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The atrial conduit flow component is blood entering the left atrium through the pulmonary veins during ventricular diastole and leaving through the mitral valve within that same ventricular diastole. The same-phase requirement distinguishes it from the atrial reservoir flow component and atrial delayed-ejection flow component.^[ref]

An original two-history example shows why a volume curve alone cannot identify it. During a selected interval, suppose 20 mL enters the left atrium and 20 mL leaves through the mitral valve, leaving chamber volume unchanged. In history A, all 20 mL entering during that interval passes straight onward; it can qualify as atrial conduit flow component if the interval lies within one ventricular diastole. In history B, the 20 mL leaving is entirely older atrial blood, while the newly entering 20 mL stays behind. The same inflow, outflow and chamber volume change are compatible with different parcel identities.

The boundary balancecannot distinguish these histories. Here $Q_{\mathrm{PV}}$ is net inward volumetric flow rate at the pulmonary veins and $Q_{\mathrm{MV}}$ is net outward volumetric flow rate at the mitral valve. The additional information is the mapping between entry and exit, represented by pathlines.

This distinction also explains why atrial conduit function, a physiological role, and a particular volume-based proxy need not equal the atrial conduit flow component. A parcel volume collected over an interval need not be contained in the chamber simultaneously, so normalization requires an explicitly stated denominator rather than an assumed fraction of maximum atrial volume. The six histories below are constructed examples, not patient measurements.

The atrial delayed-ejection flow component comprises blood leaving the left atrium during ventricular diastole after entering during the preceding diastole or earlier. The Bäck study calls this delayed outflow. In contrast, the atrial reservoir flow component enters during the intervening ventricular systole.^[ref]

This is a transport-history definition, not a statement that ventricular electrical activation was delayed. A constructed timeline illustrates it. Number successive one-second cardiac cycles so the current ventricular systole occupies $0\le t<0.4$ s and its following diastole occupies $0.4\le t<1$ s. Suppose a parcel entered in the previous diastole at $t=-0.2$ s and crosses the mitral valve at $t=0.6$ s. Its 0.8 s passage spans a ventricular systole. A second parcel entering at $t=0.5$ s and leaving at $t=0.8$ s instead belongs to the atrial conduit flow component.

For represented parcels with exit times $t_i^{\mathrm{out}}$, entry times $t_i^{\mathrm{in}}$ and volume weights $w_i$, a volume-weighted transit time within a selected exiting category isTwo hypothetical parcels representing 1 and 3 mL and taking 0.8 and 1.2 s give $\overline{\tau }=1.1$ s, rather than the unweighted mean of 1 s. If an entry occurred before the available record, the exact transit time is unknown: an observed exit can establish a category without recovering an exact entry date.

The atrial delayed-ejection flow component thus complements the atrial retained-inflow component: one identifies an observed exit of older atrial blood, while the other identifies entered blood still present at the specified endpoint.

Atrial flow component analysis classifies blood transport through an atrium by the entry, exit and retention histories of represented fluid parcels. It asks a different question from measuring only maximum and minimum left atrium volumes: which blood passed onward, remained within the chamber, or crossed a boundary in another direction?

A primary study used time-resolved computed tomography to reconstruct moving atrial anatomy, computational fluid dynamics to estimate the flow field, and modeled particle trajectories to classify six components. It distinguished atrial conduit flow component, atrial reservoir flow component, atrial delayed-ejection flow component, atrial retained-inflow component, atrial residual-volume component and pulmonary-vein backflow component.^[ref] These are model-derived transport categories, not six separate anatomical chambers.

A typical trajectory calculation followswhere $\mathbf{u}$ is the modeled fluid velocity. Boundary crossings and their timing within a defined cardiac cycle determine the category. For example, the study's conduit definition requires entry and exit within the same ventricular diastolic phase.^[ref] Changing the time window or category definition changes what is counted.

An original weighting example shows why counting markers alone can be misleading. Suppose three modeled markers represent 1, 1 and 8 mL, with the first two assigned to one component. That component contains 2 of 10 mL, or $20\%$, although it contains two-thirds of the markers. The relevant fraction iswith $w_i$ the represented volumes for the chosen analysis. The denominator must be stated; an analysis that includes entering flow is not automatically a partition of one instantaneous chamber volume.

The results depend on reconstructed boundaries, flow assumptions, numerical tracking and phase definitions. Atrial reservoir function and atrial conduit function can be investigated with these categories, but a volumetric proxy and a parcel-based component need not be identical observations.

The atrial retained-inflow component is blood that entered the left atrium during the previous cardiac cycle but has not left by the specified endpoint of the Bäck tracking scheme. Its documented entry distinguishes it from the atrial residual-volume component, whose blood remains throughout both tracked cycles.^[ref]

A simple parcel ledger makes the distinction concrete. In a constructed one-second cardiac cycle, let a tracked parcel enter through a pulmonary vein at $t=0.4$ s during the first of two cycles and remain inside until observation ends at $t=2$ s. Its observed residence time at that endpoint is 1.6 s. If that parcel crosses the mitral valve at $t=2.2$ s, a later observation reveals an exit; the earlier record was still correctly classified for its own endpoint.

Different amounts of retention need not produce different instantaneous chamber volumes. Consider two ideal histories, each with 40 mL inside the left atrium at the endpoint. In the first, 10 mL has an observed entry during the designated earlier cycle and remains, while 30 mL was present throughout the entire tracking window. In the second, the corresponding volumes are 25 and 15 mL. Both chamber totals are 40 mL, but their atrial retained-inflow component and atrial residual-volume component differ. A chamber outline alone cannot distinguish these histories.

The component is consequently useful for asking about replacement of atrial blood, alongside atrial delayed-ejection flow component and atrial conduit flow component. Its measured or modeled fraction must identify both the tracking interval and denominator. Dividing 10 mL by a 40 mL chamber volume gives $25\%$; dividing it by a stipulated 80 mL cycle throughput gives $12.5\%$. Those percentages express different comparisons.