What is one of the primary functions of the kidneys?

The kidneys play a vital role in maintaining overall health and homeostasis in the body, and one of their primary functions is the removal of waste from the blood. They achieve this by filtering blood, where they remove metabolic waste products such as urea, creatinine, and excess ions. This filtration process occurs in the nephron units of the kidneys, where waste is collected and eventually excreted as urine.

By efficiently filtering out these wastes and excess substances, the kidneys help regulate fluid balance, electrolyte levels, and acid-base balance, which are crucial for normal physiological function. This waste removal is essential not just for waste excretion, but also for preventing the buildup of potentially harmful substances in the bloodstream, thus contributing to the body's overall health and well-being. The other options do not accurately reflect the primary functions of the kidneys, as they are involved in processes that are either performed by different organs or are not main functions of the kidney itself.