Protein

• To survive, you must consume three macro-nutrients in your diet -- carbohydrates, fats and proteins. The digestive system converts protein to amino acids, which it can burn for energy or use to build body structures, such as muscle. If you have more amino acids than you can use, the body must break them down further and convert them to something else, as it can't store amino acids.
  
  

Ammonia

• If you eat more protein than your body needs at any one time, the extra amino acids that emerge from your stomach and intestines will collect in your liver, and meet up with one of six enzymes that break them down. One product of this breakdown is ammonia, a chemical compound used widely in industry. In your body, however, it's a waste product that must be eliminated.
  
  

Urea

• Ammonia from the liver enters your bloodstream, but when this ammonia-laden blood returns to the liver, a further chemical reaction combines nitrogen and oxygen with the ammonia. The result of this reaction is urea, which also leaves the liver through the bloodstream.
  
  

Urine

• When this urea-filled blood enters your kidneys, it collects in kidney structures called nephrons. The urea-free blood will then re-circulate, while the urea enters the urinary tract, where it will combine with the other components of urine and be expelled. For all of these processes to work properly, you need a healthy liver and kidneys. Liver function tests and the blood urea nitrogen, or BUN, test are part of the blood work your doctor requests to determine if your liver and kidneys are functioning properly.