What substance is formed by the kidneys and expelled from the body?

The kidneys play a crucial role in the body's excretory system, specifically in the filtration of blood and the formation of urine. Urine is a byproduct created through several processes in the kidneys, including filtration, reabsorption, and secretion. The kidneys filter out waste products, excess substances, and toxins, which are then concentrated with water to form urine. This solution is then expelled from the body through the urinary tract.

While urea is indeed a component of urine (a waste product of protein metabolism), urine itself encompasses more than just urea; it includes water, electrolytes, and various solutes. Bile is produced by the liver and aids in digestion, while plasma refers to the liquid component of blood that carries cells, nutrients, and proteins. These substances do not correspond to the excretory function of the kidneys in the same way that urine does. Therefore, recognizing urine as a mixture formed by the kidneys emphasizes its fundamental role in waste removal from the body.