A heart is an amazing muscle, but it can’t do everything. If you’ve ever had your feet all asleep, you’ve experienced how your heart can only do so much on its own as a 7- to 15- ounce pump. To save energy, the muscles in your feet and calves, combined with pressure from walking, can help push deoxygenated blood back up your legs to continue circulation. Now if our relatively short legs need this kind of assistance, consider what was necessary for a 16 ton sauropod like Diplodocus to get blood to its brain at the end of a 21-foot-long neck. How big a heart would they need to keep their blood pressure up, or could they use some of the same tricks we’re still employing in our circulatory system today?
Does more neck just need more heart?
The circulatory systems of these long-necked dinosaurs has garnered a number of different hypotheses over the years. Not only would a long neck put the animal’s brain a long distance from the heart, but every time it raised its head gravity would start working against the flow of oxygen-rich blood. With that challenge in mind, some researchers have considered everything from an oversized, two-ton heart to a series of “pseudo-hearts” acting as little relay pumps along the neck. These hypotheses were of course difficult to test, thanks to a lack of fossilized circulatory tissue. However, there may be an answer in the bones themselves.
Cervical ribs to stabilize and squeeze
As a sauropod walked, it’s long neck would naturally swing back and forth with increasing intensity. To keep their heads from turning into a giant pendulum, they had springy, rib-shaped structures along their neck vertebrae, called cervical ribs. The muscles along the neck would then have an easier time keeping the neck stable, and at the same time possibly solve the hulking creatures’ circulatory problem.
As the neck muscles flexed, they and the cervical ribs would squeeze other soft tissue in the neck, like air sacs and the vertebral artery. These small squeezes would help push blood from one end of the neck to the other, like a large-scale version of your leg muscles forcing blood back up out of your feet. Scaling up isn’t a problem for this hypothesis either, as large neck muscles and cervical ribs would provide more oomph without requiring more work from the heart.
Source: How Long-Necked Dinosaurs Pumped Blood to Their Brains by Brian Switek, Smithsonian.com