= Atrial contraction
\b[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>.<Anatomy and Physiology 2e by J. Gordon Betts and others>[§19.3, Cardiac Cycle, Atrial Systole]{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:
$$
\frac{dV_{\rm LA}}{dt}=Q_{\rm PV}-Q_{\rm mitral},
$$
where $Q_{\rm PV}$ is summed <pulmonary vein> inflow and $Q_{\rm mitral}$ 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.<Comprehensive left atrial flow component analysis reveals abnormal flow patterns in paroxysmal atrial fibrillation by Sophia Bäck and others>[Abstract; Methods]{ref} A volume change associated with contraction is informative, but its interpretation requires the concurrent inflows and outflows.
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