In just a second or two, a giraffe can lift its head from ground level to the sky, some 15 feet 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.
How it works
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.
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.
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.