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First published online June 15, 2006
Journal of Experimental Biology 209, 2515-2524 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02277

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

Graham Mitchell^1,*, Shane K. Maloney^2,3, Duncan Mitchell³ and D. James Keegan³

¹ Department of Zoology and Physiology, University of Wyoming, 1000 E University Avenue, Laramie, WY 82071, USA,
² Physiology: Biomedical and Chemical Science, University of Western Australia, Perth, Australia
³ Department of Physiology, University of the Witwatersrand, Johannesburg, South Africa

^* Author for correspondence (e-mail: mitchg{at}uwyo.edu)

Accepted 18 April 2006

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. Pumping water through the model produced positive pressures in the `carotid' limb similar to those found in giraffe. Applying an external `tissue' pressure to the `jugular' tube during pump flow produced the typical pressures found in the jugular vein in giraffe. Constriction of the lowest, `jugular cuff', portion of the `jugular' limb showed that the cuff may augment the orthostatic reflex during head raising. Except when all tubes were rigid, pressures were unaffected by a siphon.

We conclude that mean arterial blood pressure in giraffes is a consequence of the hydrostatic pressure generated by the column of blood in the neck, that tissue pressure around the collapsible jugular vein produces the known jugular pressures, and that a siphon does not assist flow through the cranial circulation.

Key words: giraffe, siphon, cranial circulation

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