= Coronary sinus
The \b[coronary sinus] is a major venous collecting channel of the <heart> that opens into the <right atrium>. It receives blood from <cardiac veins>, including the <great cardiac vein>, <middle cardiac vein> and <small cardiac vein>.<Anatomy and Physiology 2e by J. Gordon Betts and others>[Section 19.1, Coronary Veins and Figure 19.15]{ref} It belongs to the venous return pathway, distinct from the <coronary arteries> supplying the myocardium.
The collecting-channel arrangement explains its use in a regional balance. Incoming flows combine before the sample or outflow observation is taken. In a steady model with negligible local storage,
$$
Q_{\rm sinus}=\sum_jQ_j.
$$
If represented inflows are $90$ and $30\,\mathrm{mL\,min^{-1}}$, the sinus outflow is $120\,\mathrm{mL\,min^{-1}}$. This is a chosen continuity example, not a normal flow value.
A measurement at the <coronary sinus> does not automatically represent every myocardial route. If another modeled pathway returns $10\,\mathrm{mL\,min^{-1}}$ directly to a chamber, the total venous return from the represented heart is $130$, while the sinus observation remains $120$. Treating the latter as the whole total would miss about $7.7\%$ in this example.
Likewise, <venous oxygen content> sampled from a collecting channel is a mixture weighted by its contributing flows. It should be paired with flow and arterial measurements representing compatible tissue when calculating <coronary oxygen uptake>. Knowing the sampling site is part of defining the biological quantity, not simply a label attached after measurement.
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