Breathing is fundamental to life on Earth. While humans commonly understand they breathe oxygen, the atmosphere is predominantly nitrogen. This raises an intriguing question: why do our bodies specifically utilize oxygen for survival, given nitrogen’s overwhelming abundance?
The Air Around Us
Earth’s atmosphere consists of a mixture of gases, with nitrogen and oxygen being the most significant components. Dry air is approximately 78.08% nitrogen (N₂) and 20.95% oxygen (O₂), alongside smaller amounts of argon, carbon dioxide, and trace gases. Water vapor is also present in variable amounts, typically averaging around 1% at sea level.
When we inhale, both nitrogen and oxygen enter our lungs, but their fate differs dramatically. Nitrogen primarily enters and exits the respiratory system without significant chemical changes, effectively acting as an inert filler gas. This contrasts sharply with oxygen, which is actively processed and consumed by the body.
Oxygen’s Vital Role in Respiration
Oxygen plays a central role in cellular respiration, the process by which cells convert nutrients, such as glucose, into adenosine triphosphate (ATP), the primary energy currency of the body. Oxygen enters the lungs, crosses into the bloodstream, and is transported throughout the body to individual cells.
Within the cells, particularly in the mitochondria, oxygen acts as the final electron acceptor in the electron transport chain. This chain is a series of reactions that generate a large amount of ATP. Without oxygen to accept these electrons, the electron transport chain would cease to function, preventing efficient energy production. Oxygen’s high reactivity and indispensable role in generating usable energy make it essential for most living organisms.
Nitrogen’s Inert Nature
In stark contrast to oxygen, nitrogen gas (N₂) is chemically stable and largely unreactive under normal physiological conditions. This stability stems from the strong triple bond that connects the two nitrogen atoms in an N₂ molecule. Breaking this triple bond requires significant energy, which the human body cannot readily provide.
Consequently, when nitrogen is inhaled, it simply dissolves into the blood and tissues without undergoing chemical reactions. Most inhaled nitrogen is then exhaled unchanged. While living organisms require nitrogen for building proteins and DNA, they obtain it from food, not directly from the air, relying on specific bacteria to “fix” atmospheric nitrogen into usable forms.
When Nitrogen Becomes a Problem
While generally harmless at normal atmospheric pressure, nitrogen can pose risks under specific circumstances. One scenario is suffocation by displacement. If the concentration of nitrogen in the air becomes too high, it displaces oxygen, leading to an oxygen-deficient environment. Breathing pure nitrogen is dangerous not because it is toxic, but because it rapidly deprives the body of oxygen, leading to unconsciousness and death.
Nitrogen can also cause issues at increased pressures, such as during deep diving. Under high pressure, more nitrogen dissolves into the body’s tissues. This can lead to nitrogen narcosis, a reversible condition where dissolved nitrogen affects the nervous system, causing symptoms similar to alcohol intoxication, including impaired judgment and disorientation. If a diver ascends too quickly after a deep dive, the dissolved nitrogen can form bubbles in the blood and tissues, a condition known as decompression sickness or “the bends.” These bubbles can cause pain, tissue damage, and serious medical conditions.