How do the kidneys contribute to nitrogen elimination?

The kidneys play a significant role in nitrogen elimination primarily through the conversion of ammonia to urea, which is then excreted from the body. This process is crucial because ammonia is toxic at high levels and must be detoxified for safe excretion. The liver carries out the conversion of ammonia, which is a product of protein metabolism, into urea through the urea cycle. Urea is then transported to the kidneys, where it is filtered out of the blood and excreted in urine.

While the kidneys do contribute to regulating blood plasma pH by excreting ammonium, this process is secondary to their primary function of nitrogen removal via urea. The core mechanism behind nitrogen elimination occurs outside the kidneys, making the option about the conversion of ammonia to urea the primary pathway through which the body eliminates excess nitrogen.

Synthesis of non-essential amino acids and filtering nutrients from the blood pertain more to metabolic and nutritional processes rather than direct nitrogen elimination. Thus, while the kidneys perform a variety of functions, their main role in nitrogen elimination is through the formation and excretion of urea derived from ammonia.