Which fluid is excreted by the kidneys?

The kidneys are responsible for filtering waste products from the blood and excreting them in the form of urine. This process involves the removal of not only excess water but also various metabolic wastes, including urea, creatinine, and other substances. Urine is the final product of the kidneys' filtration process, which is then transported from the kidneys to the bladder and eventually released from the body.

While blood, urea, and plasma are related to kidney function, they do not represent the fluid that is actually excreted. Blood is the fluid that enters the kidneys for filtration, plasma is the liquid component of blood that contains water, salts, and proteins, and urea is one of the waste products found within urine. Thus, the excretion of urine represents the culmination of the kidneys' role in maintaining homeostasis and regulating body fluids.