BLOOD-PRESSURE AND FLOW-RATE IN THE GIRAFFE JUGULAR-VEIN

Experimental measurements in the jugular veins of upright giraffes hav e shown that the internal pressure is somewhat above atmospheric and i ncreases with height above the heart. A simple model of steady viscous flow in an inverted U-tube shows that these observations are inconsis tent with a model in which the blood vessels in the head and neck are effectively rigid and the system resembles a siphon. Instead, the obse rvations indicate that the veins are collapsed and have a high resista nce to flow. However, laboratory experiments with collapsible drain tu bing 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 flo w in a collapsible tube with non-uniform properties is used to suggest that the observed pressure distribution may be a consequence of the i ntrinsic cross-sectional area and/or compliance of the veins increasin g with distance towards the heart, or the external, tissue pressure fa lling. 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 poss ible, in which the supercritical flow in the collapsed vein cannot ret urn to the presumably subcritical flow in the open vena cava, even wit h the help of an 'elastic jump', if the flow rate is too large. The co mputed critical flow-rate, of about 89 mi s(-1), is about twice the fl ow-rate estimated to be present in the normal giraffe jugular vein. If there were circumstances in which flow limitation occurred in the jug ular veins, it would mean that the cerebral blood flow would be limite d by downstream conditions, not directly by local requirements.