While oxygen is the component the body uses for cellular respiration, it comprises only about 21% of the atmosphere. The remaining bulk of the air, roughly 78%, is made up of nitrogen gas, which is taken into the lungs with every breath. The fate of this large volume of nitrogen, which does not participate in metabolism, reveals a fascinating interplay between basic physics and human physiology.
Nitrogen’s Role in Normal Breathing
At the standard atmospheric pressure experienced at sea level, nitrogen is largely a passive participant in the process of breathing. The body lacks the necessary enzyme systems to “fix” or utilize the strong triple-bonded diatomic nitrogen molecule (\(\text{N}_2\)), meaning it is chemically inert within the human body.
The gas enters the respiratory tract, travels into the alveoli of the lungs, and then diffuses across the membranes into the bloodstream. However, it does not bind to hemoglobin like oxygen. An almost equal amount of nitrogen is subsequently diffused from the blood back into the alveoli and is exhaled. A small amount of nitrogen dissolves in the blood and tissues, establishing a state of equilibrium that causes no metabolic reaction or adverse effect under normal conditions.
How Nitrogen Dissolves in the Bloodstream
The amount of nitrogen that dissolves into the body shifts dramatically when the surrounding pressure increases. This change is governed by a physical principle known as Henry’s Law, which states that the concentration of a gas dissolved in a liquid is directly proportional to the partial pressure of that gas above the liquid. As the total pressure goes up, the force pushing nitrogen into the body’s fluids and tissues increases.
This effect is most pronounced in environments like deep-sea diving, where the ambient pressure rises significantly with depth. For every 10 meters (33 feet) a scuba diver descends, the pressure increases by approximately one atmosphere. As the pressure rises, the partial pressure of nitrogen in the breathing gas also rises, forcing more nitrogen molecules to dissolve into the bloodstream and eventually into various body tissues, including fat and muscle. This process of gas uptake continues until the tissues are saturated, meaning the pressure of the dissolved nitrogen matches the partial pressure of the nitrogen being breathed.
Health Effects of Absorbed Nitrogen
When nitrogen is forced into the tissues under high pressure, it can lead to two primary adverse health consequences. The first effect experienced at depth is nitrogen narcosis, often described as an intoxicating or anesthetic effect on the central nervous system. This condition occurs because the dissolved nitrogen interferes with nerve impulse transmission in the brain, leading to impaired judgment, reduced motor skills, and short-term memory loss.
The narcotic effect becomes noticeable for many individuals when the partial pressure of nitrogen reaches a certain threshold, typically corresponding to depths around 30 meters (100 feet) or greater. The second, more dangerous consequence is decompression sickness (DCS), commonly called “the bends.” This occurs when the ambient pressure is reduced too quickly, such as during a rapid ascent from a deep dive. The rapid pressure drop causes the nitrogen that was dissolved in the tissues to come out of solution and form bubbles, similar to opening a carbonated drink.
These nitrogen bubbles can form in any tissue, but they cause pain and injury when they lodge in joints, block blood flow in capillaries, or interfere with neurological function. To prevent DCS, divers must follow slow ascent rates and perform decompression stops, allowing the excess dissolved nitrogen to diffuse safely back into the blood and be exhaled through the lungs before the pressure change can trigger bubble formation.