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Matthew Bonnan wrote:
..."When I get a chance, if anyone's interested,we can talk about
sauropod feet and how absolutely wierd (and wonderful) they are."
Matthew, I, for one, would love to read about biomechanics of sauropod
feet. Do you know what the names of the sauropod limb muscles are? Do
you know their homologous human equivalents? I can't find this info
anywhere (reply off list if you want).
Bill Adlam wrote:
..."I thought a blood pressure below atmospheric pressure would make the
arteries and veins collapse,"
Bill, those who disagree with the siphon hypothesis have voiced similar
concerns about the negative pressure. Let me give you a few data points
and then the siphon proponents' answer. In a giraffe right atrial
pressure in a standing individual is essentially atmospheric; left
ventricular pressure is 270/150 or so. Carotid artery pressure 40cm.
below the jaw line when the animal is lying flat is 280/180, while
standing it dropped to 160/110. It was estimated that caroid pressure
at the base of the skull at this time is 125/75. Data on standing
jugular pressure show "a nearly atmospheric pressure gradient along the
length of the jugular vein." Unfortunately I could not find any data on
postural changes in the jugular venous pressure other than " it varied
in the same direction as the distal carotid pressure." Siphon
proponents, using a model of an inverted U tube with a collapsable
descending limb (representing the jugular vein), explain venous pressure
as being the sum of viscous flow (frictional) pressure and gravitational
(hydrostatic) pressure. The balance between these two pressures
determines the amount of collapse and resistance of the vein.
Gravitational pressure is a product of fluid density, gravitational
acceleration, and height of column, and is lower at the top of the
inverted U tube (where it is significantly below atmospheric) and higher
at the bottom of the column. Viscous flow pressure (a function of
viscosity of the liquid and the diameter and length of the tube) turns
out to be the opposite--higher at the top, lowest at the bottom. When
you add the two pressures together they counterbalance each other and
give you a jugular vein with an essentially atmospheric pressure which
doesn't collapse. Pressure measurements in model tubes support this
reasoning. I hope I have explained this right, because, to be honest,
it is all still somewhat fuzzy to me. Physics has never been my strong
suit. My own problem with these models is they don't allow for a
capillary bed, but perhaps this doesn't make a difference. --Ken Clay,