= Brachiocephalic vein
A \b[<brachiocephalic vein>] is one of the paired vessels that unite into the <superior vena cava>. On each side, the <internal jugular vein> and <subclavian vein> contribute major head/neck and upper-limb return to this route.<Anatomy and Physiology 2e by J. Gordon Betts and others>[§20.5, The Superior Vena Cava, Table 20.11 and Venous Drainage of the Head and Neck]{ref} The paired venous arrangement differs from the arterial naming pattern: a brachiocephalic artery is not a left/right counterpart for each of these veins.
The connection explains why a regional venous sample is not automatically a fully mixed whole-body sample. The two <brachiocephalic veins> bring together different drainage territories, and additional return enters through other routes, including the <azygos vein> into the <superior vena cava>.
An original tracer-mixing example gives the left and right streams flows 1.5 and 1.0 L/min. If an illustrative marker concentration is 10 mg/L on the left and zero on the right, complete mixing of those two streams gives
$$
C=\frac{1.5(10)+1.0(0)}{1.5+1.0}=6.0\,\mathrm{mg\,L^{-1}}.
$$
Adding a further unmarked return of 0.5 L/min reduces the mixed concentration to $15/3=5.0$ mg/L while preserving the same transported marker amount, 15 mg/min. Dilution can change concentration without destruction of the marker.
These are hypothetical well-mixed steady streams, not an injection protocol or measured human flow distribution. A sample taken before complete mixing can differ from the calculated average. The example connects vessel junctions with <indicator dilution>: the amount balance must include every stream contributing at the actual detector location.
Back to article page
