= Left atrium
The \b[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>.<How Blood Flows through the Heart by NHLBI>[Adding oxygen to blood; mitral valve connection]{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>.<Anatomy and Physiology 2e by J. Gordon Betts and others>[§19.1, Left Atrium]{ref}
A volume balance makes this more precise:
$$
\frac{dV_{\rm LA}}{dt}=\sum_i Q_{\mathrm{PV},i}-Q_{\mathrm{mitral}}.
$$
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 by
$$
\Delta V=\frac{5000}{60}\times0.10\simeq8.33\ \mathrm{mL}.
$$
The 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>.
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