What substance is primarily eliminated by the kidneys to maintain the body's nitrogen balance?

The kidneys play a crucial role in regulating the body’s nitrogen balance, primarily through the elimination of urea. Urea is a waste product formed in the liver from the breakdown of proteins. When proteins are metabolized, they release nitrogen, which must be excreted to prevent toxicity in the body. Urea is less toxic than ammonia, another nitrogenous waste product, and is soluble, allowing for easier excretion through urine.

The kidneys filter the blood, reabsorbing necessary substances and secreting waste products. Urea is filtered out of the bloodstream and concentrated in urine before being expelled from the body. Its elimination helps to regulate nitrogen levels, maintaining the body's overall balance and preventing potential damage from excess nitrogen compounds.

While ammonia is also a product of protein metabolism and can be excreted, it is quickly converted to urea in the liver due to its toxicity. Glucose, on the other hand, is not a waste product, as it is an essential energy source and is primarily reabsorbed in the renal tubules during filtration. Water is a vital substance for the body, but it is not specifically a nitrogenous waste product like urea.