[转帖] 心主神明－长颈鹿的故事

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http://sciencenow.sciencemag.org/cgi/content/full/2006/616/1?rss=1

Heads up!
A strong heart helps giraffes keep blood in their head.
Credit: ANUP SHAH/PHOTODISC GREEN

Hearts and Minds
By Briahna Gray
ScienceNOW Daily News
16 June 2006
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.
To solve the dizzying conundrum, zoologist Graham Mitchell and his team at the University of Wyoming in Laramie built a machine that replicated the length, rigidity, and pressure of the blood-flow system in a giraffe';s neck. Rubber and PVC tubes substituted for the jugular vein and carotid artery, and an electric pump became the heart. When the researchers tested the siphon model, they found that the pressures within the tubing were different from those known to exist in real giraffes. In contrast, when the team had the "heart" do all the work, the blood-vessel pressures nicely matched up with those seen in the living creature.
If the model holds, a giraffe';s blood pressure is about twice as high as that seen in people, says Mitchell. He suspects that a muscular cuff at the base of the jugular vein constricts as the giraffe stands to maintain that pressure in the brain. The team reports its results in the current issue of the Journal of Experimental Biology.
"It';s a very good paper from a modeling standpoint," says Alan Hargens, a physiologist at the University of California, San Diego. The next step, Hargens says, is more hands-on work with giraffes to put the model to the test.