It is a common question why humans cannot breathe nitrogen, despite it being the most abundant gas in the atmosphere. Understanding this involves exploring the specific biological mechanisms of respiration and the distinct properties of different gases. The human body has evolved to utilize oxygen for its metabolic needs, a process that nitrogen, due to its chemical nature, cannot support.
The Air We Breathe
The air we breathe is a mixture of gases, primarily nitrogen (about 78%) and oxygen (about 21%). The remaining 1% consists of trace gases like argon, carbon dioxide, and water vapor. While nitrogen is the most abundant gas, its role in human respiration is passive.
The respiratory system, including the nose, mouth, throat, trachea, and lungs, takes in this gas mixture. However, the body does not utilize all inhaled gases in the same way.
How Our Bodies Use Oxygen
Oxygen is essential for human survival, serving as a reactant in cellular respiration. When inhaled, air travels to tiny air sacs in the lungs called alveoli. Oxygen then diffuses across the alveoli walls into surrounding capillaries.
In the bloodstream, oxygen primarily binds to hemoglobin within red blood cells. This oxygen-rich blood is transported throughout the body to tissues and cells. Inside cells, oxygen is used in the mitochondria to break down nutrients, like glucose, generating adenosine triphosphate (ATP). ATP provides the energy for nearly all bodily functions, from muscle contraction to nerve impulses.
Why Nitrogen Isn’t Used for Respiration
Unlike oxygen, nitrogen does not participate in cellular respiration. Nitrogen gas consists of two atoms bonded by a strong triple bond, making it chemically unreactive. This stable structure means the human body lacks the enzymes and mechanisms to break these bonds and utilize nitrogen for energy.
When inhaled, nitrogen enters the lungs and dissolves into the blood plasma. However, it does not readily bind with hemoglobin or engage in metabolic reactions within cells. Consequently, inhaled nitrogen is largely exhaled unchanged. While nitrogen is an important element for proteins and DNA, this is obtained from food, not directly from breathing nitrogen gas.
Consequences of Breathing Pure Nitrogen
Breathing an atmosphere composed solely or predominantly of nitrogen poses a serious danger. The risk stems not from nitrogen being toxic, but from the absence of oxygen. When oxygen levels in inhaled air drop significantly, the body rapidly experiences hypoxia, a condition of oxygen deprivation.
Without sufficient oxygen for cellular respiration, cells cannot produce the ATP required for bodily functions. Oxygen deprivation quickly leads to loss of consciousness, often within seconds or a few breaths, as the brain is highly sensitive to a lack of oxygen. Prolonged hypoxia can result in brain damage and, ultimately, death due to the failure of physiological processes. The body provides little warning because the natural sensation of suffocation is primarily triggered by rising carbon dioxide levels, which may not occur when breathing an inert gas like nitrogen.