Is the flow in the giraffe's jugular vein a “free” fall?

The origin of mean arterial and jugular venous blood pressures in giraffes

“…This idea was confirmed (Badeer, 1997) by using a collapsible tube as the siphon tube in the descending limb of a model. Flow through the system was greater than if there was no descending limb ('free fall' flow).…”

“…The origin of these pressures is controversial. The controversy has been reviewed at least six times (Badeer, 1986;Seymour and Johansen, 1987;Pedley, 1987;Badeer, 1988;Badeer and Hicks, 1992;Seymour et al, 1993) and evaluated empirically five times using mechanical models (Holt, 1959;Hicks and Badeer, 1989;Pedley et al, 1996;Badeer, 1997;Seymour, 2000).…”

“…This lower than predicted average pressure may be because some of the animals were anaesthetized at the time of measurement, or were holding their heads at an average angle less than vertical, or did not have two meter long necks, but it is also possible that mechanisms exist that reduce the work of the heart. Thus, Badeer (Badeer, 1986;Badeer, 1988;Badeer, 1997) and Hicks and Badeer (Hicks and Badeer, 1989) have suggested that as the giraffe cranial circulation can be regarded as an inverted U-tube that functions as a siphon, gravitational effects are neutralized and the high pressure results from high Using a mechanical model of the giraffe neck and head circulation consisting of a rigid, ascending, 'carotid' limb, a 'cranial' circulation that could be rigid or collapsible, and a descending, 'jugular' limb that also could be rigid or collapsible, we have analyzed the origin of the high arterial and venous pressures in giraffe, and whether blood flow is assisted by a siphon. When the tubes were rigid and the 'jugular' limb exit was lower than the 'carotid' limb entrance a siphon operated, 'carotid' hydrostatic pressures became more negative, and flow was 3.3·l·min -1 but ceased when the 'cranial' and 'jugular' limbs were collapsible or when the 'jugular' limb was opened to the atmosphere.…”

“…Applying an external peripheral resistance. Badeer (Badeer, 1997) further suggested that high peripheral resistance is a consequence of arteries with small lumens plus high sympathetic nervous system-mediated vasoconstriction, the latter resulting from the absence of a functional baroreceptor-mediated depressor mechanism.…”

Neck length and mean arterial pressure in the sauropod dinosaurs

“…In contrast, experiments performed by Goetz et al (1960) have shown that when giraffes raise their head, the increase in systolic pressure is not enough to compensate for the change in orthostatic pressure. Badeer (1988Badeer ( , 1997 has shown that the high aortic pressure of the giraffe is required not to overcome gravitational pressure but rather to 'push' against the high peripheral resistance of the systemic circulation. This effect largely cancels the gravitational reduction in venous pressure so that the venous pressure stays close to atmospheric pressure without collapse of the veins (Badeer, 1986 A recent finding relevant to this discussion is that the left ventricle of giraffe hearts is able to develop high pressure with normal wall tension at the expense of a reduced stroke volume and cardiac output (Smerup et al, 2016).…”