The bends are caused by gas bubbles forming inside your body when you move too quickly from a high-pressure environment to a lower-pressure one. Most commonly, this happens when a scuba diver ascends too fast from depth. Nitrogen that dissolved harmlessly into your blood and tissues while you were under pressure comes out of solution as bubbles, much like carbonation fizzing out of a soda bottle when you crack the cap. Those bubbles can block blood vessels, damage tissue, and trigger dangerous inflammatory reactions throughout the body.
How Pressure Forces Gas Into Your Tissues
The underlying physics is straightforward. The amount of gas that dissolves into a liquid is directly proportional to the pressure of that gas. At sea level, the nitrogen in the air you breathe dissolves into your blood at a low, stable rate. When you descend underwater, the water pressure compresses the air you’re breathing, raising the partial pressure of nitrogen. Your blood and tissues absorb more and more nitrogen the deeper you go and the longer you stay.
This extra nitrogen isn’t a problem while you remain at depth. Your body tolerates dissolved gas just fine. The danger starts when you ascend. As the surrounding pressure drops, your tissues become supersaturated with nitrogen, meaning they hold more gas than the lower pressure can keep in solution. If you ascend slowly enough, your lungs can gradually filter out that excess nitrogen with each breath. If you ascend too quickly, the gas comes out of solution faster than your body can eliminate it, forming bubbles in your blood, joints, spinal cord, brain, and other tissues.
What Those Bubbles Do Inside Your Body
Gas bubbles aren’t just passive obstructions. They set off a chain of mechanical, inflammatory, and clotting responses that can affect nearly every organ system. The damage depends on where the bubbles form and how large they are.
When a bubble enters an artery that branches into smaller and smaller vessels, it eventually gets stuck. The surface tension at the leading edges of the bubble, pressed into tiny branch vessels, creates enough resistance to halt blood flow entirely. Tissue downstream of that blockage loses its oxygen supply. In the brain, this produces a stroke-like injury. In the spinal cord, bubbles forming in the surrounding veins can trigger localized clotting, cutting off blood flow and causing what amounts to a spinal cord infarction.
Bubbles also cause direct mechanical harm. They can stretch and distort tissue, compress nearby blood vessels from the outside, rupture small vessels and cause bleeding, or stimulate pain receptors in sensitive areas like joints. As bubbles travel through blood vessels, they scrape and damage the vessel walls, stripping away the protective lining of cells. This damage activates platelets, white blood cells, and the body’s clotting and inflammation systems. What starts as a physical obstruction quickly becomes a widespread inflammatory event.
Symptoms: From Joint Pain to Paralysis
The bends are classified into two types based on severity. Type I, the more common form, involves the skin, lymph nodes, muscles, and joints. The hallmark symptom is deep, aching joint pain, most often in the shoulders, though any joint can be affected. You may also notice swollen lymph nodes or a mottled, marbled discoloration of the skin.
Type II is the neurological form, and it’s far more serious. Bubbles that reach the brain or spinal cord can cause headaches, blurred vision, hearing loss, ringing in the ears, nausea, poor coordination, and sensory or motor weakness. Some people present with altered consciousness. In severe cases, paralysis or unconsciousness can develop. Type II decompression sickness is strongly associated with venous bubbles crossing into the arterial side of circulation, which can happen if you have a specific heart defect (more on that below).
Why Some Divers Are More Vulnerable
Two divers can do the exact same dive profile and have very different outcomes. Several individual factors explain why.
Body fat. Nitrogen is five times more soluble in fat than in blood. If you carry more body fat, your tissues absorb significantly more nitrogen at depth and take longer to off-gas during ascent. This makes higher body fat percentage one of the most well-established risk factors for the bends.
A hole in the heart. About 20 to 34 percent of adults have a patent foramen ovale, a small opening between the upper chambers of the heart left over from fetal development. In most people it causes no issues, but in divers it can allow venous bubbles to bypass the lungs (which normally filter them out) and pass directly into arterial circulation. Once arterialized, those bubbles can travel to the brain, spinal cord, or other organs and cause Type II symptoms.
Water temperature. Cold water constricts blood vessels near the skin and extremities, reducing blood flow to those tissues. If you’re cold during the deepest part of your dive, your tissues actually absorb less nitrogen because less blood is flowing through them. But if you’re warm at depth and then cold during your ascent, the pattern reverses in the worst possible way: you’ve loaded up on nitrogen while warm, and now reduced blood flow during decompression slows the rate at which your body can eliminate it.
Other contributing factors include dehydration, fatigue, age, and the intensity of physical exertion during and after a dive. Any condition that impairs circulation makes it harder for your body to clear dissolved gas efficiently.
How the Bends Are Treated
Treatment centers on getting you back under pressure so the bubbles shrink and redissolve, then bringing you back to normal pressure gradually enough for your lungs to clear the gas safely. This is done in a hyperbaric chamber, where you breathe pure oxygen at roughly 2.8 times normal atmospheric pressure. The oxygen accelerates nitrogen elimination while also delivering extra oxygen to tissues that were starved by bubble blockages. Treatment sessions can last several hours, and some cases require multiple sessions over days.
The bends are diagnosed entirely on clinical grounds. There’s no blood test or imaging scan that confirms it. If symptoms appear after a dive, treatment typically begins based on the diver’s history and presentation alone, which is why recognizing the symptoms early matters so much.
How Divers Prevent the Bends
Prevention comes down to controlling how fast dissolved nitrogen leaves your tissues. The single most important rule is ascending slowly. The U.S. Navy and NOAA recommend a maximum ascent rate of 30 feet per minute. Recreational dive training agencies generally recommend between 30 and 60 feet per minute. Many divers use the simple guideline of rising no faster than their smallest exhaled bubbles.
Safety stops, typically three to five minutes at about 15 feet, give your body extra time to off-gas before you reach the surface. For deeper or longer dives, dive tables and computers calculate mandatory decompression stops at specific depths.
Flying after diving is another critical consideration. Even after you’ve surfaced and feel fine, residual nitrogen remains in your tissues. Cabin pressure in a commercial aircraft is lower than sea-level pressure, which can push that remaining nitrogen out of solution. The Divers Alert Network recommends waiting at least 12 hours after a single no-decompression dive before flying, at least 18 hours after multiple days of repetitive diving, and substantially longer than 18 hours after dives that required decompression stops or used specialized gas mixtures.