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Apotropaic_Sphinx t1_jaxqii0 wrote

In our bodies? No, it's not from the air, it comes from food you eat. Pure Nitrogen is too strongly bonded together to be useful in plants and animals. Microorganisms break down the N2 into nitrites and nitrates so that plants can use it, then animals eat the plants (or animals eat the animals that eat the plants.) Nitrogen compounds are also manufactured industrially for fertilizers.

See: The Nitrogen Cycle

Ultimately the main Nitrogen compound in your urine (urea) is produced in the liver from the breakdown of proteins in the blood. This would normally create ammonia (like what aquatic animals do) but Ammonia is a powerful oxydizer and highly toxic, so our livers bond two Ammonia molecules to a Carbon Monoxide molecule to make it safer for our urinary track.

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CrazyisNSFW t1_jaxz3dw wrote

And urea requires less water for excretion, something important when you're living on land. Although urea requires more energy, it's excretion is safer and conserves water.

Also, around 10% of kidney nitrogen excretion is in the form of ammonia (in normal conditions)

Further read: Urea excretion in humans

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Apotropaic_Sphinx t1_jay0ge6 wrote

You are absolutely right. I wanted to touch on that but didn't want to get too wordy. Plus other land animals (like birds) have a different solutions for the toxic Ammonia problem.

This is one of those things that can get super complicated the further down you get into the minutia. OP's question was fairly simple so it's basically food+metabolism+liver=Nitrogen in urine.

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