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Atrial booster-pump function is the active contribution of atrial contraction to ventricular filling. In the left atrium, contraction late in ventricular diastole helps drive blood through the open mitral valve. It supplements earlier filling associated with atrial conduit function, rather than accounting for all filling during the cardiac cycle.^[ref]

A volumetric descriptor compares the chamber volume just before contraction, $V_{\mathrm{preA}}$, with the subsequent minimum, $V_{\min }$. Their difference is the observed active-phase volume loss; the left atrial active emptying fraction usesFor constructed values 50 and 40 mL, the volume loss is 10 mL and this fraction is $20\%$. Choosing the denominator is part of defining the descriptor.

That loss is not automatically all of the blood flow through the mitral valve during the interval. Integrating the left atrium's balance giveswhere $V_{\mathrm{PV}}$ is signed net pulmonary vein inflow over the same interval. If $V_{\mathrm{PV}}=8$ mL in the constructed example, 18 mL leaves through the mitral valve despite a chamber-volume decrease of only 10 mL.

A primary atrial flow component analysis reconstructed flow from time-resolved computed tomography and computational fluid dynamics, distinguishing modeled passage and residence histories.^[ref] This helps separate a phase-associated volume measurement from the route taken by each parcel. Likewise, an observed contraction-associated volume loss is not itself a controlled measurement of how much less ventricular filling would have occurred without contraction; pressure, timing and continuing inflow can also differ.

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.

Atrial reservoir function is the temporary storage of returning blood in an atrium during a phase when its inflow exceeds its outflow. For the left atrium, it is especially relevant while the mitral valve is closed during ventricular systole. It differs from passive onward passage, or atrial conduit function, and from the active late-filling contribution of atrial contraction.^[ref]

A direct volume measure is the excursion $\Delta V=V_{\max }-V_{\min }$ over a specified cardiac cycle. Its denominator matters when it is converted into a ratio. An original example with volumes 70 and 40 mL gives $\Delta V=30$ mL, but $\Delta V/V_{\min }=75\%$ and $\Delta V/V_{\max }\approx 42.9\%$. These ratios describe different normalizations of the same excursion, not contradictory measurements.

The governing storage balance isA constant net inflow of 60 mL/s for 0.4 s stores 24 mL in this idealized interval. Neither the peak inflow rate nor a single chamber volume alone supplies the required integral. Over a repeating cycle with no net accumulation, total inflow and outflow must balance even though storage changes within the cycle.

The volume excursion also does not identify how long each parcel remains in the left atrium. A primary study reconstructed atrial flow using time-resolved computed tomography and computational fluid dynamics, then followed modeled particles to distinguish conduit, reservoir and longer-residence components.^[ref] Two atria can have similar maximum and minimum volumes while differing in which blood passes directly onward or remains through later phases. Storage volume and parcel residence are related questions, but they are different measurements.

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.

The left atrium is the heart chamber receiving blood through the pulmonary veins and delivering it through the mitral valve to the left ventricle. It is the receiving chamber on the oxygenated return side of the pulmonary circulation.^[ref]

Its role changes during the cardiac cycle. The chamber can accumulate incoming blood while its outlet is closed, pass blood onward during ventricular filling, and contribute to late filling through atrial contraction. The storage role is called atrial reservoir function.^[ref]

A volume balance makes this more precise:Here the sum includes the incoming pulmonary vein blood flow rates, and the outlet term is signed net flow through the mitral valve. In a constructed interval with total inflow $5$ L/min and zero outlet flow for $0.10$ s, the stored volume increases byThe example describes transient storage, not an indefinitely sustained steady state. Over a repeating cycle with no net accumulation, average inflow must equal average outflow.

The entering streams can also differ in oxygen content. Their mixed content is weighted by their blood flow rates, not by the number of vessels. Thus the same anatomical chamber supports two distinct balance questions: how its volume changes with time, and how transported oxygen combines. Its volume alone determines neither the stroke volume of the left ventricle nor the oxygen content entering the left ventricle.