An adult giraffe's head is about six feet above its heart. This means that to pump enough blood up to the brain the circulatory system must be strong enough to keep the blood at very high pressure. Biologists have known for some time that giraffes solve this problem by having unusually high blood pressure, about double that of human beings. But an international team of biologists began to wonder about this. If giraffes have such high blood pressure, they should have a terrible problem with swelling in their legs and feet. Why don't giraffes have swollen feet? Giraffes should have another problem, too. Every time they bend heads down to drink, the blood should rush to their heads and have a hard time flowing back up (when the head is down)to the heart. How come giraffes don't black out when they drink? The answer to the swollen feet problem, the researchers found, is that giraffes have what the researchers call a 'natural anti-gravity suit'. It turns out that the skin and other tissues in their legs and feet are much stiffer and tougher than those of other animals. As a result, the blood vessels in the leg cannot swell. Therefore, the blood has nowhere to go but back to the heart. What about blood rushing to the head whenever the giraffe bends down to drink? The researchers found that the giraffe's jugular vein, which carries blood from the head back to the heart, has lots of one-way valves in it. In the giraffe's neck, there are lots of muscles that flex and relax repeatedly as the animal moves its head and sucks up drinking water. By squeezing the valved jugular vein, they keep blood moving back to the heart even while the animal is drinking. Giraffes do not have swollen legs because______.