What is the primary function of the kidney?

The primary function of the kidney is to filter blood and excrete waste products through urine, making the correct choice the one that points to the excretion of urine. The kidneys play a critical role in maintaining homeostasis within the body, primarily through two functions: filtration of blood to remove toxins and excess substances, and regulation of fluid and electrolyte balance. As blood passes through the kidneys, nephrons filter out waste materials, toxins, and excess ions, which are then excreted as urine.

While the production of urine involves excreting waste, it is the filtering of blood that is fundamental to the kidney's role in the body. Each kidney consists of approximately one million nephrons, which conduct the filtration process and ensure that the body retains necessary substances while eliminating the unnecessary ones. This crucial function supports overall health by regulating bodily fluids and preventing the build-up of harmful waste products.

The other options refer to functions that are not associated with the kidneys. Digestion of food occurs in the gastrointestinal tract, and insulin production is the function of the pancreas. Therefore, understanding the kidneys' role in filtering blood and producing urine helps clarify their essential contribution to bodily functions.