= End-systolic volume
{title2=$V_{\mathrm{ES}}$}
\b[End-systolic volume] is the volume of <blood> remaining in a specified ventricle at the end of its ejection phase. It must be associated with a particular chamber and <cardiac cycle>. Subtracting it from the preceding <end-diastolic volume> gives the chamber's volumetric <stroke volume>:
$$
\mathrm{SV}=V_{\rm ED}-V_{\rm ES}.
$$
For example, $V_{\rm ED}=140$ mL and $V_{\rm ES}=60$ mL give $\mathrm{SV}=80$ mL. The residual 60 mL is not a separate compartment of permanently trapped <blood>; it is a chamber-volume measurement at one phase of a recurring cycle.
Measuring <end-systolic volume> requires both a phase choice and a cavity boundary. A primary <magnetic resonance imaging> study used cine images and three-dimensional modelling to measure right ventricular volumes and function in 120 healthy subjects.<Reference right ventricular systolic and diastolic function normalized to age, gender and body surface area from steady-state free precession cardiovascular magnetic resonance by Alicia M. Maceira and others>[Abstract, Methods and results]{ref} An image's smallest-looking cavity in one view is not automatically the smallest three-dimensional volume.
An original sensitivity calculation makes the boundary issue tangible. If a selected cavity volume $V=60$ mL has surface area $S=100$ cm$^2$, a hypothetical uniform outward boundary shift of $\delta=0.01$ cm gives a first-order volume error $\delta V\simeq S\delta=1$ mL. This geometrical approximation does not prescribe how a real image should be segmented, but shows how many small boundary displacements can accumulate into a volume error.
At fixed <end-diastolic volume>, increasing <end-systolic volume> reduces both <stroke volume> and <ejection fraction>. That algebra does not identify the physiological cause: changing contraction, loading and valve behavior can affect the observed volumes. A single residual-volume number is therefore a measurement of state, not a complete mechanism.
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