The volume of a given mass of gas is inversely proportional to the pressure being exerted on it (temperature remaining steady). For every 10 metres of descent the pressure increases by one atmosphere (atm). Therefore, total lung volume during a breath-hold dive at 10 metres is one-half that at the surface. At 20 metres it is 1/3, at 30 metres it is 1/4 and at 40 metres it is 1/5. On surfacing these figures are reversed. However when breathing compressed gases as in diving the mass of gas in the lungs is increased to fill the normal volume. An ascent from 30 metres to the surface without venting (exhaling) would cause the gas, in already full lungs, with minimal ability, to expand further to increase its volume to three times normal with the greatest change occurring in the last 10 metres where it would double. This is the key law to explain pressurisation and depressurisation issues and injuries.
As a diver descends the total pressure of breathing air increases in accordance with Boyles Law; therefore, the partial pressures of the individual components of the breathing air are increased proportionally. As the individual descends deeper under water, nitrogen dissolves in the blood and is carried to all body tissues until a new equilibrium is reached. Long before this however Nitrogen at the higher partial pressures in blood alters the electrical properties of cerebral nerve cell membranes, causing an anaesthetic effect termed nitrogen narcosis. For every 15 metres of depth this is roughly equivalent to one alcoholic drink. At 50 metres divers may experience alterations in reasoning, memory, response time, and other problems such as idea fixation, overconfidence, and calculation errors. During descent the partial pressure and hence the amount of dissolved oxygen increases. Breathing 100% oxygen at 2.8 atmospheres absolute (1.8 atm or 18 metres) may cause oxygen toxicity in as little as 30-60 minutes. At 100 metres, the normal 21% oxygen in compressed air can become toxic, because the partial pressure of oxygen is approximately equal to 100% at 10 metres. For these reasons deep divers (usually professional or military, but increasingly sport divers as well) use specialised mixtures that replace nitrogen with helium and allow for varying percentages of oxygen depending on depth. The percentage is small and provides a partial pressure which supports life and strenuous activity without inducing oxygen toxicity.
With increasing depth, nitrogen in compressed air equilibrates through the alveoli of the lungs into the blood and thence into the tissues. Over time nitrogen dissolves and accumulates initially in the mainly aqueous tissues or those with a high rate of blood flow e.g. the brain, and progressively in the lipid or fatty component of tissues. On longer dives some or all tissues become saturated and will not take up any more nitrogen. As an individual ascends, there is a lag before saturated tissues start to release nitrogen back into the blood. It is this delay that creates problems. When a critical amount of nitrogen is dissolved in the tissues, reduction of pressure caused by ascending induces the dissolved gases to outgas and form small but myriad bubbles in tissue cells, tissue spaces and blood. Ascending too quickly causes the dissolved gases - nitrogen - to return to gas form more quickly increasing the number and size of the bubbles and while still in the blood or tissues causing local damage which may be felt as symptoms of DCI Further reductions in pressure through flying or ascending to a higher altitude also contribute to bubble formation. The average airline cabin is pressurised only to 8000 feet or 0.8 atm. If a person flies too soon after diving, this additional decrease in pressure may be enough to precipitate bubbling or enlarge any bubbles already in existence. With or without the effects of flying If the bubbles are in the blood in some divers paradoxical embolisation may occur through a PFO.