116 Seymour 2004 Snake tilting

116 Seymour 2004 Snake tilting - 1305 The Journal of...

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1305 When a terrestrial animal assumes an upright posture, gravity tends to pull blood away from the head toward the lower parts of the body and superimposes a hydrostatic pressure gradient on the circulatory system. Because this gradient is related to the absolute vertical height of the blood columns, the effect becomes more pronounced in larger or longer animals. To maintain perfusion pressure in the head, the system raises central arterial blood pressure by a combination of changes in heart rate, stroke volume and peripheral resistance, the so-called baroregulatory reflexes (Gauer and Thron, 1965). The ability of the heart to increase cardiac output is influenced by the arterial pressure that it must produce and the amount of blood available to it (the ‘venous return’). In upright posture, the arterial hydrostatic pressure gradient increases, and the venous return initially decreases, because blood collects in distended vessels below the heart. Stability of cephalic blood pressure, and hence blood flow, therefore partly depends on the vertical height of the head above the heart and blood pooling below it. Animals subject to orthostatic problems have evolved morphological and physiological adaptations that help maintain circulation to the head in the upright posture (Lillywhite, 1995). These include a characteristically high arterial blood pressure, a heart closer to the head, powerful vasomotor responses and stiffer vessels and surrounding tissues in lower parts of the body. The best examples come from studies of the giraffe (Hargens et al., 1987) and snakes (Lillywhite, 1996; Young et al., 1997). Snakes, in particular, are instructive because they have evolved in aquatic, terrestrial and arboreal habitats in which gravity has vastly different influences on the cardiovascular system. Aquatic snakes are in an essentially gravity-free environment because the hydrostatic pressure gradient of the medium approximately equals that in the vascular system; terrestrial snakes are subject to gravity, especially in arboreal species that climb vertically. Resting systemic arterial blood pressures in horizontal snakes is lowest in aquatic species and increases progressively in semi-aquatic, terrestrial and arboreal species (Seymour and Lillywhite, 1976). Arterial blood pressure increases in relation to head–heart distance in terrestrial species, and the heart moves relatively closer to the head from aquatic through terrestrial and arboreal species (Seymour, 1987). Terrestrial and arboreal species show more effective baroreflexes in response to tilting (Lillywhite and Donald, 1994; Lillywhite and Pough, 1983; Lillywhite and Seymour, 1978; Seymour and Lillywhite, 1976; Young et al., 1997), and they have more effective mechanisms to prevent blood pooling and oedema in the dependent end, including less compliant vessels or tissues, narrower bodies The Journal of Experimental Biology 207, 1305-1311 Published by The Company of Biologists 2004 doi:10.1242/jeb.00882 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.
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116 Seymour 2004 Snake tilting - 1305 The Journal of...

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