Final answer:
Human beings convert toxic ammonia into urea, which is then accumulated before being excreted as the primary form of nitrogenous waste.
Step-by-step explanation:
Human beings accumulate urea before excreting nitrogenous waste. During the catabolism (breakdown) of proteins, nitrogen-containing macromolecules such as proteins and nucleic acids are broken down into amino acids. The amino acids are then deaminated to remove the nitrogen groups, resulting in the formation of ammonia, which is toxic to the body. This ammonia is rapidly converted into urea in the liver, making urea the primary nitrogenous waste in humans. Uric acid and creatinine are also present in smaller amounts in human urinary wastes. Terrestrial organisms, including humans, need to convert the toxic ammonia into a less harmful substance such as urea or uric acid due to the limited availability of water to dilute and excrete ammonia directly. Notably, mammals, including humans, are classified as ureotelic animals because urea is their primary nitrogenous waste material.