= Right atrium
The \b[right atrium] is the <heart> chamber receiving <blood> from the <superior vena cava>, <inferior vena cava> and <coronary sinus>. Its principal outlet is the <tricuspid valve>, leading to the <right ventricle>.<Anatomy and Physiology 2e by J. Gordon Betts and others>[§19.1, Right Atrium]{ref} This places it between systemic venous return and the pump supplying the <pulmonary circulation>.
The <right atrium> is both a junction and a chamber that can store <blood>. For a simple volume balance,
$$
\frac{dV_{\rm RA}}{dt}=Q_{\rm in}-Q_{\rm out},
$$
where $Q_{\rm in}$ sums the incoming <blood flow> rates and $Q_{\rm out}$ is net outward flow through the <tricuspid valve>. If a constructed inflow exceeds outflow by $0.5$ L/min for $0.1$ min, atrial volume increases by 50 mL. Such an imbalance cannot persist indefinitely in a fixed-volume steady-state model. Over a repeating <cardiac cycle>, the time-averaged inflow and outflow must balance if there is no net accumulation.
The joining streams can have different <venous oxygen content>. Their location in one chamber does not guarantee instantaneous perfect mixing at every sample position. Calculating <mixed venous oxygen content> requires a representative mixture, with the entering <blood flow> rates supplying the weights. This connects an anatomical chamber to the sampling assumptions behind the <Fick principle>.
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