 External iliac vein

An 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.^[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 $A=0.75\mathrm{c}{\mathrm{m}}^2$. Mean axial velocity isIf the area increases by 25 percent while flow remains fixed, mean velocity falls to $33.3/1.25\approx 26.7$ 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.