As the tallest animal, coming in at around 18 feet, being a giraffe can have its advantages. However, the heart of such a tall animal must be extraordinarily equipped to handle the daunting challenge of pumping blood not only to the legs, but also against gravity all the way up their long necks to the brain. They achieve this by maintaining blood pressures that are twice that of other mammals.
Autopsies of giraffes have found that in order to maintain such high blood pressures, giraffe hearts have strengthened left ventricles, adding extra muscle, but these ventricles also hold a smaller volume relative to other mammals. These two adaptations allow the giraffe to pump less blood with a greater pressure, and keep a high blood pressure without stressing out their hearts.
To learn more about the amazing anatomy and physiology of the giraffe, check out this series of videos on YouTube. [Warning: The videos involve a giraffe autopsy for those with weak stomachs]
[Via Life Lines]