Biomedical Engineering

I'm always thinking of why things happen and breaking down daily functions to find the root cause or purpose. Maybe that's why I chose engineering. My degree and working for GlobalSpec has allowed me to surround myself with people who are as curious about life as I am. This is a rather long-winded introduction to another "medical (and life) mystery" post. This weekend I was talking to a friend and he asked the question, "What happens to the Nitrogen we breathe?" So for all you curious minds, here is the answer.

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The air we breathe is comprised of about 21% oxygen (O₂), 78% nitrogen (N₂), and trace amounts of other assorted gases. So if we breathe in O₂ and exhale carbon dioxide CO₂, what happens to all that nitrogen?

Where Does The Nitrogen Go?

Nitrogen is an inert gas, which means that it is unreactive, colorless, odorless, and tasteless. It is naturally occurring in the atmosphere but it is not used in respiration at all. In short, almost all the nitrogen you breathe in, you breathe back out again, unchanged. The fact that the percentage of nitrogen drops from 78% (inhaled) to 75% in the exhaled breath, is because of the addition of water vapor in the gases you breathe out. Dry air picks up moisture from your lungs, so the air you exhale is humid. This also accounts for the "clouds" you see when you exhale on a cold day. The water vapor in your breath condenses.

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Dangers of Nitrogen

Inhaling nitrogen alone is dangerous. Build up to too much nitrogen can cause various issues including muscle cramps, and general fatigue. Nitrogen is a natural asphyxiant. Each breath exchanges about .6 liters from an active lung volume of 3 liters. After 2 or 3 breaths of N₂ , the O₂ concentration in the lungs would be so low that O₂ in the blood will exchange back into the lungs and be exhaled before new O₂ could be supplied to the blood. Oxygen in the blood would reach zero in 3 minutes. Unconsciousness will between 40seconds to 60seconds depending on the oxygen concentration. In very low O₂ environments, the parts of the brain responsible for breath die, causing the person to asphyxiate in less than 7 minutes. There is a particular mixture of nitrogen and oxygen which is used as a breathing gas during scuba diving.

But nitrogen does play a role for the growth of living things. It constituents a part of human DNA, and therefore is part of our genetic code. You probably learned about the nitrogen cycle in middle school science class. Nitrogen in the atmosphere ( N₂) is inert and can't be incorporated into the molecules of living things. Therefore, all the organisms on Earth depend upon the activity of bacteria in the soil, lightning strikes, and factories to convert nitrogen into useable forms.

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This was a difficult area to find any clear answers on. Do you know what happens to the nitrogen we breathe?

Resources

Nitrogen

Biology for Dummies

Why do you exhale?

A text-book of human physiology

Nitrogen Asphyxiation