The Bends

An interactive simulation of decompression sickness and Henry's Law

The Bends: Understanding Decompression Sickness

Illustration of decompression sickness showing nitrogen bubbles forming in a diver's bloodstream during rapid ascent

What is Decompression Sickness?

Decompression sickness, commonly known as "the bends," is a potentially fatal condition that can affect scuba divers, high-altitude pilots, astronauts, and others exposed to rapid changes in ambient pressure.

The Science Behind It

When a diver descends underwater, the increased pressure causes nitrogen from breathing air to dissolve into the body's tissues and bloodstream. This follows Henry's Law, which states that the amount of gas that dissolves in a liquid is directly proportional to the partial pressure of that gas.

During ascent, if a diver rises too quickly, the rapid decrease in ambient pressure causes the dissolved nitrogen to come out of solution too quickly, forming bubbles in the tissues and bloodstream. These bubbles can block blood vessels, damage surrounding tissues, and disrupt normal bodily functions.

Symptoms and Effects

Symptoms can range from mild joint pain and skin rashes to severe neurological problems, paralysis, and even death. The name "the bends" comes from the characteristic posture adopted by sufferers due to joint pain.

Prevention

Divers prevent decompression sickness by ascending slowly, making decompression stops at certain depths, and following dive tables or using dive computers that calculate safe ascent profiles based on depth and time spent underwater.

The Science Behind "The Bends"

Chemical Equilibrium and Henry's Law

Henry's Law states that the concentration of a dissolved gas (C) is proportional to its partial pressure (P) above the liquid, expressed as:

C = kH × P

where kH is the Henry's law constant. Under high pressure during a deep dive, the increased partial pressure causes more nitrogen to dissolve into the diver's blood and tissues, establishing a new equilibrium state. This is a dynamic process where the rate of gas molecules entering solution equals the rate of molecules leaving solution.

Disruption of Equilibrium During Ascent

As a diver ascends, ambient pressure decreases, shifting the equilibrium established at depth. According to Le Chatelier's principle, the system responds by releasing dissolved nitrogen to establish a new equilibrium at the lower pressure. If ascent is controlled, this off-gassing occurs gradually through respiration.

However, during rapid ascent, the equilibrium is disrupted too quickly. The dissolved nitrogen becomes supersaturated in the blood and tissues, exceeding the new equilibrium concentration. This supersaturation forces nitrogen out of solution, forming bubbles that can block blood vessels and damage tissues, resulting in decompression sickness.

Practical Applications of Equilibrium Principles

Understanding gas equilibrium has led to critical safety protocols in diving:

  • Controlled Ascent Rates: Limiting ascent to 9-10 meters/minute allows gradual re-establishment of equilibrium.
  • Decompression Stops: Pauses during ascent provide time for excess nitrogen to reach a new equilibrium state and safely diffuse out.
  • Enriched Air Nitrox: Breathing gas mixtures with higher oxygen and lower nitrogen content reduces nitrogen loading at depth.
  • Dive Tables and Computers: These tools model tissue gas loading and calculate safe ascent profiles based on equilibrium principles.

References

  • Divers Alert Network. "Decompression Sickness."
  • National Oceanic and Atmospheric Administration (NOAA). "Diving Safety & Decompression Sickness."
  • Basic principles of gas solubility and Henry's Law as found in standard physical chemistry texts.
  • Brubakk, A.O., & Neuman, T.S. (2003). Bennett and Elliott's physiology and medicine of diving (5th ed.). Saunders.
  • Vann, R.D., Butler, F.K., Mitchell, S.J., & Moon, R.E. (2011). Decompression illness. The Lancet, 377(9760), 153-164.