What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Stroke Volume and Frank-Starling Law Explained

Name: Stroke Volume and Frank-Starling Law Explained
Creator: Dr. Zubair Khalid

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

Stroke volume is the volume of blood ejected by one ventricle in a single beat, calculated as end-diastolic volume minus end-systolic volume. The Frank-Starling law states that, within a physiologic range, increasing the volume of blood filling a cardiac chamber stretches its muscle fibers and increases the force of contraction, so that the heart ejects more blood when more blood returns to it [1][2].

That single relationship is the reason a healthy heart automatically matches its output to the volume returning from the body, without waiting for a nerve impulse or hormone signal. It explains why a dog with dilated cardiomyopathy can look stable at rest and then decompensate during a fluid bolus, why a racing greyhound moves enormous volumes of blood per minute, and why overfilling a failing heart floods the lungs. Students who understand stroke volume and the Starling mechanism can reason through most hemodynamic problems they will meet in practice.

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

The Core Equation

Stroke volume (SV) is defined as:

SV = EDV - ESV

where EDV is end-diastolic volume, the blood in the ventricle at the end of filling, and ESV is end-systolic volume, the blood left behind after contraction.

Two derived values follow directly:

Ejection fraction (EF) = SV / EDV. EF expresses what fraction of the filled ventricle was ejected. It is the most common clinical index of systolic pump performance.
Cardiac output (CO) = SV × heart rate. Cardiac output is total flow per minute, and it is the variable the body actually regulates.

Typical resting values give students a sense of scale. In a healthy adult human, EDV is roughly 120 mL, ESV roughly 50 mL, so SV is about 70 mL and EF about 58 percent. In a medium-sized healthy dog, EDV is on the order of 40 to 50 mL, ESV roughly 20 to 25 mL, and SV approximately 20 to 30 mL, with EF in a similar 55 to 65 percent range. These are textbook working figures for orientation, not diagnostic cutoffs. Absolute volumes scale with body size, so a Great Dane and a Chihuahua have very different stroke volumes but similar ejection fractions.

Summary Table: Key Terms and the Concepts They Are Confused With

Term	Definition	Often confused with
Stroke volume	Volume ejected per beat (EDV - ESV)	Cardiac output, which is per minute
Cardiac output	SV × heart rate	Stroke volume, which is per beat
Preload	Ventricular wall stress at end-diastole, indexed by EDV or EDP	Afterload, which is the load during ejection
Afterload	Wall stress during systolic ejection, indexed by arterial pressure and vascular resistance	Preload
Contractility	Intrinsic inotropic state independent of load	Stroke volume, which depends on load and rate
Frank-Starling law	Filling volume changes contraction force	The Bainbridge reflex, which is a rate response to atrial filling
Ejection fraction	SV as a fraction of EDV	Stroke volume itself

The Frank-Starling Mechanism, Step by Step

Step 1: Filling stretches the sarcomere

Cardiac muscle is built from sarcomeres, the repeating contractile units that contain overlapping actin and myosin filaments. When the ventricle fills during diastole, the chamber enlarges and the sarcomeres are stretched toward their optimal overlap. At that length, more myosin heads can bind actin and the myofilaments are more sensitive to calcium, so each contraction generates more force. This length-dependent activation is generally accepted as the cellular origin of the Frank-Starling mechanism [2].

Step 2: Force rises with filling, then plateaus

The relationship between filling and force is not linear forever. As end-diastolic volume increases from low to moderate, stroke volume rises steeply. Beyond a certain point the curve flattens into a plateau, and further stretch adds little or no extra force. Push beyond the plateau and force actually falls, because sarcomeres are overstretched and myofilament overlap is lost. This plateau is why the Starling curve is drawn as a rising limb followed by a flat top, not as a straight line.

Step 3: Molecular signaling fine-tunes the response

Stretch does more than physically pull filaments apart. Myocardial stretch activates NADPH oxidase 2 (NOX2), which produces low-level reactive oxygen species that act as signaling molecules. In isolated mouse cardiomyocytes, stretch-induced reactive oxygen species speed the rise of the calcium transient and increase contractility. In NOX2-knockout mice, the calcium transient peak was unchanged by stretch, but the rise was delayed and contractility was reduced, which links this pathway to the speed and strength of the Starling response [3].

Step 4: The response is multiscale

The Frank-Starling effect emerges from interactions among the cell, the ventricle, and the whole cardiovascular system. Computational modeling shows that length-dependent activation is required for both the Frank-Starling mechanism and a positive response to fluid loading, and that the cellular force increase alone does not fully explain the whole-organ behavior [2]. In practice this means the same ventricle can respond differently depending on vascular tone, heart rate, and how full the venous system already is.

Step 5: Efficiency improves with length

Starling himself linked the energy of contraction to muscle fiber length. Modern work on isolated rat ventricular trabeculae confirms that both work output and enthalpy (heat plus work) increase with initial muscle length, and that work rises more than enthalpy, so mechanical efficiency increases as preload rises [4]. The stretched heart does more work per unit of oxygen consumed, up to the plateau.

Reading a Frank-Starling Curve

A Frank-Starling curve plots cardiac performance on the vertical axis against a filling index on the horizontal axis. The vertical axis is usually stroke volume or cardiac output. The horizontal axis is usually a preload surrogate such as left ventricular end-diastolic volume (LVEDV), left ventricular end-diastolic pressure (LVEDP), central venous pressure (CVP), or right atrial pressure [5][6][7].

The curve has three regions:

Steep ascending limb. Small increases in filling produce large increases in output. This is the fluid-responsive zone. A patient here will gain stroke volume from a fluid bolus.
Plateau. Additional filling produces little extra output. Giving more fluid here adds volume without adding flow.
Descending limb. Excessive filling reduces output and raises filling pressures, which are transmitted backward into the pulmonary or systemic veins.

Clinicians use this shape every day. A patient with a flattened pressure-stroke volume slope may have low filling pressure yet fail to increase stroke volume after intravenous fluids, which identifies a fluid non-responder [6]. In a study of circulatory shock, patients whose central venous pressure fell while cardiac output rose had better 28-day survival than other hemodynamic patterns, which illustrates the clinical value of moving along the curve rather than simply raising filling pressure [7].

Shifts of the Curve Versus Movement Along It

Students must separate two things:

Movement along the curve is a pure preload change. More filling, more output, same contractile state.
A shift of the curve is a change in contractility. The whole relationship moves up or down.

Heart failure shifts the curve down and to the right. For the same filling volume, a failing ventricle ejects less. In a mouse model of heart failure with reduced ejection fraction, stroke volume and ejection fraction were both reduced versus controls, and stroke volume variability was markedly diminished, which the authors attributed to abnormal Frank-Starling behavior disrupting beat-to-beat stroke volume regulation independently of heart rate variability [5].

Recovery shifts the curve up and to the left. In patients supported by a continuous-flow left ventricular assist device, a speed ramp plus saline volume loading test separated patients who recovered native heart function from those who did not. The recovery group had a milder rise in pulmonary capillary wedge pressure during volume loading and a significantly greater rise in cardiac output, placing their Frank-Starling curve upward and to the left of the non-recovery group [8].

The Four Determinants of Stroke Volume

Stroke volume is set by four interacting variables. The first three are the classic triad of preload, afterload, and contractility, and heart rate is the fourth because it determines filling time and, through the force-frequency relationship, contractile strength.

Preload

Preload is the load on the ventricular wall at the end of diastole, indexed clinically by end-diastolic volume or end-diastolic pressure. Preload is set by venous return, blood volume, atrial contraction, heart rate (through filling time), and ventricular compliance.

Preload is the input to the Frank-Starling relationship. An increase in left ventricular end-diastolic volume leads to increased ventricular contraction, which is an intrinsic property of the heart and lets it compensate for increased venous return [5]. Preload also regulates myocardial strain. In septic patients with low left ventricular end-diastolic volume index, global longitudinal, circumferential, and right ventricular strain all improved after fluid resuscitation, showing that strain measurements themselves are preload-dependent [9].

Afterload

Afterload is the wall stress the ventricle must overcome to eject blood. It is determined mainly by arterial pressure, systemic vascular resistance, and ventricular wall thickness and radius (via the Laplace relationship). Higher afterload means the ventricle must generate more pressure to open the aortic or pulmonic valve, which reduces the fraction of EDV ejected and therefore reduces stroke volume.

Afterload is the easiest determinant to manipulate pharmacologically, which is why vasodilators help some failing hearts and why severe vasoconstriction can depress output. In a swine study, afterload and contractility were changed pharmacologically while preload was changed by bleeding and fluid or blood administration, and stroke volume tracked those changes as measured by multiple independent techniques [10].

Contractility

Contractility (inotropy) is the intrinsic strength of contraction at a given preload, afterload, and heart rate. It is increased by sympathetic stimulation and positive inotropes, and decreased by acidosis, hypoxia, ischemia, and systolic dysfunction.

Contractility is what shifts the Frank-Starling curve rather than moving along it. A useful clinical rule: if stroke volume is low and filling pressures are high, the problem is contractility or afterload, not preload.

Heart Rate

Heart rate affects stroke volume in two opposite ways. Up to a point, faster rates increase contractility through the force-frequency relationship. Beyond that point, faster rates shorten diastole, reduce filling time, and reduce EDV, so stroke volume falls. At very high rates, filling becomes so brief that stroke volume drops faster than rate rises, and cardiac output falls.

This trade-off is why resting heart rate differs so much across species and why tachycardia is poorly tolerated in animals with restrictive filling.

Factors Affecting Stroke Volume: Comparison Table

Factor	Effect on SV	Mechanism	Example
Increased preload (moderate)	Increases	Sarcomere stretch, length-dependent activation	Fluid bolus in a euvolemic patient
Excessive preload	Decreases	Overstretch past plateau, raised filling pressures	Volume overload in dilated cardiomyopathy
Decreased preload	Decreases	Less sarcomere stretch	Hemorrhage, dehydration, severe vasodilation
Increased afterload	Decreases	More pressure needed to eject	Systemic hypertension, aortic stenosis
Decreased afterload	Increases	Easier ejection	Vasodilator therapy
Increased contractility	Increases	Stronger intrinsic contraction, curve shifts up	Sympathetic stimulation, positive inotropes
Decreased contractility	Decreases	Weaker contraction, curve shifts down	Dilated cardiomyopathy, myocardial ischemia
Moderate tachycardia	Increases or maintains	Force-frequency relationship	Exercise
Severe tachycardia	Decreases	Shortened diastolic filling time	Sustained supraventricular tachycardia
Bradycardia	Decreases CO, may increase SV	Longer filling time, but fewer beats per minute	High-grade atrioventricular block

Species Differences in Resting Heart Rate and Stroke Volume

Resting heart rate and stroke volume vary widely across species, and the two trade off to produce a cardiac output matched to metabolic demand.

Species	Typical resting heart rate	Typical resting stroke volume	Notes
Adult human	60 to 100 beats per minute	About 70 mL	Larger body mass, lower rate
Dog (medium, 10 to 25 kg)	60 to 160 beats per minute	About 20 to 30 mL	Rate rises with smaller body size
Cat	140 to 220 beats per minute	Smaller than dog	High rate, small volumes
Horse	28 to 40 beats per minute	Very large	Elite stroke volume at low rate
Pig (anesthetized, experimental)	Variable	Measured by flow sensor around the pulmonary artery	Used as a translational model for stroke volume measurement [10]

These ranges are standard textbook figures and vary with breed, fitness, body size, and excitement level. A greyhound at rest may have a heart rate in the 40s, while a toy breed may sit near 180. The key concept is that cardiac output is conserved across species by adjusting the product of rate and stroke volume, not by holding either constant.

Smaller animals generally have higher heart rates and smaller stroke volumes. Larger animals have lower rates and larger stroke volumes. This is why a horse can move liters of blood per beat while a cat moves milliliters, yet both maintain adequate cardiac output for their metabolic rate.

Worked Example: Linking Preload to Output

Consider a 20 kg dog with a resting heart rate of 100 beats per minute and a stroke volume of 25 mL. Cardiac output is 100 × 25 = 2,500 mL/min, or 2.5 L/min. This is a reasonable resting value for a dog of that size.

Now the dog is given a fluid bolus that raises end-diastolic volume by 20 percent, from 40 mL to 48 mL. Because the dog is on the steep ascending limb of its Frank-Starling curve, contractile force rises and end-systolic volume falls slightly, from 15 mL to 13 mL. New stroke volume is 48 - 13 = 35 mL. If heart rate is unchanged at 100 beats per minute, cardiac output rises to 3,500 mL/min, a 40 percent increase from preload alone.

Now consider the same dog with dilated cardiomyopathy. Its curve is shifted down. The same fluid bolus raises end-diastolic volume from 60 mL to 72 mL, but the failing ventricle ejects poorly. End-systolic volume rises from 45 mL to 58 mL, so stroke volume is 72 - 58 = 14 mL, lower than before the bolus. Cardiac output falls, and the extra volume backs up into the pulmonary veins, raising pulmonary capillary pressure and risking pulmonary edema. This is the clinical trap: fluid that helps a normal heart can harm a failing one.

Clinical Relevance, Limitations and Common Mistakes

The Frank-Starling mechanism is the physiologic basis for fluid therapy, and it is also the reason fluid therapy fails. A patient on the steep limb gains output from volume. A patient on the plateau does not. A patient past the plateau loses output and develops congestion. In a prospective study of patients after cardiac surgery who were both fluid-tolerant and preload-responsive, 45 percent developed systemic venous congestion within two minutes of a 7 mL/kg crystalloid infusion, and congestion persisted in 5 percent at 20 minutes [11]. Preload responsiveness and fluid tolerance are not the same thing.

Excessive preload causes pulmonary edema. When left ventricular filling pressure rises above the oncotic pressure that holds fluid in the capillaries, fluid moves into the alveoli. This is why a dog in left-sided congestive heart failure can drown in its own fluid after an aggressive bolus, and why the therapeutic goal in that setting is to reduce preload with diuretics and vasodilators, not to increase it.

Mechanical load also remodels the heart over time. In cultured rat papillary muscles, stepwise increases in preload and afterload produced a biphasic response in t-tubule density, with the highest density at moderate loads and low density at very low or very high loads. Mice subjected to mildly elevated preload or afterload for one week developed compensated function with increased t-tubule density, while marked elevation of load in rodents and patients moved the heart down the descending limb, with t-tubule disruption and impaired calcium handling [12]. Preload is not just a moment-to-moment variable. It is a chronic structural signal.

Pregnancy provides a natural experiment in chronic volume overload. In pregnant women, acute volume overload from passive leg elevation increased inferior vena cava diameter and stroke volume in both the first and third trimesters, confirming the Frank-Starling response. Cardiovascular risk factors, but not parity or age, significantly affected stretch-induced compliance adaptation [13].

Noninvasive monitoring is changing how these concepts are applied. Wearable Doppler ultrasound over the common carotid artery can detect changes in stroke volume during a passive leg raise, with corrected flow time rising within 40 to 60 seconds of the maneuver while pulse contour stroke volume rose more gradually [14]. A wearable patch that simultaneously captures jugular venous and carotid arterial Doppler has been proposed to infer the Frank-Starling relationship in real time, which would let clinicians confirm that low filling pressure predicts poor response to volume [6].

Common mistakes students make:

Confusing preload with afterload. Preload is filling. Afterload is the resistance to ejection.
Treating the Starling curve as a straight line. It has a plateau, and past the plateau it descends.
Assuming more fluid always means more output. In a failing heart, more fluid means congestion.
Confusing stroke volume with cardiac output. Stroke volume is per beat. Cardiac output is per minute.
Forgetting that heart rate changes stroke volume by changing filling time.
Believing the Frank-Starling mechanism is a nervous reflex. It is intrinsic to the myocardium and operates in the denervated heart.
Assuming ejection fraction and stroke volume move together. A small ventricle can have a normal EF and a low stroke volume.

Limitations: this article covers principles and comparative physiology. Individual patients need a veterinarian for diagnosis and treatment decisions.

Quick Review

Stroke volume = end-diastolic volume - end-systolic volume.
Cardiac output = stroke volume × heart rate.
The Frank-Starling law: more filling stretches sarcomeres and increases contraction force, up to a plateau.
The four determinants of stroke volume are preload, afterload, contractility, and heart rate.
Preload moves you along the curve. Contractility shifts the curve.
Heart failure shifts the curve down and to the right, so the same filling produces less output.
Excessive preload raises filling pressures and causes pulmonary edema.
Smaller species have higher resting heart rates and smaller stroke volumes. Larger species have the opposite.

Frequently Asked Questions

What is the Frank-Starling law in one sentence?

The Frank-Starling law states that increasing the volume of blood filling a cardiac chamber stretches its muscle fibers and increases the force of contraction, so the heart ejects more blood when more blood returns to it [1][2].

How do you calculate stroke volume?

Stroke volume equals end-diastolic volume minus end-systolic volume. In a healthy adult human this is about 120 mL minus 50 mL, or roughly 70 mL per beat.

What are the four determinants of stroke volume?

Preload, afterload, contractility, and heart rate. Preload and afterload are loading conditions, contractility is the intrinsic inotropic state, and heart rate affects both filling time and contractile strength.

Why does the Frank-Starling curve plateau?

At high filling volumes the sarcomeres are stretched near their optimal overlap and further stretch adds little force. Beyond that point, overstretching reduces myofilament overlap and force falls, which is why the curve flattens and then descends.

Why is excessive preload dangerous?

When left ventricular filling pressure rises high enough, fluid moves into the pulmonary alveoli and causes pulmonary edema. In a failing heart, extra volume can reduce stroke volume while raising congestion, which is the opposite of the intended effect.

Do dogs and humans have the same stroke volume?

No. Stroke volume scales with body size. A healthy adult human ejects roughly 70 mL per beat, while a medium-sized dog ejects roughly 20 to 30 mL per beat. Ejection fraction, however, is similar in both, typically 55 to 65 percent.