= Superior vena cava
The \b[<superior vena cava>] returns blood from much of the head, neck, upper limbs and thorax to the <right atrium>. The right and left <brachiocephalic veins> join into this return channel, and the <azygos vein> also drains into it.<Anatomy and Physiology 2e by J. Gordon Betts and others>[§20.5, The Superior Vena Cava and Table 20.11]{ref} Much of the heart's own venous return follows separate cardiac routes, so “everything above the diaphragm” would be an inaccurate definition.
The <superior vena cava> and <inferior vena cava> collect different territories. Their blood can have different oxygen contents because regional flows and <oxygen consumption> differ. The chamber destination is shared, but the two vessels are not interchangeable sampling sites for every physiological calculation.
An original complete-mixing model combines upper-body flow $Q_U=2$ litres per minute at content $C_U=0.10$ with lower-body flow $Q_L=3$ litres per minute at $C_L=0.14\,\mathrm{mL\ O_2\,mL^{-1}}$. Ignoring separate cardiac venous return for this reduced example,
$$
C_{\mathrm{mix}}=\frac{Q_UC_U+Q_LC_L}{Q_U+Q_L}=0.124\,\mathrm{mL\ O_2\,mL^{-1}}.
$$
Neither original vessel content equals that mixture. Using the superior-vessel value with the total 5 L/min would imply 500 mL oxygen returned per minute, whereas the model's two streams return 620 mL per minute.
The discrepancy arises from mismatching a regional composition with whole-system flow. It persists even with a perfectly accurate content instrument. A whole-body <Fick principle> calculation requires <mixed venous oxygen content> on the appropriate basis; a regional specimen remains valuable when the intended question is about its actual drainage territory.
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