= Internal iliac vein
An \b[<internal iliac vein>] collects venous return from pelvic territories and joins the <external iliac vein> to form a <common iliac vein>. The paired internal routes are part of the return network leading toward the <inferior vena cava> and <right atrium>.<Anatomy and Physiology 2e by J. Gordon Betts and others>[§20.5, Veins Draining the Lower Limbs and Table 20.16]{ref} Internal identifies a regional branch, not a general category containing every vein inside the body.
The branching distinction matters to sampling. A pelvic return sample and a lower-limb return sample can have different concentrations because their tissues exchange different amounts of a substance. Their common destination does not make their compositions equal.
For an original two-tributary example, suppose an <internal iliac vein> receives flows 300 and 200 mL/min with concentrations 4 and 1 mmol/L. Complete mixing without local exchange gives
$$
Q=500\,\mathrm{mL\,min^{-1}},\qquad
C=\frac{300(4)+200(1)}{500}=2.8\,\mathrm{mmol\,L^{-1}}.
$$
The unweighted mean 2.5 would ignore how much blood each branch contributes. Multiplying the mixture concentration by total flow recovers the incoming substance rate, $0.500\times2.8=1.4$ mmol/min.
These stipulated values do not represent reference human flows or a complete map of pelvic drainage. A specimen taken before mixing can differ from the calculated average, and collateral routes can change which flows enter a selected vessel. Anatomical continuity defines the regional question; <blood flow> and <conservation of mass> determine how its transported substances combine.
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