In humans, the kidneys primarily excrete urea, which is the main nitrogenous waste product. Urea is formed in the liver from the breakdown of excess amino acids and other nitrogenous compounds. Ammonia is highly toxic and is converted to less toxic urea in the liver before being filtered by the kidneys. Uric acid and creatinine are also waste products excreted by the kidneys but are present in smaller quantities compared to urea as the primary nitrogenous waste.