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In just a second or two, a giraffe can lift its head from ground level to the sky, some 15 feet (4.5 meters) up, and never get a head rush.
"If we did that we'd certainly faint," says physiologist Graham Mitchell of the University of Wyoming.
Mitchell and his team report in the July 1 issue of the Journal of Experimental Biology that a hefty, hard-working heart and high blood pressure keep a giraffe free of fainting spells.
A giraffe's head fills with blood when it is near the ground, and blood pressure there doubles. When the beast raises its head up for a bite in the trees, the blood drains out.
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This happens to us too. You can feel light-headed if, after hanging upside down and getting red in the face, you quickly turn right-side up. If your blood pressure drops too low, not enough blood flows to your brain, and you can pass out.
With such long necks, giraffes spend a lot of time moving their heads from down low to up high, and so they need a way to keep blood flowing to the brain so they don't get woozy.
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Scientists once thought that vessels in the giraffe's neck siphoned blood from the heart to the brain. However, Mitchell's research suggests getting the blood to the brain of an erect giraffe involves a powerful pump and very high blood pressure—twice as high as ours.
26-pound heart
Giraffes have big hearts, weighing up to 26 pounds. When a giraffe lifts its head, blood vessels in the head direct almost all of the blood to flow to the brain, and not to other parts of the head such as their cheeks, tongue, or skin.
At the same time, the animal's thick skin and an unusual muscle in the jugular vein—veins don't normally have muscles—add pressure to the vein, which carries blood from the head back to the heart.
"It's a much more advanced anti-fainting mechanism than we have," Mitchell said.
Corey Binns lives in Northern California and writes about science, health, parenting, and social change. In addition to writing for Live Science, she's contributed to publications including Popular Science, TODAY.com, Scholastic, and the Stanford Social Innovation Review as well as others. She's also produced stories for NPR’s Science Friday and Sundance Channel. She studied biology at Brown University and earned a Master's degree in science journalism from NYU. The Association of Health Care Journalists named her a Centers for Disease Control and Prevention Health Journalism Fellow in 2009. She has chased tornadoes and lived to tell the tale.
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