 Brachiocephalic vein

A 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.^[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 givesAdding 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.