= External iliac vein
An \b[<external iliac vein>] continues the <femoral vein> proximally as lower-limb venous return passes beneath the <inguinal ligament>. It joins the <internal iliac vein> to form the <common iliac vein>, which connects the route with the <inferior vena cava>.<Anatomy and Physiology 2e by J. Gordon Betts and others>[§20.5, Veins Draining the Lower Limbs, external/internal/common iliac sequence]{ref} External distinguishes this branch from the pelvic internal route; it does not mean that the vessel lies outside the body.
Following the pathway separates a route name from a measured quantity. The <external iliac vein> can transport a changing <blood flow>, and its local velocity also depends on lumen area. A velocity observation is therefore not by itself a volumetric-flow measurement.
In an original incompressible-flow example, let $Q=1.50$ litres per minute and lumen area $A=0.75\,\mathrm{cm^2}$. Mean axial velocity is
$$
\bar v=\frac QA=\frac{1500/60}{0.75}\,\mathrm{cm\,s^{-1}}
\approx33.3\,\mathrm{cm\,s^{-1}}.
$$
If the area increases by 25 percent while flow remains fixed, mean velocity falls to $33.3/1.25\approx26.7$ cm/s. The vessel can carry the same volume per minute with a different mean speed. The dimensions and flows are illustrative rather than measured anatomical norms.
This distinction also matters when comparing measurements taken at different locations. At a junction, flow changes by the added tributary contributions, while vessel geometry may change at the same time. Defining the observation site and geometric model keeps an anatomical statement about lower-limb return separate from a quantitative estimate of that return.
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