The air you breathe is around 78 percent nitrogen, so nitrogen enters your body with every breath. Because nitrogen is an important part of human health, it is unfortunate that the nitrogen people inhale gets immediately exhaled. Animals, including humans, cannot absorb nitrogen in its gaseous form.
Plants and Soil
Plants also need nitrogen to survive. Many plants can absorb nitrogen from compounds in the soil like nitrates, nitrites and ammonia. Some plants -- mostly legumes and a few trees and shrubs like birch and alder trees -- have symbiotic relationships with bacteria; the microorganisms attach to the roots of the plants and make nitrogen compounds out of nitrogen gas in the soil. Plants use the nitrogen to produce proteins, enzymes, amino acids and nucleotides (components of DNA) -- all of which people absorb when they eat plants. This is the primary source of nitrogen in animals, but you also absorb nitrogen from meats you eat.
A small amount of nitrogen and other gases are absorbed into the outer layer of your skin, the epidermis. Nitrates and nitrites are added to meats to preserve the red color. Municipal water supplies are treated to remove nitrogen compounds that enter groundwater from agricultural fertilizer runoff, but small amounts still persist in most drinking water.
Most animals ingest more nitrogen than they can absorb, so much of it gets excreted. When cells use protein, the waste product is urea, which is almost half nitrogen. It travels through the bloodstream, and the urea gets filtered out in the kidneys and mixed with water to produce urine. The ammonia and urea in urine then become a valuable fertilizer for plants because of its high nitrogen compound content. Some nitrogen is lost in the shedding of hair, nails and skin as well.
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