From Science:
What do giraffes and fighter pilots have in common? They both experience extreme rushes of blood from the head: pilots (from the force created by rapid acceleration) and giraffes (merely from lifting their necks). Pilots wear special flight suits to avoid fainting. Now, a new study suggests that a powerful heart is what keeps the giraffes from swooning.
Thanks to its long neck, a giraffe’s head can rise up to 5 meters in mere seconds after the creature takes a drink. One would expect this dramatic motion to trigger a massive drain of blood from the brain, but giraffes obviously aren’t fainting all over the place. As long ago as 1955, researchers speculated that giraffes keep their head full using a sort of siphon system, whereby the pull created by blood flowing from the brain via the jugular vein draws extra blood from the heart via the carotid artery. Others hypothesized that the heart alone did the job, pumping blood at sufficiently high pressure to keep the brain running smoothly.
More here.