stroke volume, ejection fraction, and cardiac output from first principles [8]. Repeating measurements after brief exercise demonstrates an increase in stroke volume and ejection fraction, sometimes with and sometimes without a change in end-diastolic volume [8]. Students report improved understanding of the law after this exercise [8].

In research settings, the law is probed by controlled preload and afterload changes. A study in 24 healthy pigs performed preload reductions, afterload increases, and combined maneuvers, then fit end-systolic pressure-volume points with linear and bilinear regressions [9]. Afterload increases produced a biphasic course of end-systolic points that fit a bilinear relationship better (r² 0.96 and 0.943) than a linear one (r² 0.929), while preload reduction alone fit a linear relationship well (r² 0.974) [9]. The authors attribute the nonlinearity to an overlay of shortening deactivation and the Frank-Starling mechanism [9].

In the intensive care setting, the law underlies the fluid challenge. A pediatric multicenter study used transpulmonary thermodilution to measure global end-diastolic volume index, stroke volume index, and cardiac index before and after 66 fluid challenges in 75 patients, including children with cardiovascular dysfunction and dilated cardiomyopathy [10]. Global end-diastolic volume related to predicted body surface area by a power law, and four preload levels were defined from that relationship to test fluid responsiveness [10]. The clinical question is always the same: is this patient on the steep part of the curve or the flat part?

The law also operates in the atrium. Real-time three-dimensional echocardiography in 70 patients showed that active atrial stroke volume correlated with pre-atrial contraction volume but decreased at larger volumes, and active atrial emptying fraction fell significantly in the group with the largest left atrial volumes [11]. The atrium follows its own Frank-Starling relationship.

Comparative Notes Across Species

The mechanism is present in all studied vertebrate lineages, from fish to mammals [12]. The Frank-Starling law applies to all classes of vertebrates and links cardiac ejection to cardiac filling by increasing contractility as muscle stretches, up to an optimum length [2].

The differences are quantitative. Non-mammalian vertebrates vary stroke volume to a larger extent than mammals, and their sarcomere length-tension peaks occur at longer sarcomere lengths [2]. The high extensibility of certain hearts matches the extensibility of their myocytes [2]. Mammalian ventricular myocytes are wider and have higher passive stiffness, which may explain why mammals rely more on heart rate and less on stroke volume for output changes [2].

The law is not the only intrinsic autoregulatory mechanism. The Anrep effect describes the heart's ability to adjust contractile force when afterload changes, and the two mechanisms together maintain cardiac output under changing loads [5]. A modeling study examined whether the Frank-Starling mechanism is required for stability of the cardiovascular operating point and concluded that it is not necessary for stability, identifying instead three complementary roles: reducing the demand on central control, matching output to venous return, and providing short-term adjustment [12].

Clinical Relevance, Limitations and Common Mistakes

Heart failure changes the shape of the curve. The Frank-Starling relationship becomes flatter and shifts downward, so the same increase in filling pressure produces much less increase in stroke volume. Volume loading in this setting raises filling pressure and worsens congestion without raising output. That is the physiologic basis for pulmonary edema in a dog with dilated cardiomyopathy that receives aggressive intravenous fluids.

The clinical literature supports this. In heart failure patients receiving adaptive servo-ventilation, changes in left ventricular end-diastolic pressure alone did not correlate with changes in cardiac index, but changes in transmural left ventricular end-diastolic pressure (calculated by subtracting right ventricular end-diastolic pressure from left ventricular end-diastolic pressure) correlated positively with changes in cardiac index, following the ascending limb of the Frank-Starling law [13]. Higher baseline right ventricular end-diastolic pressure was a stronger predictor of acute response than left ventricular end-diastolic pressure, with an area under the curve of 0.846 versus 0.673 [13]. The takeaway for clinicians is that the effective filling pressure, not the absolute number, drives the response.

Chronic heart failure also triggers molecular adaptations. Restrictive proteolysis of cardiac troponin I removes its N-terminal extension, and transgenic mice expressing only the truncated form show an extended Frank-Starling response with reduced left ventricular end-diastolic pressure, increased systolic pressure development, and increased stroke volume without an increase in end-diastolic volume [14]. This is a compensatory remodeling of the length-sensing apparatus itself.

Two practical cautions follow from the law.

First, a measurement that changes preload can change the cardiac output reading. In anesthetized dogs, injection of a cold water bolus increased mean cardiac output measured by electromagnetic flow meter and transesophageal echo-Doppler by 26% and 27% during steady state, and by 85% and 75% during a low-output state, with the transient increase produced by a rise in stroke volume while heart rate did not change [15]. The authors attribute this to a sudden preload increase from the injectate, consistent with the Frank-Starling law [15]. Thermodilution can therefore overestimate cardiac output in low-output states when a cold bolus is used [15].

Second, the curve position depends on the individual. Thermal stress shifts the operating point. In human subjects, lower-body negative pressure at 30 mmHg reduced stroke volume more during whole-body heating (minus 48.7 mL) than during normothermia (minus 33.2 mL) or cooling (minus 10.3 mL) [16]. Relating pulmonary capillary wedge pressure to stroke volume showed that heat stress moved subjects to a less favorable position on the Frank-Starling curve [16]. Analogous shifts occur in animals with fever, shock, or anesthesia-induced vasodilation.

Common mistakes students make:

• Treating preload and afterload as interchangeable. They are not. Preload is the filling load, afterload is the ejection load.
• Assuming more fluid always means more output. It does not, once the plateau is reached.
• Reading a low ejection fraction as proof of poor contractility. Ejection fraction is load-dependent. A high-afterload ventricle can have a low ejection fraction with normal contractility.
• Forgetting that the curve is a family of curves. Contractility changes move it. Preload changes slide along it.
• Applying the law without measuring. The operating point must be estimated, not assumed.

Individual patients require veterinary assessment before any fluid or drug decision.

Quick Review

1. The Frank-Starling law is a length-tension relationship. More stretch means more force, up to a plateau.
2. Preload is end-diastolic volume or pressure. Afterload is systolic wall stress or arterial pressure. Contractility is intrinsic inotropic state.
3. Stroke volume equals EDV minus ESV. Ejection fraction equals SV divided by EDV.
4. In the worked example, EDV 60 mL and ESV 30 mL give SV 30 mL and EF 50%.
5. The curve is steep at low filling and flat at high filling. Contractility shifts the curve. Preload moves the point along it.
6. In heart failure the curve flattens and shifts down, so volume loading raises filling pressure without raising output.
7. The mechanism is present across vertebrate classes, with species differences in sarcomere length optimum and stroke volume range.

Frequently Asked Questions

What is the Frank-Starling law in one sentence?

The Frank-Starling law states that the energy of cardiac contraction is a function of the length of the muscle fiber, so a ventricle that fills more ejects more, up to a plateau.

What is the difference between preload and afterload?

Preload is the load on the ventricular wall at end-diastole, measured as end-diastolic volume or pressure. Afterload is the load the ventricle must overcome to eject, approximated by aortic pressure or effective arterial elastance.

How do you calculate stroke volume and ejection fraction?

Stroke volume equals end-diastolic volume minus end-systolic volume. Ejection fraction equals stroke volume divided by end-diastolic volume, expressed as a percentage.

Why does the Frank-Starling curve flatten in heart failure?

The failing myocardium has reduced contractility and altered length-dependent activation, so the curve shifts down and becomes flatter. Additional filling then raises pressure more than it raises output.

Does the Frank-Starling law apply to animals other than mammals?

Yes. The mechanism is found across all studied vertebrate lineages, including fish, amphibians, reptiles, birds, and mammals, though the sarcomere length at which force peaks differs between classes.

Can fluid therapy increase cardiac output in a dog with heart failure?

Sometimes, but only if the patient is on the steep part of the curve. In advanced heart failure the curve is flat, and fluid loading typically worsens congestion without improving output.

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3. Myosin head orientation: a structural determinant for the Frank-Starling relationship.
4. Stretch-induced reactive oxygen species contribute to the Frank-Starling mechanism.
5. Mechano-chemo-transduction in cardiac myocytes.
6. The physiological role of cardiac cytoskeleton and its alterations in heart failure.
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10. Cardiac preload responsiveness in children with cardiovascular dysfunction or dilated cardiomyopathy: a multicenter observational study.
11. Left atrial Frank-Starling law assessed by real-time, three-dimensional echocardiographic left atrial volume changes.
12. Frank-Starling mechanism and short-term adjustment of cardiac flow.
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14. N-terminal truncated cardiac troponin I enhances Frank-Starling response by increasing myofilament sensitivity to resting tension.
15. Overestimation of low cardiac output measured by thermodilution.
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