Stroke Volume Definition

Stroke Volume Definition is the volume of blood ejected from a ventricle during a single heartbeat, calculated as the difference between the volume of blood present in the ventricle immediately before contraction and the volume remaining immediately after contraction. Stroke volume represents the fundamental unit of ventricular output per beat and, together with heart rate, determines the total cardiac output delivered to the systemic and pulmonary circulations.

Mathematical Definition

Stroke volume is derived directly from the two boundary volumes of the ventricle across a single cardiac cycle.

End-Diastolic Volume

The end-diastolic volume is the maximum volume of blood contained in the ventricle, reached at the close of diastole immediately before ventricular contraction begins.

End-Systolic Volume

The end-systolic volume is the minimum volume of blood remaining in the ventricle, reached at the close of systole immediately after ventricular ejection ends.

The Stroke Volume Equation

Stroke volume is the arithmetic difference between these two volumes.

Physiological Determinants

Stroke volume is not a fixed quantity but varies according to three interacting factors.

Preload

The degree of ventricular filling, and the resulting stretch of myocardial fibers, directly influences the force of contraction and the resulting stroke volume through the Frank–Starling mechanism.

Afterload

The resistance the ventricle must overcome to eject blood affects how completely the ventricle empties during systole, with higher afterload tending to reduce stroke volume.

Contractility

The intrinsic strength of myocardial contraction, independent of loading conditions, modifies how much blood is ejected for a given preload and afterload.

Relationship to Ejection Fraction

Stroke volume expressed as a proportion of end-diastolic volume defines the ejection fraction, a widely used clinical index of ventricular systolic function.

Relationship to Cardiac Output

Stroke volume combines multiplicatively with heart rate to determine the total volume of blood pumped per minute.

Diagrammatic Summary

Physiological Significance

As the single-beat output of the ventricle, stroke volume is a central determinant of overall cardiac performance, and changes in stroke volume, whether from altered preload, afterload, or contractility, form the basis for the heart's continuous adjustment of blood delivery to meet the body's varying metabolic demands.