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Case Study: The Bends - Chemwiki
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ChemWiki: The Dynamic Chemistry E-textbook > Physical Chemistry > Equilibria > Case Studies > Case Study: The Bends
Case Study: The Bends
The Bends is an illness that arises from the rapid release of nitrogen gas from the bloodstream and is caused by bubbles forming in the blood when a
diver ascends to the surface of the ocean to rapidly. It is also referred to as Caisson sickness, decompression sickness (DCS), and Divers' Disease.
Introduction
People that go deep sea diving usually take an air supply tank down in the ocean to allow them to breathe. Normally transported through highly
pressurized air tanks, the packaged air is taken in by our bodies. Unfortunately, our bodies aren't used to the pressurized air (because we take in air
that is under normal atmospheric conditions). With higher air pressure, the blood is more soluble, meaning elements dissolve into the bloodstream
easily, including helium, nitrogen, and oxygen. When divers want to emerge from the water, they have to make sure they don't ascend to the surface
level too quickly because they risk creating numerous bubbles from blood-soluble gases. When nitrogen (N2) gas forms bubbles, it accumulates and
saturates the muscles and blood, causing pain. Called the Bends, this condition can also cause injuries involving the nervous system.
Figure 1. Diver with compressed air tank (yellow). Figure taken with permission from Wikipedia Commons.
The Solubility of Gases
The solubility of a gas is the ability for the gas to dissolve in a solvent (in our case, blood). Both temperature and pressure affect the solubility of a
gas.
Temperature
In water solvents, the higher the temperature, the less soluble the gas is.
In organic solvents, the higher the temperature, the more soluble the gas is.
Pressure
English chemist William Henry discovered that as the pressure increases, the solubility of a gas increases. Henry's Law is then:
where
C=solubility of a gas in a solvent at a specific temperature,
Pgas is the partial pressure of the gas, and
k is Henry's Law Constant
In the case of The Bends:
If a diver goes deeper and deeper into the water, pressure builds up in the bloodstream.
As the diver inhales highly pressurized air, the bloodstream also absorbs the gaseous particles, including nitrogen.
Following Henry's Law; as the pressure increases, the solubility of N2 the diver's bloodstream increases.
As a result, nitrogen from the compressed air stays in the bloodstream, until it is able to escape at a lower pressure through exhalation
However, since the diver is in a highy-pressurized environment, the N2 can only leave the body when the diver reaches the lower pressures
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Case Study: The Bends - Chemwiki
Ideally, this should happen during the diver's gradual rise to the surface
Unfortunately, sometimes, the diver ascends too quickly, resulting in the rapid formation of N2 bubbles, which causes interference with nerves,
blood and lymphatic vessels, and clots
Example 1
Determine Henry's Law Constant, k, with the information that the aqueous solubility of N2 at 10 degrees Celsius is 11.5 mL N2 / L and 1 atm.
Now if the Pgas of N2 increases to 5 atm:
Solve for C: C= 57.5 mL N2 /L
Therefore, both examples show that as the the pressure increases from 1 atm to 5 atm, the solubility of the N2 gas increases from 11.5 to 57.5 mL
N2 / L. This supports Henry's Law.
Symptoms of the Bends
Excessive Coughing
Chest pain
Dizziness
Paralysis
Difficulty breathing
Unconsciousness
Death
Most symptoms occur 24 hours after decompression, but can occur up to 3 days after. Prevention
Ascending to the surface slowly (rate of 60 ft/min.)
The slower the diver surfaces, the less pain impact from the bubbles on the diver
Spending time in a decompression chamber
Chambers that high-pressured divers are placed in.
Once in the chamber, the diver is immersed in high pressure until the chamber slowly reduces it, minimizing symptoms of the illness
Breathing in compressed air mixture of helium and oxygen.
Helium is less soluble in the blood stream, providing a smaller threat to divers when they come up to the surface of the ocean
References
1. Petrucci, et al. General Chemistry: Principles & Modern Applications: Custom Edition for CHEM 2 (Hardcover). Upper Saddle River: Pearson
Education, Inc., 2011.
2. Phatak, Uday. "Decompression Syndrome (Caisson Disease) in an Indian Diver." Annals of Indian Academy of Neurology. Online-Only Journal.
13.3. 2010 n.pag. web. 30 May 2011.
Outside Links
http://en.wikipedia.org/wiki/Decompression_sickness
http://www.pbs.org/wgbh/amex/eads/peopleevents/e_caisson.html
http://www.uhms.org/ResourceLibrary/Indications/DecompressionSickness/tabid/275/Default.aspx
Problems
1.
2.
3.
4.
5.
What is Henry's law?
How does Henry's law relate to The Bends illness?
How does temperature affect solubility?
How does ascending to the surface make the Bends less prevalent?
Why is breathing the compressed helium/oxygen mixture better than the air with N2?
Answers
1.
(where C=solubility of a gas in a solvent at a specific temperature, Pgas is the partial pressure of the gas, and k is Henry's Law
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2.
3.
4.
5.
Case Study: The Bends - Chemwiki
Constant)
As the pressure increases, the solubility of the diver's bloodstream increases. Henry's law states that the solubility of a gas increases when the
pressure increases.
As temperature increases, the solubility of gases decrease in water solvents. In organic solvents, the solubility of gases increase in higher
temperatures.
When diver surfaces slowly, he/she will have a reduced impact of pain from the bubbles that form. Instead of rapidly forming and causing joint
pain, the slow rise to the surface creates a steady loss of pressure, resulting in pain that is not as severe.
Helium is less soluble in the blood stream, providing a smaller threat to divers when they come up to the surface of the ocean. Less bubbles are
formed, meaning that the divers encounter less pain as they ascend.
Contributors
Dhara Shah (UCD)
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