An adult giraffe's head is about six
feet above its heart. This means that to{{U}} (67) {{/U}} enough blood
up to the brain the circulatory{{U}} (68) {{/U}} 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{{U}} (69)
{{/U}} high blood pressure, about{{U}} (70) {{/U}} that of human
beings. But an international team of biologists began to{{U}} (71)
{{/U}} about this. If giraffes have such high blood pressure, they should
have a{{U}} (72) {{/U}} problem with swelling in their legs and feet.
Why don't giraffes have swollen feet? Giraffes should have{{U}}
(73) {{/U}} problem, too. Every time they bend heads{{U}} (74)
{{/U}} to drink, the blood should{{U}} (75) {{/U}} to their heads
and have a hard time{{U}} (76) {{/U}} back up (when the head is down) to
the heart. How come giraffes don't black out when they drink?
The answer to the{{U}} (77) {{/U}} feet problem, the researchers
found, is that giraffes have{{U}} (78) {{/U}} the researchers call a
"natural anti-gravity suit". It{{U}} (79) {{/U}} out that the skin and
other{{U}} (80) {{/U}} in their legs and feet are{{U}} (81)
{{/U}} stiffer and tougher than those of other{{U}} (82) {{/U}} . 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{{U}} (83) {{/U}} the giraffe bends down to drink? The
researchers found that the giraffe's jugular vein, which{{U}} (84)
{{/U}} 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{{U}} (85) {{/U}}
drinking water. By squeezing the valved jugular vein, they{{U}} (86)
{{/U}} blood moving back to the heart even while the animal is
drinking.