Because a giraffe's head is so far above its heart, the animal's blood pressure, measured six feet lower at the level of the heart, must be roughly double that of a human in order to supply the brain with enough oxygen. This much has been known for years.

But, a team of researchers recently wondered, if this is so, giraffes should have a terrible problem with edema in their legs -- the blood pooling near the feet, roughly six feet below the heart, and causing severe swelling. Giraffes should also be in trouble every time they bend their heads down to drink. Blood should rush to the head and be unable to get back to the heart.

Nature, as might be expected, has provided giraffes with ways of preventing these problems.

Giraffes don't suffer from swollen legs and feet, the scientists discovered, because the animals have a "natural antigravity suit." The skin and other connective tissue of the long, thin legs is unusually strong and tight. Their feet don't swell because they're too strongly built.

When the animal bends down to drink, blood doesn't pool in the head because the jugular vein, which carries blood back to the heart, has lots of one-way valves that prevent blood from flowing back. Some human veins have one-way valves, especially in the legs but not the jugular.

A report on the findings was published in last week's Nature. The research was done by Alan R. Hargens, then at the University of California at San Diego; Ronald W. Millard of the University of Cincinnati; Knut Pettersson of the Swedish corporation AB Haessle and Kjell Johansen of the University of Aarhus in Denmark.