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Atrial conduit function is the passive passage of returning blood through an atrium toward the ventricle, distinguished from temporary storage in atrial reservoir function and active pumping in atrial booster-pump function. For the left atrium, the pathway connects pulmonary vein inflow with mitral valve outflow during ventricular filling.^[ref]

Two measurement descriptions need to be distinguished. A volumetric account compares chamber-volume changes with total ventricular filling. A parcel-based account asks when particular modeled blood enters and leaves the left atrium. A primary study defined conduit flow as entry and exit within the same ventricular diastolic phase and compared such components with volumetric indices.^[ref] The operational definition determines what is counted.

An original bookkeeping model illustrates the volumetric approach. Suppose a periodic system has ventricular stroke volume $S$, and suppose a reservoir release $R$ and direct onward passage $C$ form a complete, nonoverlapping partition of that delivered volume. ThenIf a further idealization identifies $R$ with the atrial excursion $V_{\max }-V_{\min }$, values $S=80$ mL and an excursion of 30 mL give a conduit proxy of 50 mL. The arithmetic is exact for the stipulated partition; the correspondence between that partition and real trajectories is an additional assumption.

For example, simultaneous inflow and outflow can exchange blood while chamber volume remains constant. In the balance $dV/dt=Q_{\mathrm{in}}-Q_{\mathrm{out}}$, equal 100 mL/s flows give zero volume change even though 20 mL passes in 0.2 s. Thus a flat volume trace does not demonstrate absent transport. Geometry, timing and modeled parcel histories supply information that volume extrema alone cannot provide.

Atrial contraction is active shortening of the atrial myocardium that contributes to moving blood from the atria into the ventricles. In the usual coordinated cardiac cycle, it occurs late in ventricular diastole, while the mitral valve and tricuspid valve permit filling. It supplements earlier passive filling; ventricular filling is not restricted to the period of atrial contraction.^[ref]

For the left atrium, this active contribution is called atrial booster-pump function, distinguishable from atrial reservoir function and atrial conduit function. A volume balance separates storage from transport:where $Q_{\mathrm{PV}}$ is summed pulmonary vein inflow and $Q_{\mathrm{mitral}}$ is net outflow through the mitral valve. During atrial contraction, a decrease in chamber volume can accompany outflow larger than the continuing inflow.

In an original example, the left atrium loses 15 mL during a chosen contraction interval while receiving 5 mL through the pulmonary veins. Integrating the balance shows that 20 mL must have left through the mitral valve. The volume decrease alone would undercount that interval's forward transport by 5 mL. Conversely, not all blood entering the left ventricle in that interval was necessarily stored before contraction began.

The distinction matters experimentally: a changing cavity volume, a velocity recorded at a valve and the trajectory of a particular parcel of blood are different observations. A primary flow-component study used time-resolved computed tomography, computational fluid dynamics and modeled particle paths to distinguish atrial transport routes.^[ref] A volume change associated with contraction is informative, but its interpretation requires the concurrent inflows and outflows.

Left atrial active emptying fraction is a volumetric descriptor of the relative decrease in left atrium volume during its active emptying phase. With volume $V_{\mathrm{preA}}$ immediately before atrial contraction and subsequent minimum volume $V_{\min }$,The phase choice distinguishes this quantity from a fraction formed using the maximum atrial volume over the entire cardiac cycle. Atrial contraction normally contributes to late ventricular filling after earlier passive filling.^[ref]

An original comparison uses $(V_{\mathrm{preA}},V_{\min })=(50,40)$ mL and $(25,15)$ mL. Both chambers lose 10 mL, but their fractions are $20\%$ and $40\%$. The denominator converts an absolute volume decrease into a relative one; it does not add an independent measurement of forward transport.

Timing is part of the measurement. With the first pair, choosing an earlier or later frame that gives $V_{\mathrm{preA}}=48$ mL instead of 50 mL changes the calculated fraction to $8/48\approx 16.7\%$, even if $V_{\min }$ is unchanged. Consistent boundaries and phase identification therefore matter alongside image segmentation precision.

The fraction describes chamber-volume change, not all concurrent mitral valve flow. Pulmonary vein inflow during the interval contributes to the balance $V_{\mathrm{out}}=V_{\mathrm{preA}}-V_{\min }+V_{\mathrm{in}}$. Nor does the observed volume decrease isolate the causal effect of contraction from every other changing condition. A primary CT-based atrial flow component analysis compared volumetric indices with modeled transport components, illustrating the separation between volume measurements and parcel histories.^[ref] Atrial booster-pump function is the physiological contribution; this fraction is one specified descriptor of its associated volume behavior.