[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
Re: Mamenchisaurus Posture Paper
> What do you think of the idea that various tricks might have helped? Valves
> in the neck arteries? Perhaps even contractile arteries?
Our 2000 paper on sauropod necks dispels all previous attempts to help
the heart pump blood to the brain. This includes accessory hearts in
the neck. These would have had to evolve from contractile walls and
valves in the arteries to work. Arteries do have muscular walls, but
none has valves outside of the heart (they would serve no purpose in
arteries). Even if they had valves, it is difficult to imagine how a
less than fully functioning hearts would evolve and how it would have
been coordinated with the main heart. Probably the most appealing aid
is the syphon principle, championed by Jim Hicks and Henry Badeer. But
we have disproved that on intuitive, hydrodynamic, fluid mechanics and
modelling grounds. All hearts must be able to develop enough pressure
to support the arterial blood column above the heart. If they didn't
the arotic valve would never open. See:
67. Seymour, R.S., A.R. Hargens and T.J. Pedley. 1993. Invited
Review: The heart works against gravity. Amer. J. Physiol. 265
(Regulatory Integrative Comp. Physiol. 34): R715-R720.
78. Pedley, T.J., B.S. Brook and R.S. Seymour. 1996. Blood pressure
and flow rate in the giraffe jugular vein. Phil. Trans. R. Soc. Lond. B
97. Seymour, R.S. 2000. Model analogues in the study of cephalic
circulation. Comp. Biochem. Physiol. 125:517-524.
> Ignorant question: How does heart power scale with heart size? Muscle
> strength is supposed to scale with muscle cross-sectional area, and this
> should scale with the... volume of the heart? In this case the absolute size
> of the heart would merely need to scale with the height of the head above
> the heart. This in turn would mean that there would be a limit on relative
> but not absolute head-above-heart height (as long as the blood pressure
> wouldn't blow anything apart, of course). Is it this convenient?
No question is ignorant. Failure to ask questions is ignorant.
I needed good data on what vertebrate cardiac muscle is capable of in
order to write the sauropod neck paper. The data in this paper were
collected over 25 years:
99. Seymour, R.S. and A.J. Blaylock. 2000. Invited perspective: The
principle of Laplace and scaling of ventricular wall stress and blood
pressure in mammals and birds. Physiol. Biochem. Zool. 73:389-405.
It answers your question. In general, the work of a ventricle over one
cycle is proportional to the muscle mass. Work depends on the blood
volume inside and the pressure required in the systemic circulation
(which depends on the height above the heart). All of this is related
to the Law of Laplace which says that the thickness of walls of a vessel
are related to internal volume and pressure. Pressure at the base of a
blood column is related to the absolute vertical distance. It does not
depend at all on the size of the blood vessels. Thus, the pressure is
> > Two meters above the heart would probably have
> > been about the limit. That's what giraffes do.
> Isn't that more like three meters, or perhaps four in big bulls?
I believe that the height of a bull giraffe is less than 5 m in total.
The heart is about half-way up, leaving about 2.5 m for the neck. (It
is highly significant that the giraffe attains about half of its height
with its legs. I believe that it is important in reducing the need for
an even heavier heart if it had relatively short legs and a longer
neck. Sauropods didn't appear to take the long legged approach.
Probably it's because they didn't raise their heads high.)
Roger S. Seymour
Environmental Biology (Darling Building D418)
School of Earth and Environmental Sciences
University of Adelaide
Adelaide, SA 5005, Australia