Humans have proven to be able to adapt to just about every sort of unique environment the planet earth has to offer. 
Given that our planet has far more space covered by water than land, it is not surprising that we are able to explore underwater with the use of the right tools. Being able to explore the underwater environment requires specialized gear and training. Scuba gear includes a tank of air for a diver to breathe while underwater.
Physics places restraints on how deep the human body can function underwater. The deeper a diver goes, the more pressure is exerted on the human body and this has implications for the air the diver is breathing from a scuba tank. When the human body is subjected to increased pressure followed by decreased pressure, alarming things can happen to gas molecules trapped in tissues and the bloodstream. This happens due to the physics of increased pressure on the body thereby causing more uptake of gasses into the body fluids and tissues.
Air is about 20% oxygen and 80% nitrogen. As a diver descends into deeper water, the pressure there increases the amount of gas that enters the tissues of the body. Oxygen is used by cells to generate energy by converting sugar to ATP. So the excess oxygen is partly used up and converted to carbon dioxide which is lost by breathing out. Nitrogen however is inert, it is not metabolized, so the concentrations slowly build up in the fluids of the body as pressure increases.
Biology of the Bends
Problems begin when the diver begins to ascend and the pressure lessens. It takes time for the gasses to leave the body tissues slowly by breathing them out. If the pressure drops suddenly due to a rapid ascent, the dissolved gasses quickly form bubbles which cause all sorts of physiological complications. The effect is not unlike opening a can of soda that has been shaken. Opening the sealed container causes an immediate change in pressure and the bubbles violently form and fizz out all over the place. If a diver comes up from the depths slowly enough, the gasses will have time to make their way to the lungs to be expelled when the diver breathes out. By accidentally skipping this precaution, gas bubbles will form.
The sudden formation of bubbles in blood vessels and body tissues can have significant effects. A person can come down with a set of potentially very serious symptoms collectively referred to as “the bends”. This condition was originally seen in workers constructing underwater foundations for bridges on a riverbed in specially constructed chambers called caissons.
When returned suddenly to the surface the workers would be in intense pain in the legs and knees and would bend over, thus the name. Eventually, this was understood to be caused by gas bubbles in the tissues leading to a condition called decompression sickness.
Decompression Sickness Explained
The broad term for complications due to changes in ambient pressure on the human body is Decompression Illness. There are actually two primary complications involving the formation of gas bubbles inside the fluids of the body. One is Decompression Sickness (DCS), a localized formation of painful gas bubbles. The other is Arterial Gas Embolism (AGE) where gas bubbles enter into pulmonary circulation and are distributed throughout the cardiovascular system by the heart via large, then small, arteries. These gas bubbles can interrupt blood flow to vital organs, much like plaque formation in the vessels of the heart or brain leads to interrupted flow in cases of heart attack and stroke.
Decompression sickness, comes about when gas bubbles form in the tissues and cause localized damage. Gas will collect in joints, distorting them and causing pain. Muscle pain is another symptom as is mottling of the skin as shown here:
These symptoms are commonly called the bends or in diving slang, the person “got bent”. However, DCS can be more severe than just the basic symptoms. Nitrogen gas is readily dissolved in the myelin sheath of nerves and gas bubble formation there can pinch the nerve leading to neurological problems. Some cases have demonstrated dizziness, confusion, even amnesia and unconsciousness. High concentrations of nitrogen can stimulate inflammation responses in tissues. Depending on the concentrations involved, nitrogen narcosis for example, death is a possibility.
As bad as having the bends can be, even worse is an arterial gas embolism (AGE). The leading cause of death in diving accidents, this condition comes from gas bubbles forming in blood vessels which then travel to (or originate in) the lungs. If the gas is not passed on to the alveoli and expelled, the bubbles can make their way to the arterial circulation which interferes with blood delivery to vital organs.
A more dramatic way to introduce bubbles into the arterial circulation is Pulmonary Overpressure Syndrome (basically a burst lung) During an ascent to the surface, external pressure drops and air trapped in the lungs will expand, potentially stretching the tissues to the point of rupture (a pneumothorax). Known as pulmonary barotrauma, it could easily cause a lung to collapse. In this way gas bubbles can make their way into pulmonary capillaries and then to the arterial portion of the circulation to be distributed to arteries leading to the brain and heart.
Just like air in the water pipes of your house can make them knock, ping and make strange noises, bubbles cause uneven flow of fluids which disrupts, or even blocks, the delivery of oxygen in the bloodstream. An arterial gas embolism can block blood flow leading to death by heart attack or stroke.
Treatments
The speed at which treatment is required depends on the severity of the symptoms. Giving a person oxygen immediately is a good first step. The only real effective treatment is to get the person suffering DCS into a hyperbaric chamber. By increasing the pressure again it causes the gas bubbles to disperse into smaller ones that can more easily be cleared of the system slowly over time just by normal breathing. These facilities are rarely close by in such emergencies so rushing the patient to one is a time sensitive necessity. Ironically, flying a person subjects them to even lower ambient air pressure the higher the aircraft goes into the sky. This can exacerbate the condition, at least temporarily for the duration of the flight. Still the speed of getting treatment outweighs the risks and if medevac is the fastest option then that is the preferred method. Even so the aircraft may attempt to fly lower than usual if possible.
Additional Information
- Scuba Diving Explained : Lawrence Martin
- Article on Arterial Gas Embolism
- Decompression Illness: What Is It and What Is The Treatment? (Divers Alert Network)
- What is Decompression Sickness and How Can You Treat it?
- Decompression Sickness (MedScape)
Article by Michael Troyan. Michael has spent 20 years teaching non-majors biology and microbiology and currently works as an online instructor at Penn State University. He can be reached at [email protected]. Michael is a frequent contributor to Ricochet Science, and this work was prepared to support the recent release of our Human Biology textbook.
Image Credits:
- Historic Photo of the Bends: This file comes from the Historic American Buildings Survey (HABS), Historic American Engineering Record (HAER) or Historic American Landscapes Survey (HALS).
- Skin of a Bends Patient: By Huckfinne (Own work) [CC0], via Wikimedia Commons
- Decompression Chamber: By Jayme Pastoric (http://www.news.navy.mil/view_single.asp?id=41011) [Public domain], via Wikimedia Commons
