Re: where does the nitrogen go when it gets into the human body?

Two answers (probably more, but two to keep it simple).

If by nitrogen you mean nitrogen gas (N2, the stuff we breathe), then it 
doesn't go anywhere really.  You breath it into the lungs with the 
oxygen.  The oxygen binds to hemoglobin in the red cells in your blood.  
However the nitrogen has no affinity (liking) for the hemoglobin or for 
much of anything else and so the very, very large majority of nitrogen is 
simply breathed right back out.  A very, very little nitrogen simply 
dissolves in the blood and into any cell/tissue.  However, the organism 
(you or a plant for example) are at steady state (meaning there is no net 
change) so the actual amount of nitrogen dissolving in the blood with each 
breath is equal to the amount that comes back out of the blood into the 
air within the lungs.  Now the nitrogen dissolved in the blood and tissues 
is simply inert.  Our bodies have no ability to react chemically with that 
nitrogen.  We lack the necessary genes encoding the necessary enzymes.  
Certain bacteria however have enzymes that can reach with nitrogen to 
break it down and make use of it.  We don't.  So any nitrogen dissolved in 
our bodies is just there.  Doesn't do anything.

If, however, by nitrogen you mean chemical compounds with nitrogen in them 
(such as an amino acid, which has carbon, hydrogen, oxygen, nitrogen and 
sometimes sulfur or more rarely selenium), then that nitrogen comes from 
other chemicals that we eat or make.  It is called "fixed" nitrogen to 
distinguish it from gaseous (and non-fixed) nitrogen.  It can be obtained 
via food, or we can use the fixed nitrogen already in our bodies and make 
new chemicals we might need from other chemicals.  This does not in any 
way involve nitrogen as a gas, but as, for example, the amino group (NH2) 
on amino acids.

If this doesn't answer your question or I didn't understand it, write back.