Editing Decompression illness (section)

== Mechanism ==
The mechanism of decompression sickness is different from that of arterial gas embolism, but they share the causative factor of depressurization.

===Decompression sickness===
{{See also|Decompression sickness#Mechanism}}

Depressurisation causes [[inert gas]]es, which were dissolved under higher [[pressure]], to come out of physical [[Solution (chemistry)|solution]] and form gas [[liquid bubble|bubble]]s within the body. These bubbles produce the symptoms of decompression sickness.{{r|38uhms | Ackles}} Bubbles may form whenever the body experiences a reduction in pressure, but not all bubbles result in DCS.{{sfn|Nishi Brubakk & Eftedal|p=501}} The amount of gas dissolved in a liquid is described by [[Henry's Law]], which indicates that when the pressure of a gas in contact with a liquid is decreased, the amount of that gas dissolved in the liquid will also decrease proportionately.

On ascent from a dive, inert gas comes out of solution in a process called "[[outgassing]]" or "offgassing". Under normal conditions, most offgassing occurs by [[gas exchange]] in the [[lung]]s.{{r|Kindwall1 | Kindwall2}} If inert gas comes out of solution too quickly to allow outgassing in the lungs then bubbles may form in the blood or within the solid tissues of the body. The formation of bubbles in the skin or joints results in milder symptoms, while large numbers of bubbles in the venous blood can cause lung damage.{{sfn|Francis & Mitchell, ''Manifestations''|pp=583–584}} The most severe types of DCS interrupt — and ultimately damage — spinal cord function, leading to [[paralysis]], [[Sensory system|sensory]] dysfunction, or death. In the presence of a [[right-to-left shunt]] of the heart, such as a [[patent foramen ovale]], venous bubbles may enter the arterial system, resulting in an [[arterial gas embolism]].{{r|Francis}}{{sfn|Francis & Mitchell, ''Pathophysiology''|pp=530–541}} A similar effect, known as [[ebullism]], may occur during [[explosive decompression]], when water vapour forms bubbles in body fluids due to a dramatic reduction in environmental pressure.{{r|Landis}}

===Arterial gas embolism===
{{see also|Air embolism#Decompression illness}}
When a diver holds their breath during an ascent the reduction in pressure will cause the gas to expand and the lungs will also have to expand to continue to contain the gas. If the expansion exceeds the normal capacity of the lungs, they will continue to expand elastically until the tissues reach their tensile strength limit, after which any increase in pressure difference between the gas in the lungs and the ambient pressure will cause the weaker tissues to rupture, releasing gas from the lungs into any permeable space exposed by the damaged tissue. This could be the pleural space between the lung and the chest walls, between the pleural membranes, and this condition is known as pneumothorax. The gas could also enter the interstitial spaces within the lungs, the neck and larynx, and the mediastinal space around the heart, causing interstititial or mediastinal emphysema, or it could enter the blood vessels of the venous pulmonary circulation via damaged alveolar capillaries, and from there reach the left side of the heart, from which they will be discharged into the systemic circulation. On the way out through the aorta the gas may be entrained in blood flowing into the carotid or basilar arteries. If these bubbles cause blockage in blood vessels, this is [[arterial gas embolism]]. Vascular obstruction and inflammation caused by gas bubbles causes [[end organ damage]] to most tissues.<ref name="Cooper" /> Sufficient pressure difference and expansion to cause this injury can occur from depths as shallow as {{convert|1.2|m|ft}}.<ref name="DAN FAQ" />