The bends is the common name given to decompression sickness or caisson disease. The bends occur when the pressure surrounding a person’s body drops quickly. Most commonly, the bends result when a diver resurfaces too quickly, but they may also occur when unpressurized aircraft flies quickly into lower-pressure air, or when a miner comes out of a mine that has been heavily pressurized to hold water out.
Gasses are often dissolved in liquids, such as carbon dioxide being dissolved in soda in order to make a carbonated drink. When the pressure on those gasses is decreased – such as when you open a bottle of soda and the high-pressure inside stabilizes to that of the outside air – they are released from the liquid and escape. When you open a carbonated soda, you can hear the carbon dioxide escaping and may even see the bubbles coming up.
The human body also keeps gasses dissolved in liquids, most notably nitrogen. Nitrogen is kept in the body in blood, other liquids, and various tissues. If the body depressurizes, some of that nitrogen is released as a gas in the body, and it is this release of nitrogen that causes the bends. Depending on the severity of the bends, the victim may experience pain, itching, and rashes on the gentler side of the spectrum, to full-blown paralysis or death at the worst.
When you dive, your body is subjected to rather impressive pressure. Because of the weight of water, it takes only 33 feet (10 meters) of water to double the amount of pressure exerted on your body at sea level – one atmosphere, or roughly 14.7 pounds per square inch. When a person is breathing through a tank, he is breathing air that is pressurized to the same level as the surrounding water, so that at 100 feet (30 meters) deep, roughly 60 PSI are compressing the lungs. The deeper a diver goes, the more nitrogen will enter her body as solution, and the longer she will need to take to resurface. By pausing slowly, the diver allows the nitrogen to release from the body slowly, causing little or no experience of the bends or the physical pain and damage they cause – if a diver is forced to rise quickly, the only way to avoid the bends and possible damage is to enter a pressurized chamber and slowly depressurize.
The bends may also be the result of changing one's pressure dramatically in ways other than diving. Many mines, for example, pressurize their shafts to keep water out of the space being excavated. Some of the earliest reported cases of the bends come from mines in the early 19th century, with miners reporting severe muscle aches and cramping after being underground for extended periods of time. Ascending quickly to high altitudes, and the subsequent reduction in atmospheric pressure, may also cause the bends. Most aircraft guard against this eventuality by pressurizing their passenger chambers to a pressure analogous to that of sea level, but if the hull is breached and air pressure is lost, the bends may occur.