if we inhale 78% Nitrogen and Exhale 78% does this mean we dont use any of it?

Yeah,our cells don't use any of the nitrogen and for the 3.9% of CO2,it is given out by the process of cellular respiration, which is needed for producing energy.

The nitrogen source for the body is food-mainly proteins.From the total air we inhale only oxygen is used for respiration and all other components are exhaled out,including N,Co2 and other gases.The N inhaled is not used in any biological reactions..

As a result of cellular respiration,CO2 is produced in the body,this is excreted out in 2 ways,1)through exhalation and 2)by combining with NH3 and formation of urea and excretion through urine..This why eventhough we inhale 0.03% CO2,we exhale 4% CO2

correct. The N2 we inhale is not reactive with any body processes. It does flow around in our blood stream, though, and can cause problems for divers going down below a couple of bars (atmospheres, about 32 feet).

The CO2 comes from metabolic processes that combine oxygen with carbon, releasing energy.

Our bodies don't require external nitrogen. Our lungs take in oxygen and release carbon dioxide from the blood. The CO2 in exhaled air came from metabolism of lipids, proteins and carbohydrates within our bodies.

Yep. The 3.97% comes from the chemical reactions inside our bodies where oxygen is used by the cells.

Yeap. We don't use nitrogen. It is a elatively nonreactive gas.

we don't use nitrogen