Experimental measurements in the jugular veins of upright giraffes have shown that the internal pressure is somewhat above atmospheric and increases with height above the heart. A simple model of steady viscous flow in an inverted U-tube shows that these observations are inconsistent with a model in which the blood vessels in the head and neck are effectively rigid and the system resembles a siphon. Instead, the observations indicate that the veins are collapsed and have a high resistance to flow. However, laboratory experiments with collapsible drain tubing in place of the down arm of the U-tube show internal pressure to be exactly atmospheric and uniform with height. A model of viscous flow in a collapsible tube with non-uniform properties is used to suggest that the observed pressure distribution may be a consequence of the intrinsic cross-sectional area and/or compliance of the veins increasing with distance towards the heart, or the external, tissue pressure falling. Finally, the effect of fluid inertia on steady flow in vertical collapsible tubes with uniform intrinsic properties is analysed, and it is shown that a phenomenon of flow limitation is theoretically possible, in which the supercritical flow in the collapsed vein cannot return to the presumably subcritical flow in the open vena cava, even with the help of an 'elastic jump', if the flow rate is too large. The computed critical flow-rate, of about 80 ml s-1, is about twice the flow-rate estimated to be present in the normal giraffe jugular vein. If there were circumstances in which flow limitation occurred in the jugular veins, it would mean that the cerebral blood flow would be limited by downstream conditions, not directly by local requirements.
