Humans evolved in an atmosphere—a blanket of gases that gravity holds in place around Earth. The weight of these gases is called air pressure. Air pressure is measured in units called atmospheres. One atmosphere is equal to the weight of a one-inch-wide column of air reaching from sea level to the edge of space, or 14.7 pounds per square inch.

One of the discoverers of air pressure, Evangelista Torricelli, once noted that "we live submerged at the bottom of an ocean of the element air." The human body has adapted to life in this ocean of air by maintaining an internal pressure that balances air pressure. However, with sufficient oxygen to breathe, we can exist comfortably in pressures down to one-third of an atmosphere, which is roughly the pressure atop Mount Everest. At pressures lower than this, though, gases start to boil, bubbling out of the body's cells and tissues, especially in the lungs and bloodstream, starving the body of oxygen.

A liquid boils when the pressure of the gases dissolved in it exceeds the pressure outside, which allows the gases to escape. Thus, boiling can be caused by increasing the inside pressure (heating the liquid) or decreasing the outside pressure (climbing to a higher altitude). Because boiling depends on the difference in pressure, the temperature at which it happens can vary. At a pressure of one atmosphere, water boils at 100°C/212°F. At a higher pressure, the boiling temperature is higher, too. At 4.5 atmospheres, the temperature at which water boils rises to 150°C/300°F.

In the vacuum of space, where there is no outside pressure, normal body temperature (37°C/98.6°F) far exceeds the boiling point of water. So a sudden drop in outside pressure causes gases to begin to bubble out of the blood. This is similar to the decompression sickness, also known as "the bends," experienced by scuba divers who ascend too quickly after breathing in high-pressure gases at depth.

Humans can survive very brief exposures to a vacuum (less than 90 seconds). However, they would lose consciousness as gas bubbles blocked blood circulation. If not rescued immediately, their tissues would continue to swell as more of their bodily fluids boiled, quickly causing death. To venture beyond the Earth's atmosphere, and leave the confines of a pressurized vehicle, humans need space suits that not only produce the temperatures and pressures needed to survive in the vacuum of space, but also provide all life support functions and protect them from the hazards of space.

To redesign the bulky, inflated space suits, some engineers have looked to nature for inspiration. At MIT, Dava Newman is designing a lightweight, form-fitting, flexible space suit to give astronauts mobility. By studying how the giraffe maintains blood pressure when lifting and lowering its head, she discovered a new way to apply pressure directly to the body. Newman's Bio Suit is made from a compression garment woven with strong elastic threads that increase pressure without constricting joints. This suit may one day allow astronauts to walk easily on Mars.

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