Stroke Volume

Stroke volume is the volume of blood ejected by one ventricle during a single heartbeat. It is an important measure of cardiac pump function and, together with heart rate, determines cardiac output.

Stroke volume depends on how much blood fills the ventricle before contraction, how forcefully the myocardium contracts, and the load against which the ventricle must eject. These factors are commonly described as preload, contractility, and afterload.

Because these determinants change continuously with venous return, autonomic activity, arterial pressure, exercise, posture, and cardiovascular disease, stroke volume is a dynamic physiological variable rather than a fixed quantity.

Stroke Volume Equation

Stroke volume is calculated as the difference between ventricular end-diastolic volume and end-systolic volume:

Stroke Volume = End-Diastolic Volume − End-Systolic Volume

End-diastolic volume represents the amount of blood in the ventricle immediately before systole. End-systolic volume represents the amount remaining after ventricular ejection.

Example Calculation

If a ventricle contains 120 mL of blood at the end of diastole and 50 mL remains at the end of systole, the stroke volume is:

120 mL − 50 mL = 70 mL

The ventricle therefore ejects 70 mL during that cardiac cycle.

Major Determinants of Stroke Volume

Determinant	Physiological Meaning	General Effect
Preload	Ventricular myocardial stretch before contraction	Greater physiological preload generally increases stroke volume
Contractility	Intrinsic force-generating ability of myocardium at a given loading condition	Greater contractility generally increases stroke volume
Afterload	Load opposing ventricular ejection	Greater afterload generally reduces ejection if other factors are unchanged

End-Diastolic Volume

End-diastolic volume (EDV) is the volume of blood within a ventricle at the end of ventricular filling, immediately before systole.

EDV is influenced by venous return, ventricular compliance, filling time, atrial contraction, and other factors affecting ventricular filling.

End-Systolic Volume

End-systolic volume (ESV) is the blood remaining within a ventricle after systolic ejection.

ESV is strongly influenced by myocardial contractility and afterload. More complete ventricular emptying decreases ESV and, if EDV remains unchanged, increases stroke volume.

Preload

Preload refers to the degree of myocardial fiber stretch before contraction. At the ventricular level, it is related to the loading conditions present near the end of diastole.

End-diastolic volume and end-diastolic pressure are often used as practical indicators of preload, although preload itself more precisely refers to myocardial wall stress or sarcomere stretch before systole.

Venous Return

Venous return is the flow of blood returning to the heart through the venous circulation.

Increased venous return generally increases ventricular filling, raising EDV and preload. Within physiological limits, this tends to increase stroke volume.

Frank-Starling Mechanism

The Frank-Starling mechanism describes the intrinsic ability of the heart to increase the force of contraction when ventricular filling increases.

Within physiological limits, increased filling stretches cardiac muscle fibers before contraction. This allows the subsequent contraction to generate greater force and eject a larger volume of blood.

Importance of the Frank-Starling Mechanism

The Frank-Starling mechanism helps the heart automatically match its output to venous return.

It also helps maintain balance between the right and left ventricles. If one ventricle temporarily receives more blood, increased filling tends to increase its subsequent stroke volume.

Length-Tension Relationship

The Frank-Starling response reflects, in part, the relationship between myocardial sarcomere length and force generation.

Within the physiological operating range, increased initial fiber length improves the mechanical and calcium-sensitive conditions for force development.

Contractility

Contractility, also called inotropy, describes the intrinsic force-generating ability of cardiac muscle independent of changes in initial fiber length.

An increase in contractility allows the ventricle to eject more blood at a given preload and afterload.

Positive Inotropy

A positive inotropic effect increases myocardial contractile force.

Increased contractility generally decreases end-systolic volume because the ventricle ejects more completely, thereby increasing stroke volume.

Sympathetic Stimulation

Sympathetic stimulation is an important physiological regulator of ventricular contractility.

Norepinephrine released from sympathetic nerve endings and circulating epinephrine stimulate beta-adrenergic receptors, increasing intracellular calcium availability and enhancing contraction.

Calcium and Contractility

Cardiac contraction depends on intracellular calcium.

Changes in calcium entry, calcium release from the sarcoplasmic reticulum, and calcium handling within cardiomyocytes can alter the force of contraction and therefore stroke volume.

Afterload

Afterload is the load that opposes ventricular shortening and ejection during systole.

For the left ventricle, arterial pressure and aortic impedance contribute importantly to afterload. For the right ventricle, pulmonary arterial pressure and pulmonary vascular resistance are important determinants.

Effect of Increased Afterload

If afterload increases acutely while preload and contractility remain unchanged, the ventricle ejects less blood.

End-systolic volume rises and stroke volume falls.

Effect of Reduced Afterload

A reduction in afterload makes ventricular ejection easier.

When other conditions remain comparable, the ventricle can empty more completely, reducing ESV and increasing stroke volume.

Preload, Contractility and Afterload

Change	Typical Immediate Effect on Stroke Volume
Preload increases	Increases
Preload decreases	Decreases
Contractility increases	Increases
Contractility decreases	Decreases
Afterload increases	Decreases
Afterload decreases	Increases

Stroke Volume and Cardiac Output

Stroke volume is one of the two direct determinants of cardiac output:

Cardiac Output = Heart Rate × Stroke Volume

If heart rate remains constant, an increase in stroke volume increases cardiac output.

Stroke Volume and Heart Rate

Heart rate can indirectly influence stroke volume by changing ventricular filling time.

At very rapid rates, diastole becomes shorter and ventricular filling may decrease, potentially reducing EDV and stroke volume.

Ventricular Filling

Most ventricular filling normally occurs during diastole as blood flows from the atria into the ventricles.

Filling depends on atrioventricular pressure gradients, ventricular relaxation and compliance, venous return, heart rate, and atrial contraction.

Atrial Contraction

Atrial systole contributes additional blood to ventricular filling near the end of diastole.

The importance of this contribution increases in some circumstances, particularly when ventricular relaxation or compliance is impaired.

Ventricular Compliance

Ventricular compliance describes how readily the ventricle expands in response to filling.

A less compliant ventricle may develop a higher filling pressure without a proportionate increase in volume, altering preload and stroke volume relationships.

Blood Volume

Circulating blood volume influences venous return and ventricular filling.

Reduced blood volume can decrease EDV and stroke volume, while increased effective circulating volume may increase filling when cardiac function permits.

Venous Tone

Veins act as capacitance vessels and contain a large fraction of the circulating blood volume.

Sympathetic venoconstriction reduces venous capacitance and can shift blood toward the heart, increasing venous return and preload.

Skeletal Muscle Pump

Contraction of skeletal muscles compresses veins, particularly in the limbs.

Venous valves help direct this blood toward the heart, increasing venous return during physical activity.

Respiratory Pump

Respiratory changes in intrathoracic and abdominal pressures influence venous return.

During inspiration, reduced intrathoracic pressure generally promotes systemic venous flow toward the right side of the heart.

Posture

Posture affects the distribution of blood within the circulation.

Standing causes gravitational pooling in dependent veins, which can transiently reduce venous return, EDV, and stroke volume compared with the supine position.

Exercise

Stroke volume commonly increases during dynamic exercise.

This results from increased venous return, enhanced sympathetic contractility, and changes in vascular resistance and ventricular loading.

Exercise and Preload

The skeletal muscle pump, respiratory pump, and sympathetic venoconstriction increase venous return during exercise.

These mechanisms help maintain or increase ventricular filling despite the shorter diastolic interval associated with increased heart rate.

Exercise and Contractility

Sympathetic stimulation increases ventricular contractility during exercise.

This allows the ventricles to eject more completely and helps maintain a large stroke volume as cardiac demand rises.

Training Adaptations

Endurance training can increase ventricular filling and stroke volume at rest and during exercise.

A larger stroke volume allows a trained individual to maintain a given cardiac output at a lower heart rate under many conditions.

Ejection Fraction

Ejection fraction (EF) is the fraction of end-diastolic volume ejected during systole.

It provides information about ventricular systolic performance but is not identical to stroke volume.

Ejection Fraction Equation

Ejection Fraction = Stroke Volume ÷ End-Diastolic Volume × 100

Two individuals can have similar ejection fractions but different stroke volumes if their ventricular volumes differ.

Stroke Volume Versus Ejection Fraction

Measure	Definition
Stroke volume	Absolute volume ejected per beat
Ejection fraction	Percentage of end-diastolic volume ejected per beat

Pressure-Volume Loop

A ventricular pressure-volume loop displays changes in ventricular pressure and volume during one cardiac cycle.

The width of the loop represents stroke volume because it corresponds to the difference between EDV and ESV.

Effect of Increased Preload on the Pressure-Volume Loop

An increase in preload generally increases EDV.

When contractility and afterload are unchanged, the pressure-volume loop becomes wider because stroke volume increases through the Frank-Starling mechanism.

Effect of Increased Contractility

Increased contractility allows the ventricle to eject more completely.

ESV decreases, and the pressure-volume loop generally widens, reflecting an increased stroke volume.

Effect of Increased Afterload

Increased afterload initially reduces ventricular emptying.

ESV rises and the pressure-volume loop becomes narrower if other variables are unchanged, indicating a reduced stroke volume.

Right Ventricular Stroke Volume

The right ventricle ejects blood into the pulmonary circulation.

Its stroke volume is influenced by right ventricular preload, myocardial contractility, and pulmonary vascular afterload.

Left Ventricular Stroke Volume

The left ventricle ejects blood into the systemic circulation.

Its stroke volume is influenced by left ventricular filling, contractility, systemic arterial pressure, and other components of left ventricular afterload.

Balance Between the Ventricles

Over time, right and left ventricular stroke volumes must be essentially equal because the pulmonary and systemic circulations are connected in series.

Persistent imbalance would cause blood to accumulate in one portion of the circulation.

Transient Differences

Small temporary differences between right and left ventricular stroke volumes occur normally.

Changes in ventricular filling and the Frank-Starling mechanism help restore balance between the two sides.

Measurement of Stroke Volume

Stroke volume can be measured or estimated using several clinical and physiological techniques.

The method selected depends on the clinical setting, required accuracy, invasiveness, and available equipment.

Echocardiography

Echocardiography can estimate stroke volume using Doppler measurements of blood-flow velocity and the cross-sectional area of a ventricular outflow tract.

It can also estimate ventricular volumes and calculate stroke volume from EDV and ESV.

Cardiac Magnetic Resonance Imaging

Cardiac magnetic resonance imaging can provide detailed measurements of ventricular volumes and function.

Stroke volume can be calculated from measured end-diastolic and end-systolic volumes or assessed using flow measurements.

Thermodilution

Thermodilution can estimate cardiac output invasively using a pulmonary artery catheter.

When cardiac output and heart rate are known, stroke volume can be derived by dividing cardiac output by heart rate.

Stroke Volume Calculation From Cardiac Output

Stroke Volume = Cardiac Output ÷ Heart Rate

This relationship follows directly from the cardiac output equation.

Factors That Increase Stroke Volume

• Increased physiological preload
• Increased venous return
• Increased myocardial contractility
• Sympathetic stimulation
• Reduced afterload under appropriate conditions
• Exercise

Factors That Decrease Stroke Volume

• Reduced venous return
• Blood loss or severe volume depletion
• Reduced myocardial contractility
• Markedly increased afterload
• Impaired ventricular filling
• Very rapid heart rates that substantially shorten diastole

Clinical Significance

Heart Failure

Heart failure can reduce stroke volume through impaired contractility, abnormal ventricular filling, excessive afterload, or combinations of these mechanisms.

Compensatory mechanisms may temporarily maintain cardiac output by increasing heart rate, filling pressures, or neurohormonal activation.

Systolic Dysfunction

In ventricular systolic dysfunction, impaired contractile performance can increase ESV and reduce stroke volume.

Ejection fraction may also decrease when the proportion of EDV ejected during systole is reduced.

Diastolic Dysfunction

Impaired ventricular relaxation or reduced compliance can limit ventricular filling.

Stroke volume can therefore be reduced even when ejection fraction is preserved.

Hypovolemia

Loss of circulating blood or fluid reduces venous return and ventricular preload.

The resulting reduction in EDV can decrease stroke volume and cardiac output.

Hemorrhage

Significant hemorrhage reduces circulating blood volume.

Reflex tachycardia and vasoconstriction can partially compensate, but severe blood loss can cause a major decline in stroke volume and tissue perfusion.

Hypertension

Elevated systemic arterial pressure increases the load against which the left ventricle ejects.

Acute increases in afterload can reduce stroke volume, while chronic hypertension can produce structural ventricular adaptations.

Aortic Stenosis

Aortic stenosis creates an obstruction to left ventricular outflow and increases the pressure load on the left ventricle.

Severe obstruction can limit effective forward stroke volume, particularly as ventricular compensation fails.

Mitral Regurgitation

In mitral regurgitation, part of left ventricular stroke volume is ejected backward into the left atrium rather than forward into the aorta.

Total ventricular stroke volume and effective forward stroke volume are therefore not necessarily the same.

Aortic Regurgitation

In aortic regurgitation, blood returns from the aorta to the left ventricle during diastole.

The ventricle may eject a large total stroke volume, but part of that volume represents regurgitant rather than effective forward flow.

Cardiogenic Shock

Severe impairment of ventricular pump function can markedly reduce stroke volume.

If cardiac output becomes inadequate to maintain tissue perfusion, cardiogenic shock may develop.

Pericardial Tamponade

In cardiac tamponade, elevated pericardial pressure restricts cardiac filling.

Reduced ventricular preload decreases stroke volume and can cause severe circulatory compromise.

Arrhythmias

Arrhythmias can alter stroke volume by changing ventricular filling time, atrioventricular coordination, and the sequence of ventricular contraction.

The effect depends on the type and rate of the arrhythmia and the underlying condition of the heart.

Stroke Volume, Preload and Afterload Summary

Variable	Main Physiological Relationship
Venous return	Influences ventricular filling and preload
EDV	Represents ventricular volume before systole
Preload	Influences contraction through the Frank-Starling mechanism
Contractility	Influences the extent of ventricular emptying
Afterload	Opposes ventricular ejection
ESV	Volume remaining after systole
Stroke volume	EDV minus ESV

Key Features of Stroke Volume

Feature	Key Point
Definition	Volume ejected by one ventricle per heartbeat
Equation	SV = EDV − ESV
Typical unit	Milliliters per beat
Major determinants	Preload, contractility and afterload
Cardiac output relationship	CO = HR × SV
Preload relationship	Greater physiological filling generally increases SV
Contractility relationship	Greater contractility generally increases SV
Afterload relationship	Greater afterload generally opposes ejection
Clinical relevance	Reflects ventricular filling and pumping performance

Physiological Importance

Stroke volume links ventricular filling with ventricular ejection. The amount of blood entering the ventricle establishes the initial loading condition, while myocardial contractility and afterload determine how effectively that blood can be expelled during systole.

The Frank-Starling mechanism provides intrinsic beat-to-beat regulation, allowing increased venous return to produce increased ventricular output within physiological limits. Autonomic regulation adds another layer of control by altering contractility, venous tone, heart rate, and vascular resistance.

Because stroke volume directly contributes to cardiac output, changes in ventricular filling or pumping performance can have important effects on systemic and pulmonary blood flow. Understanding stroke volume therefore provides a foundation for interpreting cardiac mechanics, pressure-volume relationships, exercise physiology, heart failure, shock, valvular disease, and other cardiovascular conditions.