Urine is the substance that is filtered out of the blood by the kidneys, and is expelled through
the urinary tract.
Urine is composed mainly of water, salt, urea and other organic materials.
The main component is water which gets
absorbed into the blood through the walls of the small intestine. The colour of your urine is determined by how much water
you drink, and how much of that water you use. Your urine should normally be clear, and if it is yellow, it means you are
dehydrated. Urea, also known as carbamide, is the next largest component of urine. It is a neutralized version of ammonia,
because ammonia is too toxic for the body. The process which does this is called the urea cycle, and it is performed in the
liver. Creatinine is the next largest component of urine. It is a byproduct of creatine phosphate in muscle tissue. Creatine
phosphate is used in the production of ATP from ADP, and afterwards turns into creatinine, which is filtered by the kidney,
and expelled as part of urine. The next largest component of urine is uric acid. It is created in the liver by a process called
xanthine oxidase. Without xanthine oxidase, the reactants, xanthine and hypoxanthine would be found at high concentrations
in the blood and can cause renal failure, also known as kidney failure.