sauropod necks again

[Richard Ellis <rellis@idt.net>]

In response to the ongoing question about sauropod necks, I
submit an article that appeared in the British medical journal The Lancet
in 1992. Dr. Daniel S.J. Choy (senior author) can be contacted at
<DSJC@aol.com>

The cardiovascular system of barosaurus: an educated guess

D. S. J. Choy and P. Altmans

The Lancet, Vol. 340, August 29, 1992.

In 1991 the Museum of Natural History in New York City put on display a reconstructed skeleton of Barosaurus, a longnecked dinosaur that stands 15m from foot to head, Barosaurus belongs to the group of large herbivorous dinosaurs known as sauropods, which flourished in the Jurassic period, 200 to 150 million years ago.
There is some controversy about the exact posture of Barosaurus. Some claim that it raised its head no higher than the top of its back, but others argue that the shape of the facets of The cervical vertebrae indicate a weightbearing function and an erect giraffelike stance.
With its head raised Barosaurus would have been able to nibble vegetation up to 15m above ground level. But how would the animal's cardiovascular System have coped with The need to pump blood up 12m or so from the thorax to the head?
                     In the giraffe, the nearest living model of the longnecked dinosaur, Goetz et al. found the highest standing blood pressure to be 353/303 mm Hg and the lowest 260/158 mm Hg. Intraventricular pressures were 260-286 mm Hg systolic and 1018 mm Hg diastolic. The jugular veins were collapsed in the standing position. Goetz et al. postulated that a siphon effect aided arterial perfusion of the brain. Lowering the head to heart level did not result in a great change of heart rate, a finding that is in keeping with the absence of a carotid sinus in the giraffe. Of particular interest was the 1.5 cm thick skin around the neck, which was so tough that a linoleum cutter was needed to effect entry to the carotid artery and jugular vein. The viscosity of giraffe blood was found to be 4.9 times that of

water; the erythrocyte
count 11,950,000) per
m)
was similar to that of the camel and llama
and
double that of man,
indicating a greater oxygen carrying capacity. The heart of an old
bull giraffe weighed 11.3 kg; the left ventricular wall was 7.5 cm thick
and the right 2.5 cm. The jugular veins were 2.5 cm in diameter and had
tricuspid valves
as did their tributaries. When
a giraffe lowers its head to drink, the jugular system acts as a large
blood reservoir, The wall of the ascending aorta was 1.5 cm thick
and that of the pulmonary artery
0.7
cm. The walls of the carotid
arteries consisted primarily of elastic tissue, whereas the peripheral
arteries had very small lumens and very
thick
walls,  consisting mainly
of smooth muscle and little elastic tissue. This anatomical
arrangement would help the legs to withstand hydrostatic pressures of 500
mm Hg, but another contributing factor may be the thick tight skin
covering the giraffe's legs, which probably acts like strong elastic
stockings or the lateral balloons of a fighter pilot's
"G-suit"
To pump blood 12m from the thorax to the
top
of the head the heart of
Barosarus would need to achieve a systolic pressure of 12,000 mm
of water, or about 880 mm Hg. Such an enormous pressure
would
require a very large and
strong heart and very thick walls in the arterial system to prevent
rupture, Indeed, zoologist Roger Seymour (cited by Lillywhite) estimated
the heart size of large saurorpods to have been more than 1.6 metric
tonnes, or eight times that of a whale of similar size. In man, the
higher the systolic pressure, the thicker the myocardium becomes, and
pulmonary hypertension is accompanied by thickening of the walls of the
pulmonary arteries.
The larger the heart, the longer the myocardium takes to complete a
systolic contraction. The whale heart normally contracts at 30 to 40
heats per minute, compared with the  hummingbird's 300 or more. To
generate 880 mm Hg pressure, the Barosaurus heart would have had
to
be very large and very
powerful, and it
would have had to beat very
slowly because of its size. With a long diastolic interval, there would
have had to be check valves in the neck arteries supplying the brain (the
two cerebral arteries in reptiles) to prevent the column of
blood
from falling back to the heart
during diastole.
The walls of the arterial tree would have
had
to be very thick to prevent
rupture. Originally, we thought that we
could
estimate arterial size from
the vertebral artery channels in the cervical vertebrae and also
ascertain the outer diameter of carotid arteries from the carotid artery
foramens at the base of the skull. But the reptilian cerebral blood
supply differs from that of mammals in having only a single pair of
cerebral arteries, which divide into the internal carotid and a much
larger spatial artery inside the skull. There are no carotid foramens in
the reptilian skull and the channels alongside the cervical vertebrae
through which these cerebral arteries course are very large, measuring 5
cm x 30 cm at the level of C8 in Barosaurus and 13 cm
x
26 cm at C 13,
and clearly bear no relation to the size or shape of these
vessels. Thus we have no means of assessing the thickness of the cerebral
artery walls.
We have already postulated that the cerebral arteries would have had
check valves to lighten the load on the pumping mechanism. Perhaps there
were multiple pumps, in series so
that the primary pump
(heart)
would have had to
generate only sufficient pressure to drive the fluid column to the
next pump, and so on. How many pump relays would there have been? If the
heart produced a systolic pressure of, say, 200 mg Hg, five pump levels
would have been needed, We postulate that the cardiovascular system of
Barosaurus
consisted of single primary and secondary hearts and three pairs of hearts in series, each being 2.44 m higher than the one below The primary heart would be in the thorax. The second heart would be in the thoracic inlet. The next three pairs of hearts would be smaller and situated in the neck in the openings lateral to the spinal cord. With the exception of the first heart, all the hearts would be singlechambered, with valves located at the inlet and outlet of the aortic trunk.
The sinoatrial node in

the right atrium of the primary heart would initiate systolic contraction, generating a single P wave. The sinus nodes in each heart would be linked by parasympathetic and sympathetic fibres so that when the primary heart started systolic contraction the next heart would begin diastolic relaxation. When the second heart was in systole, the thirdlevel hearts would be in diastole, and so on up the chain. In this cascade each heart would have to pump blood only to its next higher neighbour. There would thus be only one P wave, originating in the primary heart All the hearts would have QRS complexes.
                 A single cerebral artery would arise from the aortic arch of the primary heart, subclavian arteries would supply the upper extremities, and the descending aorta would supply the lower body. The single cerebral artery would enter the apex of the secondary heart, which would have a oneway valve at the entry point. A single aortic trunk would arise from the secondary heart and bifurcate at the thoracic inlet into two cerebral arteries that would enter the apices of the first pair of hearts located in the lateral apices of the cervical vertebrae. These hearts would in turn send cerebral arteries to the apices of the next level of cervical hearts. The oneway valves at the entry and exit points of these hearts would act as check valves and prevent downward flow of blood, Only the primary heart would have four chambers, the right side receiving venous blood and pumping it through the pulmonary circuit for oxygenation. A set of pulmonary veins

would return the oxygenated blood to the left atrium, then to the left ventricle Each heart would be served by its own coronary tree, with the ostia located just distal to the oneway ("aortic") valves. Thus each heart would be provided with oxygenated blood. Venous blood from all the     hearts except the primary one would return to the jugular venous system through directly connecting "coronary   jugular" veins. In essence all the hearts with the exception
        of the primary heart, would be "satellite" hearts located at intermediate points of the cerebral blood flow to provide a        sequential forward pumping action. Thus, each heart level.      only has to generate enough pressure to move a column of fluid 2.44 mm high  i.e., 179 mm Hg.   

I
It is known that high systolic pressures predispose to atherosclerosis, Barosaurus would therefore have been subject to early coronary artery disease as well as stroke. Unfortunately, soft tissues are not preserved as fossils, so we can only speculate on, and not reconstruct, the cardiovascular dangers facing Barosaurus.
What happened when Barosaurus stooped to ingest a tasty morsel? Perhaps cardiac output was reduced by a slowing of the heart rate. On resumption of the erect position baroreceptors would constrict the peripheral arteries to prevent pooling, and cardiac output would increase as a result of noradrenaline release to prevent loss of perfusion to the cerebral cortex, now suddenly 12 m higher than the thorax. An older Barosaurus would experience slowing of these physiological responses and would tend to faint from postural hypotension. On falling to the ground, however, it would quickly regain consciousness as the pressure differential disappeared.

REFERENCES

1. Goetz R.H., Warren J.V. Gauer, O. et al. 1960. Circulation of the giraffe. Circ. Res.
8: 1049-57.

2. Goetz R.H, Berne M.D. Budtz-Olsen, O. 1995. Scientific safari: The Circulation
of the Giraffe. S. Afr. Med. Jouir. 1955:29: 2773-77.

3.  Lillywhite H.B.1991.  Sauropods and Gravity. Nat. Hist. 1991. 100:33