= Atrial retained-inflow component
The \b[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.<Comprehensive left atrial flow component analysis reveals abnormal flow patterns in paroxysmal atrial fibrillation by Sophia Bäck and others>[Methods, Flow Component Analysis; Figure 1]{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.
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