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When a scuba diver breathes compressed air, about 75 % of the gas mixture is nitrogen. The external pressure of the water forces nitrogen molecules into the body, primarily into fatty tissues. The deeper and longer a dive, the greater the nitrogen load. As the diver ascends, the pressure drops, and dissolved nitrogen is released into the bloodstream. If the gas comes out too quickly, bubbles form—this is what causes decompression sickness or “the bends.”
For the average person, nitrogen enters the body mainly through the diet. In drinking water with less than 10 mg N L⁻¹ of nitrate, the contribution is negligible. In regions where nitrate concentrations exceed 50 mg L⁻¹, water can become a significant source. However, most people obtain nitrogen from the foods they eat.
All protein‑containing foods contain nitrogen, but the amounts vary. Vegetables and fruits—such as lettuce, radish, rhubarb, and spinach—are high in nitrogenous compounds. Dairy products, meats, and fish also provide substantial nitrogen, although usually less than the leafy greens. Cured meats (sausages, ham) are especially nitrogen‑dense because they are preserved with sodium nitrate or nitrite.
Proteins are the body’s primary nitrogen carriers, composed of carbon, hydrogen, oxygen, and nitrogen. Humans and other animals cannot absorb atmospheric nitrogen; instead, they rely on consuming plants or animals that have incorporated nitrogen from the environment.
Plants do not produce nitrogen themselves; they acquire it from the soil. The cycle starts when animals eat plants. The animal’s waste—rich in nitrogenous compounds like urea and amino acids—is broken down by bacteria, which convert the nitrogen into atmospheric nitrogen gas or nitrates. Rain then delivers these nitrates back into the soil, where plants absorb them and synthesize new proteins. This continual loop ensures a steady supply of nitrogen to the food chain.
Certain crops can increase the amount of nitrogen available in the food chain. Legumes (alfalfa, beans, peas) host nitrogen‑fixing bacteria in root nodules. These bacteria convert atmospheric N₂ into ammonia, enriching the soil with bioavailable nitrogen. As the plant grows, it incorporates this nitrogen into its tissues, and when consumed, humans receive a richer nitrogen source.
