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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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