When a giraffe holds its neck in a vertical position, its brain is about 2 meters above its heart. Contraction of the ventricle must create enough pressure to raise the blood to the level of the head and perfuse the brain (i.e

Question

, circulate blood through the brain). The arterial pressure at the aorta of a giraffe is approximately 200 mm Hg, twice that of a human. The brain of a sauropod dinosaur, such as Barosaurus, would have been about 9 meters above the level of its heart if it held its neck vertically. Calculate the aortic pressure required to perfuse the brain using the following information: • The density of blood is 1.055 g/cm3 and the density of mercury 13.554 g/cm3 • An aortic pressure of 10 mm Hg is needed to overcome resistance to blood flow in the carotid arteries leading from the heart to the brain. • A blood pressure of 50 mm Hg at the head is required to perfuse the brain. What inferences about the biology of sauropods can you draw from your calculation?

Answer 1

Calculation: Aortic pressure = gravitational force + resistance to flow + perfusion pressure at the brain; that is,
Aortic pressure = gravitational force + 10 mm Hg + 50 mm Hg
Gravitational force: The ratio of the density of blood to mercury is 1.055/13.554 = 0.078 . Each centimeter of blood is equivalent to 0.078 cm Hg, so 1 m of blood = 0.078 m of Hg or 78 mm Hg. The 9 m of blood between the heart and the brain = 702 mm Hg.
Aortic pressure (mm Hg) = 702 + 10 + 50 = 762 mm Hg.
Inferences: This is an open-ended question. Here are some starting points for discussion that are presented in the sources cited.
Cardiovascular physiology:
A sauropod would have required an unrealistically large and thick-walled heart to generate that pressure.
The energy required to pump blood to the head would have been about half of the total daily energy expenditure of the dinosaur, perhaps offsetting the benefits of the additional food that would be available.
Ecology and behavior:
If sauropods held their necks horizontally, the brain would have been approximately at the level of the heart. In this posture they could have browsed through a large arc without moving their bodies and that might have been an energetically efficient mode of feeding.
Sources
Christian, A. 2010 . Some sauropods raised their necks—evidence for high browsing in Euhelopus zdanskyi. Biology Letters 6:823-825.
Ruxton, G. D., and D. M. Wilkinson. 2011 . The energetic of low browsing in sauropods. Biology Letters 7:779-781 .
Seymour, R. S. 2009 . Raising the sauropod neck: It costs more to get less. Biology Letters 5:317-319
Seymour, R. S., and H. B. Lillywhite. 2000 . Hearts, neck posture and metabolic intensity of sauropod dinosaurs. Proceedings of the Royal Society London B 267:1883-1887.