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(sigh)sauropod necks again--long!



Okay, I know you were all hoping this thread was dead and buried but I
got stuck in the local university library over the weekend and looked up
a few papers on giraffe necks, which the more compulsive physiology
buffs out there may find interesting. May the gods help me, I will try
to summarize.  There seems to be two schools of thought on the subject:
1) The traditional:  Giraffes have derived  high blood pressures in
order for the heart to drive blood up the long neck to counteract
gravitational forces which are lowering neck arterial pressure, and 2) a
siphon principle is functioning in giraffe necks; that although
gravitational forces lower the blood pressure of the neck arteries, they
are also lowering the pressure of the neck veins and thus perfusion
pressure (arterial pressure minus venous pressure) is unchanged.  Along
with this it is noted that the cerebral veins and arteries are
surrounded by cerebrospinal fluid in a rigid cranium and that these
vessels do not collapse from postural changes (which alter transmural
pressure)--when upright the cerebral vessels are subject to low
transmural pressure within the cranium--the cerebrospinal fluid is
affected by gravity as well.  The conclusion: "It is no harder, in the
circulation, for blood to flow uphill than downhill," and that the heart
does not have to spend extra energy to raise the blood to the head.
Bored? Confused? Good, now for more details.
So how do postural changes affect circulation?  Why does your hand turn
pale and get fatigued easily when you raise it over your head for
awhile?  The siphon people say it is not because the direction of
arterial flow is against gravity but because of gravitational effects on
the transmural pressure of extra-vascular tissue fluid on collapsable
vascular structures.  The arteries and arterioles are more muscular and
less distensble and are affected to only a minor extant. The venous
system is the most distensible and is affected the most.  The
gravitational effects alter vascular resistance and subsequent flow.  If
the veins collapse completely then a siphon effect cannot take place.
They say that the reason a giraffe's neck does not tire like your raised
arm is that the jugular veins only partly compress in the giraffe, thus
maintaining the siphon.  Attachment of  the veins in the head and neck
to the surrounding structures by collagenous tissue would tend to reduce
the amount of collapse.  Also, the reason giraffes have high blood
pressures is not to drive the blood to the brain but to minimize the
collapse of the vessels of the head and neck in the upright position.
The mechanism by which giraffes maintain this high blood pressure is by
having long legs with thick arteries and arterioles containing very
narrow lumina (an adaption to high gravitational pressure as a way of
preventing edema, or swelling of the legs, from a high arterial
pressure) thus increasing peripheral resistance.  Baroreceptors in the
neck may induce a high sympathetic tone to the peripheral vessels which
would also keep blood pressure raised.
Okay, we're almost home. If you've made it this far pat yourself on the
back.
The traditional people counter that with terrestrial and arboreal
snakes, blood pressure increases in proportion to the length between the
heart and head, and the heart mass of longer species is larger than
predicted from body mass.  They also argue that a siphon that involves
very low pressures in the veins of the head could not function in
animals, even if their veins were rigid.  If the blood pressure in a
vascular bed were lower than the tissues surrounding it, low capillary
blood pressure would result in a net reabsorption of tissue fluids,
i.e., the brain would dehydrate.  They conclude: "A heart capable of
producing adequate perfusion to the head does not need a siphon to help
get the blood up there." In other words, the heart must work harder to
pump the blood up a long neck.
Who's right?  Actual measeurements of giraffe vein/artery pressures are
used by both sides to bolster their arguments.  The siphon principle is
championed by a physiologist named Bader at Creighton who has writtern
the most on the subject, and has constructed numerous inverted U tube
models to support his theory.  The way I see it, if the siphon people
are right, the heart is still going to have to pump harder in animals
with a long neck in order to fight the increased total peripheral
resistance caused indirectly by the long neck.  Having said that, I
still think that sauropods could rear up on their hind legs, with or
without siphon.  Any clarifying comments out there on this subject are
welcome. --Ken Clay, M.D.