Independent effects of hearthead distance and caudal blood pooling on blood pressure regulation in aquatic and terrestrial snakes
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
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Summary |
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Key words: blood pressure regulation, gravity, orthostasis, snake, tilting, Liasis fuscus, Acrochordus arafurae
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Introduction |
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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
headheart 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 and behavioural responses that facilitate venous
return (Lillywhite,
1985a
,b
,
1987b
,
1993a
). Aquatic snakes and some
non-climbing vipers have such poor baroregulation that blood pressure in the
head becomes negative, and circulation ceases in head-up tilting in air
(Seymour and Lillywhite, 1976
;
Lillywhite, 1993b
;
Young et al., 1997
; R. S.
Seymour and J. O. Arndt, unpublished). The result of this research is a
picture of evolutionary loss of pressure-regulating mechanisms as snakes
invaded the gravity-free aquatic habitat, and enhancement of these mechanisms
as they began to climb in trees.
A remaining question, however, concerns the relative influences of the
vertical blood column in arteries above the heart, and blood pooling in the
dependent vasculature, on blood pressure regulation in snakes from different
habitats. Specifically, is blood pressure in the head of a snake more
influenced by the headheart distance or by blood pooling and decreased
venous return during head-up tilting? The shape of snakes offers a unique way
of answering this question by permitting tilting of the anterior and posterior
segments independently. By bending the snake at the heart, the head can be
raised, or the tail lowered, to separate the two influences. This study
compares the results from pythons (Liasis fuscus Peters; Boidae) and
file snakes (Acrochordus arafurae McDowell; Acrochordidae). Although
commonly called the `water python', L. fuscus is essentially
terrestrial, frequenting floodplains and feeding on rodents and birds, while
A. arafurae is totally aquatic in fresh and brackish estuaries, where
it feeds on fish (Shine,
1993).
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Materials and methods |
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The animals were anaesthetised by cooling their body temperature below 5°C in a bed of chipped ice, preventing direct contact between the head and the ice. They were weighed and measurements made of body length and heart position, as determined by palpation. Small (2 cm) incisions were made in the ventrolateral body wall, between the midline and the ends of the ribs. Appropriately sized PVC catheters were fixed in a major artery and vein. The aorta was occlusively catheterised just anterior to the vent. Collateral circulation was sufficient to maintain perfusion of the tail, which retained mobility and sensation after the operation. A non-occlusive catheter was placed in the inferior vena cava, by passing it through the wall toward the heart and fixing it in place with a purse-string ligature and cyanoacrylate tissue adhesive. Both catheters passed out of the body through a puncture away from the incision, and enough loose tubing was left inside the body to accommodate shifts in the viscera. The sites of the catheter tip and the location of the heart were marked on the skin with a permanent marking pen. The catheters were flushed with 0.85% saline (heparinised 250 units ml1) and sealed.
Catheterised snakes were transferred to the tilting apparatus for recovery
overnight. They were restrained in clear acrylic tubes of appropriate size to
allow free ventilation but to prevent coiling and reversing direction. Each
tube had a full-length slit for passage of the catheters, and it was divided
by a 10 cm gap at the location of the snake's heart so that the animal
could be flexed at this point to raise or lower the head or tail while
maintaining the rest of the body horizontal
(Fig. 1). To prevent escape
through the gap, the entire snake was placed inside a loose hosiery sleeve,
inside the acrylic tube and attached to its distal ends. We found that the
snake's ventilation was affected if the sleeve contacted its head, so this was
prevented with a short length of rigid tube inside the sleeve around the
head.
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The restraining tube was attached to a bar that pivoted vertically in the middle of a large board equipped with an indicator of tilt angle. Half of the tube could be clamped horizontally while the other half was tilted. Like the two hands of a clock, therefore, the two halves of the snake could be oriented independently around the pivot point at the heart. We chose tilt angles of 30°, 45° and 70°. Because the effect of tilting relates to the sine of the tilt angle, these angles represented an added hydrostatic pressure equivalent to 50%, 71% and 94%, respectively, of that present at the bottom of a vertical column of blood equal in height to the length to the tilted segment. Thus, a 70° tilt had practically the same effect as a 90° tilt but did not bend the animal at a stressful angle.
The protocol consisted of tilting the snake to the desired angle within
5 s, maintaining the tilt for 2 min and then returning to horizontal for
at least 2 min. In some cases, the pressure transducer was attached to the
tube being tilted, so measured blood pressure at the site of the catheter
required conversion to pressure at heart level by calculation from the tilt
angle and the linear distance between the transducer and the heart. Density of
blood was assumed to be 1.05 kg l1. In other cases, when the
catheter was long enough, the transducer was placed at heart level. Tilting
often slightly shifted the location of the snake, so differences in elevation
were noted and used to correct pressure to heart level.
Blood pressure was measured with Gould-Statham P23 transducers [Grass-Telefactor (Astro-Med Inc.), West Warwick, RI, USA] connected to Grass Model 7P1E low-level DC preamplifiers and Model 7DAF driver amplifiers within a Model 79D oscillograph (Grass-Telefactor). The signal was also recorded rectilinearly on a flat bed recorder, from which the reported data were taken. The transducers were calibrated with a water manometer before every experiment, and data were converted to the traditional units of mmHg for ease of understanding (1 mmHg=13.6 mmH2O=133 Pa). Mean arterial blood pressure was taken as diastolic blood pressure plus one-third of the difference between systolic and diastolic blood pressure.
All measurements were made on healthy, conscious animals at body temperatures between 22°C and 27°C. After the tilting experiments, the animals were euthanized with an overdose of the anaesthetic Hypnomidate (Janssen GmbH, Neuss, Germany) and inspected to confirm the location of catheters and the heart.
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Results |
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Pythons showed high systemic arterial pressures and crisp regulation during straight, head-up tilts (Fig. 2). Immediately upon tilting, there was a large drop in CVP, but CAP remained almost normal because of a rapid increase in heart rate. CVP generally remained positive and increased during the tilting period as heart rate decreased. This is interpreted as an immediate caudad flow of venous blood that reduced cardiac filling. Increased heart rate compensated for this while the posterior circulation filled with blood and reflexogenic venous constriction gradually raised CVP and allowed the heart rate to decrease.
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Straight head-up tilts in file snakes resulted in a different pattern of
regulation (Fig. 3). Tilting
immediately resulted in a fall in CVP, tachycardia and an initial rise in CAP.
Then CAP began to fall back to normal, and the rises in CVP were small. In
some cases, CVP became negative but recovered to positive values after 1
min at 30° and 45° tilts. When CVP became positive, CAP stabilised at
levels not greatly below normal. However, at 70°, CVP remained negative
and CAP continued to fall. A loss in pulse pressure in CVP trace indicates
that the blood in the posterior veins had lost connection with the heart.
Pulse pressure almost disappeared in the CAP trace at 2 min, but CAP remained
positive.
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The temporal patterns of blood pressure reflexes were consistent within each species, but there were large differences between species. Blood pressure stabilised after 2 min of full and partial tilting in both species, and the separate effects of hearthead distance and blood pooling can be observed in the mean values from all snakes. CAP in pythons was much higher than in file snakes in the horizontal orientation (Fig. 4). In pythons, CAP tended to rise during head-up tilts and fall during tail-down tilts but did not change during full tilts (Fig. 4; Table 2). In file snakes, CAP increased significantly during head-up tilts but did not change consistently under other conditions.
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HAP of individual snakes were calculated from the vertical distance between the heart and the head, ignoring any pressure drop due to viscous resistance. Consequently, these were identical to CAP in horizontal or tail-down tilted snakes (Figs 4, 5). HAP always fell significantly in both pythons and file snakes during head-up and full tilts but decreased significantly in pythons only during tail-down tilts to 30° and 45° (Fig. 5; Table 2). Nevertheless, HAP remained relatively high in pythons at all angles but dropped to much lower values in file snakes. File snake HAP became negative in full tilt at 45° and 70° and reached zero in head-up tilts to 70°.
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CVP was positive and increased slightly during head-up tilts, but it decreased to values insignificantly different from zero during tail-down or full tilts (Fig. 6). In pythons, CVP could become negative, but pulse pressures were always evident. In file snakes, however, CVP pulse pressures disappeared at full tilts to 45° and 70°.
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Heart rates were significantly higher in pythons than in file snakes resting horizontally (Fig. 7). In pythons, heart rate did not change much, increasing significantly only at 70° tail-down and full tilting; in file snakes, heart rate increased significantly at all angles of tail-down and full tilting (Fig. 7; Table 2). Within each species, there was no significant difference between full and tail-down tilts (Table 2). There was also no change in heart rate during head-up tilting in pythons, and a slight significant rise in file snakes at 45°.
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Partial and full tilting in the same animals permitted partitioning of the effects of arterial blood column and blood pooling, shown for example in 45° tilting data. In horizontal pythons, mean CAP and HAP were 73 mmHg, and CVP was 5 mmHg (Fig. 8). With a head-up partial tilt, CAP and CVP were unchanged, but HAP decreased 27%, to 53 mmHg. With a tail-down partial tilt, CAP decreased to 64 mmHg, CVP went to approximately zero but the decrease in HAP was only 14%. In full tilt, however, CAP was 65 mmHg, CVP went to zero and HAP dropped 42%, to 42 mmHg. Thus, the decreases in HAP brought about by partial tilts (27+14%=41%) approximately added up to the decrease resulting from full tilt (42%) and showed that the vertical distance to the head was about twice as important as blood pooling. The sum of changes to HAP in partial tilts was also close to the change in full tilts to 30° and 70° in pythons.
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The effects of tilting were more extreme in file snakes. Horizontal snakes had a mean CAP and HAP of 24 mmHg, and a CVP of 2 mmHg (Fig. 9). Upon head-up partial tilting to 45°, CAP increased to 29 mmHg, HAP dropped 79%, to 5 mmHg, and CVP rose to 3 mmHg. Tail-down partial tilting resulted in CAP and HAP decreasing only 17%, to 20 mmHg, while CVP became zero. Full tilting resulted in CAP remaining at 21 mmHg, and a calculated value of 3 mmHg for HAP. In this case, the vertical distance to the head was over four times more important than blood pooling in diminishing head blood pressure.
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Discussion |
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Totally aquatic species are protected from the effects of gravity while in
water. This is correlated with hearts closer to the centre of the body
(Seymour, 1987) and inherently
low arterial blood pressure (Seymour and
Lillywhite, 1976
), both of which reduce the energy cost of
circulation. When the animals are removed from water and tilted, the
consequences are obvious and dire. A long hearthead distance, coupled
with a low central arterial pressure, causes head arterial pressure to
decrease greatly, with calculated values even below zero. This is shown in
file snakes in head-up partial tilts to 70°
(Fig. 5). In this case, there
is no blood pooling, yet blood flow to the head apparently ceases, because
unprotected arteries and microvessels close under negative blood pressure.
Blood flow in the carotid artery of snakes has been shown to cease when the
central arterial blood pressure equals or falls below the equivalent of the
hydrostatic blood column above the heart (Lillywhite, 1993;
Lillywhite and Donald, 1994
).
On its own, blood pooling in file snakes has little influence on head arterial
pressure, but when combined in a full tilt exacerbates the problem by causing
head arterial pressure to become negative at lower tilt angles
(Fig. 5). Pooling appears to
increase during a full tilt, probably because dependent vasculature is further
distended by increased hydrostatic pressures generated in longer vertical
blood columns in arteries and valve-less veins
(Lillywhite, 1987a
).
It has been proposed that the anterior heart placement in terrestrial and
arboreal snakes is not related to the hearthead distance but is related
to the filling pressure of the systemic veins
(Badeer, 1998). The principle
of the siphon is thought to make blood flow to the head independent of the
height of the vascular loop. This notion is derived from the idea that the
heart does not work against gravity in the circulatory system but works only
against viscous resistance (Hicks and
Badeer, 1992
), although the idea and its experimental evidence
have been shown to be seriously flawed
(Pedley et al., 1996
;
Seymour et al., 1993
).
Nevertheless, Badeer proposes that the hearts of arboreal snakes are closer to
the head because they can fill better in this location when the animal is
tilted head-up (Badeer, 1998
).
This appears to be false, because the heart would fill even better if it were
further back in the body, where systemic venous pressures are higher. If
filling pressure affected heart placement, then there would be no selective
advantage of an anteriorly placed heart. On the other hand, if a given level
of arterial blood pressure were necessary at the entrance to vascular beds in
the head in order to perfuse them, then an anterior heart would have to
produce less pressure and consequently expend less energy.
The effects of blood pooling on the heart are mediated through a fall in CVP in the post-cava (Fig. 6). In horizontal snakes, CVP was slightly positive in file snakes and more so in pythons. Despite the long post-heart length in pythons (Table 1), they regulated CVP well and it dropped to about zero only during tail-down partial tilts (Fig. 6). Nevertheless, pulse pressures were always evident in pythons, which is evidence that they continued to fill the heart from posterior vessels. In file snakes, on the other hand, calculated CVP went slightly negative in acute tail-down and full tilts and pulse pressure oscillations disappeared (Figs 2, 6). This might indicate that circulation to the posterior part of the snake ceased, but it is apparent that venous return from the posterior parts in fact continued, even at CVP down to about 2 mmHg and loss of venous pulse pressure, because central arterial pressure remained substantially positive (Fig. 4), and arterial pulse pressure oscillations were evident (Fig. 3).
It is interesting that heart rate changes were more pronounced during full
tilts and tail-down tilts than head-up tilts
(Fig. 7), despite considerable
falls in arterial blood pressure in the neck and head (Figs
8,
9). This implies that the
effective site of baroreception lies near the heart rather than further up the
neck. It is also consistent with morphological and physiological evidence of
baroreceptors in the truncus arteriosus and central arteries of snakes
(Lillywhite and Donald,
1994).
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Acknowledgments |
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References |
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Badeer, H. S. (1998). Anatomical position of heart in snakes with vertical orientation: a new hypothesis. Comp. Biochem. Physiol. A 119,403 -405.
Gauer, O. H. and Thron, H. L. (1965). Postural changes in the circulation. In Circulation, vol.3 (ed. W. F. Hamilton and P. Dow), pp.2409 -2439. Washington, DC: American Physiological Society.
Hargens, A. R., Millard, R. W., Pettersson, K. and Johansen, K. (1987). Gravitational haemodynamics and oedema prevention in the giraffe. Nature 329, 59-60.[CrossRef][Medline]
Hicks, J. W. and Badeer, H. S. (1992). Gravity and the circulation: "open" vs. "closed" systems. Am. J. Physiol. 262,R725 -R732.[Medline]
Lillywhite, H. B. (1985a). Behavioral control of arterial pressure in snakes. Physiol. Zool. 58,159 -165.
Lillywhite, H. B. (1985b). Postural edema and blood pooling in snakes. Physiol. Zool. 58,759 -766.
Lillywhite, H. B. (1987a). Circulatory adaptations of snakes to gravity. Am. Zool. 27, 81-95.
Lillywhite, H. B. (1987b). Tissue free fluid pressures in relation to behavioral and morphological variation in snakes. Am. Zool. 27,117A .
Lillywhite, H. B. (1993a). Subcutaneous compliance and gravitational adaptation in snakes. J. Exp. Zool. 267,557 -562.[Medline]
Lillywhite, H. B. (1993b). Orthostatic intolerance of viperid snakes. Physiol. Zool. 66,1000 -1014.
Lillywhite, H. B. (1995). Evolution of cardiovascular adaptation to gravity. J. Gravit. Physiol. 2,1 -4.
Lillywhite, H. B. (1996). Gravity, blood circulation, and the adaptation of form and function in lower vertebrates. J. Exp. Zool. 275,217 -225.[CrossRef][Medline]
Lillywhite, H. B. and Donald, J. A. (1994). Neural regulation of arterial blood pressure in snakes. Physiol. Zool. 67,1260 -1283.
Lillywhite, H. B. and Pough, F. H. (1983). Control of arterial pressure in aquatic sea snakes. Am. J. Physiol. 244,R66 -R73.[Medline]
Lillywhite, H. B. and Seymour, R. S. (1978). Regulation of arterial blood pressure in Australian tiger snakes. J. Exp. Biol. 75,65 -79.[Abstract]
Pedley, T. J., Brook, B. S. and Seymour, R. S. (1996). Blood pressure and flow rate in the giraffe jugular vein. Philos. Trans. R. Soc. Lond. B 351,855 -866.[Medline]
Seymour, R. S. (1987). Scaling of cardiovascular physiology in snakes. Am. Zool. 27, 97-109.
Seymour, R. S., Hargens, A. R. and Pedley, T. J. (1993). The heart works against gravity. Am. J. Physiol. 265,R715 -R720.[Medline]
Seymour, R. S. and Lillywhite, H. B. (1976). Blood pressure in snakes from different habitats. Nature 264,664 -666.[Medline]
Shine, R. (1993). Australian Snakes: A Natural History. Chatswood, NSW: Reed.
Young, B. A., Wassersug, R. J. and Pinder, A. (1997). Gravitational gradients and blood flow patterns in specialized arboreal (Ahaetulla nasuta) and terrestrial (Crotalus adamanteus) snakes. J. Comp. Physiol. B 167,481 -493.[Medline]