What substance is carried through the bloodstream to the kidneys for excretion?

The substance that is primarily carried through the bloodstream to the kidneys for excretion is urea. Urea is a waste product formed from the breakdown of proteins in the liver. Once proteins are metabolized, amino groups are converted into ammonia, which is then converted to urea, a less toxic compound. This urea is released into the bloodstream and transported to the kidneys, where it is filtered out and excreted in urine.

This process is vital for regulating the body’s nitrogen balance and preventing the accumulation of toxic wastes in the bloodstream. The kidneys play a crucial role in maintaining homeostasis by filtering blood, removing waste products like urea, and ensuring that essential substances are reabsorbed back into the body as necessary.

The other substances listed—glucose, cholesterol, and protein—have different roles in the body and are not primarily waste products meant for excretion. Glucose is a critical energy source for the body's cells, cholesterol is important for cellular structures and hormone production, and protein serves as a fundamental building block for tissues, all of which are not directly excreted as waste in the same manner as urea.