The rate at which this diffusion occurs is largely driven by gas concentrations (the amount of a gas in a particular space) and the partial pressure exerted on that gas.
When pressure decreases gas molecules expand and thus take up more space within a given area. So while the percentage of gases is the same, the actual concentration is higher as we drop below sea level.
The increase in pressure has another effect on gases, as predicted by Henry's Law, which states that the amount of gas dissolving in a liquid, e.g. blood or tissue, is proportional to the partial pressure of that gas.
For the typical SCUBA diver these laws may not mean a lot, but taken together, Boyle's Law (see box) and Henry's Law have important implications for the predominant gas in air, Nitrogen (N2).
Unlike O2 or CO2, N2 is inactive in the body, so the greater levels of N2 breathed in are dissolved into the various fluids and tissues of the body while under pressure, and are released as the pressure decreases during ascension. When dive times are matched for depth, meaning N2 levels don't build up too much, and ascension is slow, there is plenty of time to allow the N2 to diffuse into the blood and be exhaled.
DCS arises when the rate of ascension exceeds the divers ability to exhale the N2, and the loss of pressure allows the N2 to dissolve out in bubbles large enough to cause problems.
DCS encompasses a broad range of maladies with varying severity involving large bubbles blocking blood flow to the nervous tissue, the brain and the joints--joint pain is often the first symptom and forces individuals into a bent posture, hence the phrase "the bends".
In simple terms, the more nitrogen you push into tissues/blood the more time you need to exhale it out.
Come up too fast and those gas bubbles block blood flow to areas like joints and even the brain. Repeated or long-term exposure leads to permanent damage.
The potentially life threatening consequences of DCS have led to specific diving guidelines and recommendations for divers to minimize their risk for DCS.
One such recommendation has been the long-held view that moderate to strenuous exercise increases a divers risk for developing DCS. However, in a 2008 review, Dujic and colleagues discuss a block of recent research that indicates exercise before, during and after decompression decreases your risk for DCS.
Exercise and DCS
Much of the concern raised was based on a small study conducted back in the 1940s by the U.S. Navy and has simply been reiterated for more than 70 years. Emerging research now suggests otherwise.
In a series of studies Dujic et. al. studied the role of exercise before during and after a dive. Regarding prior exercise, it appears that timing is less important than intensity, with data demonstrating that exercise 24 hours to just 2 hours prior to a dive, with an intensity up to 90 percent of maximum heart rate (MHR), reduced nitrogen bubble formation after a dive; exercise actually prevented DCS in all subjects.