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First published online March 9, 2004
Journal of Experimental Biology 207, 1305-1311 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00882

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Independent effects of heart–head distance and caudal blood pooling on blood pressure regulation in aquatic and terrestrial snakes

Roger S. Seymour^1,* and Joachim O. Arndt²

¹ Environmental Biology, University of Adelaide, Adelaide, SA 5005, Australia
² Department of Experimental Anaesthesiology, Heinrich-Heine-Universität, Universitätsstrasse 1, 40225 Düsseldorf, Germany

^* Author for correspondence (e-mail: roger.seymour{at}adelaide.edu.au)

Accepted 19 January 2004

Changes in orientation in a gravitational field markedly alter the patterns of blood pressure and flow in animals, especially tall or long ones such as giraffes or snakes. Vertical orientation tends to reduce blood flow and pressure in the head for two major reasons. First, the increased vertical blood column above the heart creates a gravitational hydrostatic pressure against which the heart must work. Second, expansion of dependent vessels in the lower extremities causes blood pooling and reduces return of venous blood to the heart, thereby lowering flow and pressure. For most animals, it is difficult to separate these two effects, but snakes offer the possibility of bending the animal in the region of the heart and manipulating the two ends of the body independently. We studied baroregulatory responses in terrestrial pythons (Liasis fuscus) and aquatic file snakes (Acrochordus arafurae) by tilting only the front or rear parts and then the whole animal. Changes in head blood pressure during partial tilts added up to the change during full tilt. The vertical distance to the head had twice as much influence on head blood pressure than did blood pooling in the pythons and four times as much in file snakes. This accounts for the cephalad location of the heart in terrestrial species compared with aquatic ones.

Key words: blood pressure regulation, gravity, orthostasis, snake, tilting, Liasis fuscus, Acrochordus arafurae